STORM CHASE MANUAL FOREWORD

This is the fourth edition of the Storm Chase Manual. I initially started the manual in 1979 when I was at TexasTech University being part of the Tornado Intercept Team. It has been 12 years since the last revision and muchinformation has been gathered during that time by storm chasing. Thus, I have updated the manual considerablysince then. The deployment of Doppler Radar and advancement in computer technology have brought tons ofreal time data to the storm chaser. We have learned a great deal about storms during the past twenty years,however, the problem still exists of trying to accurately forecast where tornadic storms will form.

Over the years, storm spotters and chasers have played an important role in our understanding of severe storms.Spotters and chasers were the first to recognize certain cloud precursors to tornado development. Words likebeaver tail, wall cloud, rear flank downdraft, and gustnado were developed by storm chasers and spotters todescribe certain storm features. More detailed definitions and applications of such terms can be found in mybook entitled STORM TALK. I’ve used bold type herein to emphasize important words.

Tips on forecasting for tornadoes are presented but by no means is comprehensive. Traditional methods areshown but there are many exceptions to the general rules of thumb. You will find that forecasting when andwhere tornadoes will strike is mostly trial and error, with failure being the most common result. Readers areencouraged to seek out and study the reference material listed herein. To test your forecasting skills further, trymy just published TORNADO FORECASTER’S WORKBOOK #1.

I’m glad our knowledge about storms continues to grow. By watching the sky and trying to understand what wesee, we not only gain insight into the world around us but we also advance the science. It is still a new science,so we must continue to update the writings on this subject. Thus, this revised manual (now in its fourth edition)is larger and incorporates a lot of new material. I thank all the spotters and chasers who have contributed to ourunderstanding of storms. I am especially grateful to Roy Britt, Don Burgess, Dr. Charles Doswell, Carson Eads,David Hoadley, Les Lemon, Jim Leonard, Alan Moller, Gene Rhoden, Phil Sherman, and Tim Vasquez who haveprovided information, photographs, or suggestions to this manual and who excite and motivate others to look atthe sky.

Before setting out to chase storms, it is important to have knowledge about them. Talk to others who have thesame fascination. If you haven't chased storms before, it is highly recommended that you chase with someonewho has more experience. Planning a chase involves more than the preparation of the car. Are you prepared?

The author disclaims any responsibility for persons who use any portion of this manual.

c 1979, 1983, 1986, 1998 - 4th Edition

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TABLE OF CONTENTS . l. Chase Objectives 2. Planning to Chase. 2.1-2.3 Personal, Vehicle, and Photographic Items

2.4-2.5 Other Items, Computer/Television Equipment. 3. Personnel. 3.1 Driver 3.2-3.3 Navigator and Observer

4. Documentation 4.1 Storm Structure 4.2-4.3 Tornadoes and Local Weather. 4.4-4.6 Hail, Lightning, and Photographs

4.7 Radio4.8-4.10 Television, Weather Data On-line, Weather Software

5. Thunderstorm Development 5.1 Growth Stages of Storms. 5.2 Multicell Storms

5.3 Supercell Storms 5.4 Storm-scale Features

6. Field Strategy 6.1 Long Range. 6.2 Medium Range 6.3 Close Range 7. The Tornado. 7.1 Life cycle. 7.2-7.3 Tornado-scale Features and Cyclic Tornadogenesis.

7.4 Tornado Perception 8. Observational Summary. 8.1 Rear Flank Downdraft Theory 8.2 Horizontal Vortex Tube Theory 9. Photography. 9.1 Photograph Procedures 9.2 Photograph Problems. 9.3 Photographing Lightning. 10. Forecasting a Chase. 10.1 The Severe Weather Outlook 10.2 Surface Mesoanalysis 10.3 Upper Air Maps.

10.4 Soundings and Stability. 10.5 Radar

10.6 Satellite 10.7 Climatology. 11. Post-Storm Damage Investigation. 11.1-11.2 F-scale rating and Photographing Damage.

11.3 Eyewitness Interviews. 12. Safety Rules. REFERENCES ii

1. CHASE OBJECTIVES

Knowing your chase objectives before you leave town is very important to insure a successful and enjoyablechasing experience. Regardless whether your approach to chasing is scientific or non-scientific, the followingobjectives should be kept in mind:

a. To return all participants safely to base. This is the first and most important objective. You should drivecarefully and always have an escape route when closing in on a storm. Many times the biggest hazard isslick roads, other cars, and lightning.

b. To photograph storm structure from birth to maturity. Photographs taken at great distances from thestorm can be used to document anvil growth, overshooting storm tops, flanking lines and overall stormstructure. At closer ranges, photographs can document storm features such as cloud base rotation, wall andtail clouds, and tornadoes. These photographs can be used to study the growth and maintenance ofthunderstorms and tornadogenesis.

c. To photograph, at close range, the motion of tornadic debris clouds. Equipment should be mounted on tripods to enhance the value of the footage.

d. To photograph unique storm features such as large hail, lightning, gustnadoes, mammatus, etc. and document the storm structure/environment.

e. To take meteorological data in and around storm environments which include sampling temperature,dewpoint, wind speed and direction. The purpose of taking weather measurements will be to sample theinflow regions of the storm's updraft, dry line cross section and rear flank downdraft.

f. Document of all weather observations and film shots using a hand held tape recorder noting the time andlocation of each. This makes each chase more meaningful and helps you recall pertinent items about the chase.

2. PLANNING TO CHASE

There are several must items to bring along on a chase. You will find some items are more important thanothers. Being organized is the key. I have segregated these items into four categories: personal, vehicle,photography, and instrumentation. Personal items can be carried in your luggage and placed in the trunk. It is agood idea to place vehicle items in a sturdy box in the trunk. Camera bags or cases are a must to carryphotograph equipment in the vehicle with you. Another storage case can be used for weather equipment. Makesure you have plenty of room to spread the cases out so that you aren’t fumbling around for cameras orequipment. It is a good idea to anchor the cases to the floor or seats with bungee cords so your equipment willnot move around in case you stop quickly. Cameras should be loaded with film and tested prior to starting thechase. Keep your camera equipment out of direct sunlight to avoid heat damage to the film.

When stopped, make sure your vehicle is in plain sight at all times. If you are at a restaurant or conveniencestore, cover your equipment with a blanket. There have been chasers who have had their equipment stolen whileaway from their vehicle by leaving the vehicle doors unlocked or windows down. When stopped for the evening,make sure you take all of your camera equipment out of your vehicle. If you have to make several trips to yourmotel room and you are not close to your vehicle, lock your vehicle in between each trip.

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2.1 PERSONAL ITEMS

There are essential personal items to have on a chase. Plan to stay overnight a day or two longer than youexpect. The following personal items are deemed essential:

___ Toothbrush ___ Wallet ___ Socks ___ Shaver or razor ___ Credit cards ___ Shoes ___ Small soap bars ___ Telephone card ___ Underwear ___ Sunglasses ___ First aid kit ___ Belt ___ Wrist watch ___ Cassette tapes ___ Pen/Pencils ___ Money and/or ___ Shirts ___ Paper Travelers checks ___ Kleenex ___ Light coat ___ Snacks ___ Bug spray ___ Small pillow ___ Hair dryer ___ Cotton swabs ___ Small alarm clock

2.2 VEHICLE ITEMS

You depend greatly on your vehicle. Make sure it is reliable and has undergone a thorough pre-chase inspection.Having your car serviced on a moderate or high risk day can ruin your chasing experience. Check all fluid levels:gas, oil, transmission.

___ Tire pressure gauge ___ Tire inflator ___ Road flares ___ Extra fan belts ___ Small wood board ___ Gallon of water ___ Extra oil ___ Car jack ___ Plastic funnel ___ Jumper cables ___ Flashlights ___ Duct tape ___ Coolant ___ Road Maps ___ Road Emergency ___ Fire Extinguisher ___ AAA Service Card Service Kit ___ Extra Car Keys ___ Tool Kit ___ Extra hose ___ Rain-X ___ Bungee cords ___ Spare tire ___ Window cleaner ___ Paper towels ___ Extra wipers

2.3 PHOTOGRAPHIC ITEMS

Next to your vehicle, your photographic equipment is critical for the chase. Try keeping an eye on your vehiclewhen you are in a restaurant or store. Lock your vehicle or have your chase partner stay with the vehicle even ifyou are running into the store for a minute. Cover your equipment when not in use. The followingphotographic items are deemed essential:

___ Camera and backup ___ Camera case ___ Camcorder and backup ___ Film for camera ___ Blanket ___ Battery pack(s) ___ Camera batteries ___ Plastic bags ___ Videotapes ___ Lenses 28-400mm ___ Cotton swabs ___ Soft cloth ___ Tripod- Large/small ___ Lens cleaner ___ Lighter adapter ___ Cable release ___ UV Filter ___ Polarizer ___ Masking tape ___ Tissue paper ___ Lens cases

Use plastic bags to place spent film and cassettes. Rolls of film have been lost on past chases and spent cassetteshave been taped over.

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2.4 OTHER EQUIPMENT

The following other equipment has been helpful on chases:

___ Tape recorder ___ Scanner and spare fuses ___ Detailed road maps ___ AA Batteries ___ ARRL (ham radio) book ___ Compass ___ Cassette tapes ___ External antenna ___ Barometer ___ Weather radio ___ Binoculars ___ Wet bulb ___ 9-volt battery ___ Anemometer ___ Vial of water ___ Transistor radio ___ Thermometer ___ Stakes/spray paint ___ Ruler/baseball ___ Blank Wx maps ___ Motel directory ___ Address book ___ AM Weather guide ___ Radio directory

Detailed road maps include those from the “Roads of” series as well as the Gazetteer series. A Rand McNally orGousha road atlas are good backups. Use the ruler or baseball for scale next to hailstones. Stakes or spray paintcan mark the edge of the road where you have been. Many motels, airlines and rental car agencies have 1-800numbers. A list of favorite motels and restaurants can be found on the Storm Chaser Home Page athhtp://taiga.geog.niu.edu/

2.5 COMPUTER/TELEVISION EQUIPMENT

A laptop computer can access the latest weather maps and model data. Thus, the need to visit a local weatherservice on each day of your chase becomes secondary as you can get any weather product on your laptopcomputer. Having television access can provide additional real time information when you are out in the field.The following computer and television items are recommended:

___laptop computer ____extra phone cord ____ blank floppy disks___small television ____power inverter ____ Cable TV dish___F-jacks ____remote control ____ satellite receiver___phone access numbers ____RCA plugs ____ acoustic coupler___cellular phone ____DeLorme Map CD

3. PERSONNEL

A chase team should be composed of at least two members with a third person highly recommended. Eachperson will be responsible for their own documentation of photographs. At least one member should haveextensive experience in storm chasing.

3.1 DRIVER

The driver assumes the utmost responsibility for the vehicle and crew. This includes all safety decisions and theproper operation of the vehicle. It is easy to become distracted during a chase and a few chasers have beeninvolved in accidents. Do not stop on the road. The driver:a. Is responsible for all traffic tickets.b. Must keep the vehicle in reliable working order.c. Must keep their eyes and the vehicle on the road while moving.d. Ultimately determines the best route decision.e. Must park completely off the road or pull onto road aprons when stopping.

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3.2 NAVIGATOR

As a team, the navigator coordinates with the driver on the heading and the route of the chase. The navigatorshould know where the vehicle is at all times and should plan for a good escape route in case evasive action isneeded. The navigator:

a. Documents the vehicles direction on tape and/or video recorder by noting road intersections, towns, riversand other landmarks according to odometer reading, heading, and time. b. Documents all starts and stops by odometer reading. c. Continues to document the surroundings when the car is stopped by noting the local landmarks such asbuilding types and colors, trees, hills, cattle guards, radio towers, oil derricks, windmills etc. d. Should know where they are on the road map at all times. e. Briefs the driver on the best route to take. f. Assumes the observers role if not present. g. Books motel/airline reservations. 3.3 OBSERVER

The observer has their eyes to the sky and gives input to the driver and navigator on areas of interesting weather.The observer can listen to scanners and radio for the latest bulletins and possible warnings. The observer:

a. Should document all interesting weather phenomena. b. Checks out the equipment before the chase. c. Maintains equipment in a state of readiness. d. Can take psychrometer readings every ten to fifteen minutes while the vehicle is in motion. e. At stops, records wind speed and direction in addition to taking psychrometer measurements. f. Helps navigator determine the best route to the storm. g. Listens to scanners and radios for weather updates.

4. DOCUMENTATION

Good documentation is essential to the success of the chase. It is important to establish a chronology of eventsso that a final chase summary can be prepared. Record the time and location of your observations. A hand heldtape recorder is best to document all the pertinent chase information.

4.1 STORM STRUCTURE

It is important to look around and view the entire sky. There is a lot out there to describe. This becomesdifficult when there is a focus on wall clouds and tornadoes. However, interesting cloud features may be presentoverhead or in back of you without being noticed. Simply, document the weather around you. Which way is thewind blowing? Is the air warm or cold? Also note any rapid changes in the storm such as the size and shape ofthe inflow bands or beaver tail. What changes are there in the anvil structure? Is there another tower developingin back of the main updraft? Such items are important to document as we can learn about how storms behave.For example, it has been found that tornado occurrence has some correlation with the collapse of overshootingtops. So even documentation of storm behavior on the horizon can be important. Once again, it is critical torecord the time and location of your observation.

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In identifying storm structure, be sure to mention your position relative to the storm or weather feature bydirection of the compass (i.e. 030 to 090 degrees). The amount of time between observations is left to thediscretion of the chaser. It is recommended that observations be made once every half hour in non-severe timesand nearly continuous when severe weather is occurring. The following storm structure information should bedocumented:

a. Storm type (multicell, supercell, squall line) b. Cloud tower orientation (vertical, tilted by wind shear) c. Cloud base (smooth, ragged, striations, and location) d. Anvil (backshear, overshooting, mammatus, and hardness) e. Wall cloud (rotation, tail, tilt, and location) f. Precipitation area (extent, type, and location) g. Gust front/shelf cloud (orientation, and location) h. Rear flank downdraft (location and distance) i. Inflow bands (number, orientation, and location)

4.2 TORNADOES

During the tornadic stage, excitement is high. It is easy to forget to document what is actually going on at thistime. The best thing to do is to talk to the tape recorder. Record as much of the following information aspossible: a. Appearance (V-shaped cone, cylinder, rope) b. Heading (yours and the storms, estimate speed) c. Tornado structure (core, sheath, base behavior, ripples) d. Vortices (number, formation cycle, and rotation) e. Time of touchdown and liftoff f. Time of stage changes (i.e. cone to cylinder, rope) g. Environment (audible noise, lightning, hail proximity)

4.3 LOCAL WEATHER

When approaching the storm, describe the local weather. Though skies overhead maybe relatively clear, cloudfeatures like altocumulus castellanus can indicate local instability. You may find the boundary of dry lines andoutflow boundaries simply by observing wind direction and dew point. Record the following general weatherinformation at least every half hour and note any large changes in the weather:

a. Sky cover (cloudy, broken, scattered, clear) b. Cloud types (cirrus, alto, strato, cumulus types) c. Cloud layers (high, middle, low) d. Visibility (blowing dust, haze, smoke) e. Dry bulb thermometer (increasing/decreasing) f. Wet bulb (increasing/decreasing) g. Dew point (increasing/decreasing) h. Wind speed (increasing/decreasing, gusts) i. Wind direction (backing/veering) j. Altimeter (rising/falling)

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4.4 HAIL

The size of hail gives an indication of storm severity. Usually, the larger the hail, the more severe the storm.Note the following information about hail:

a. Time hail begins and ends. b. Size ( Hail size is described by the weather service as the following approximate dimensions: pea =

<.25 inch, marble = .75 inch, golfball = 1.75 inches, and baseball = 2.75 inches). c. Shape (round, oval, plate, spiked) d. Hardness (mushy hit, solid hit, car permanently dented) e. Amount of ground covered (include depth) f. Amount of elapsed time on ground g. Weight and volume displaced - optional h. Hail crusher deflection - optional

4.5 LIGHTNING

Lightning is most common in the precipitation area. However, bolts can suddenly occur without warning justabout anywhere around a thunderstorm. It is not unusual to see lightning under the rain free base or anvil toground. Thus, be aware that lightning NOT the tornado is the storm chasers greatest hazard. The followingcharacteristics of lightning should be recorded:

a. Location (from your position, and in the storm) b. Distance (from you, from rain free base) c. Type (cloud-to-cloud, cloud-to-ground, in cloud) d. Structure (branching, single stroke, repeating, staccato) e. Color (white, blue, red, etc.) f. Thunder (sudden clap, long rumbling, continuous)

4.6 PHOTOGRAPHS

Photograph documentation is important to establish the time the picture was taken. With more than one cameraor lens, this documentation becomes more critical. Record the following photograph information.

a. Camera type (8mm, 16mm, 35mm, video) b. Time of picture (Local time) c. Direction of view (N, NE, E, SE, S, SW, W, NW) d. Description of subject- What are we looking at? Identify any landmarks in the picture. e. Type of lens (28mm, 50mm, 135mm, etc.) f. ASA film (25,64,100,200,400,1000) g. Aperture setting (1/1000 to bulb) h. Movie framing rate (How many frames/sec?)

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4.7 RADIO

Radios are very important in providing crucial information to maximize a chasers odds in seeing a tornado. Achaser without radio equipment is almost blind as tornado ten miles a way can go unnoticed sitting behind a rain-shaft. There are a variety of radios that a chaser can use. Record the following information directly on the taperecorder.

a. Time of report b. Storm location and heading c. Radar summary (time, intensity, cell heading) d. Town(s) mentioned near severe weather e. County(s) mentioned with severe weather f. Watches and warning locations

4.7.1 HAM RADIO/SCANNERS

Storm spotter nets are usually activated in a severe weather situation. These ham radio operators areexperienced and can give very useful information about the storms structure and position. This information inreal time is most useful to the storm chaser. Ham radio frequencies throughout the country are published by theAmerican Radio Relay League, 225 Main Street, Newington, CT 06111. You can also obtain this informationfrom the internet on the Storm Chaser Home Page at http://taiga.geog.niu.edu/

Depending on the chase area, some spotter nets have more experienced than others. Caution must be used whenlistening to these reports. Pay careful attention to the wind directions. Many times scud is reported as a wallcloud.

4.7.2. NOAA WEATHER RADIO

Weather radios can be bought at most department and Radio Shack stores. Bulletins on the NOAA WeatherRadio can be helpful if they are timely. It is important to recognize the time the report was issued. Radar reportsare updated every hour or two. However, be advised that the lag time in Weather Service Reports can be severalhours old. One of the drawbacks with the weather radio is that its range is limited, usually 40 miles in radius,more on a hilltop, less in a valley.

In Oklahoma, a NOAA Weather Radio product termed the Oklahoma Thunderstorm Outlook (OTO) is veryuseful to storm chasers. OTO provides detailed nowcasts on severe thunderstorm potential giving the chances ofsevere storms in specific areas of the state. OTO is broadcasted daily between noon and 4 p.m. from Clinton,Enid, Lawton, McAlester, Oklahoma City, and Tulsa. Other National Weather Service Offices are now airingsimilar thunderstorm outlooks. NOAA Weather Radio has three main frequencies depending on your location:162.400, 162.475, or 162.550 Mhz.

4.7.3 LOCAL RADIO STATIONS

Gathering pertinent weather information off commercial radio stations is difficult and usually untimely, althoughsome stations are better with severe storm information. Reports are usually too general and the warning lag timetoo long. Keep in mind that many stations are restricted to interrupt normally scheduled programming like TexasRangers Baseball and especially commercials. A.M. Radio stations

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are subject to static from electrical discharges whereas F.M. stations usually are static free. However, majorA.M. radio stations can carry a signal farther than F.M. especially at night. The major radio stations in tornadoalley, outputting greater than 10,000 watts day or night are as follows: KGNC (710 A.M) in Amarillo, KRLD(1080 AM) in Dallas, KOA (850 AM) in Denver, KRVN (880 AM) Lexington, NE, KOMA (1520 AM)Oklahoma City, KFAB (1110 AM) Omaha, KCKN (1020 AM) Roswell, NM, and KVOO (1170 AM) Tulsa. InKansas, I listen to KFDI in Wichita (101.3 FM and 1070 AM) and KGGF (690 AM) in Coffeyville. A list ofpopular A.M. stations can be obtained from The National Radio Club, P.O. Box 164, Mannsville, NY 13661-0661 or Storm Chaser Home Page at http://www.stormtrack.org/

4.8 TELEVISION

Some chasers are now using a portable television to monitor the severe weather updates. Most valuable are theradar and satellite displays that are occasionally flashed on the screen on local television stations. Many timesthese stations will break in to regular programming especially if severe weather is close to the area. Also, somechasers carry along a portable satellite dish and receiver which can download The Weather Channel onsubscriptions services like Direct-TV (dial 1-800-DIRECTV,347-3288). It is important to know if the radar orsatellite images are current or whether they are hours old. Old information can result in a wrong chase decision.

4.9 WEATHER DATA ON-LINE

The internet has provided a fast, inexpensive way of getting a lot of weather information. Raw METAR andsounding data can be obtained from a number of universities and organizations. My favorite locations are asfollows:

UCAR - http://www.rap.ucar.edu/weather/

SPC - http://www.spc.noaa.gov/exper/mesoanalysis/

K5KJ - http://www.k5kj.net/forecast.htm

COD - http://www.kamala.cod.edu/svr/

STSCH - http://www.stormtrack.org/

Note: URL's are subject to change. If your looking for more information and software, there are a number ofweather data services on-line. Of course this service is expensive and can run several hundred to severalthousand dollars per year depending on the amount of use. The following services can provide you with the kindof weather data you need:

Accu-data, 619 W. College Ave.,State College,PA. ph: 814-237-0309.Alden Electronics, 47 Washington Street, Westborough, MA 01581 phone: 617-366-8851.WSI, 41 North Road, Bedford, MA 01730. telephone: 617-275-5300.

In planning a chase, real time hourly surface weather observations can be obtained and plotted right in your motelroom. Morning weather soundings, upper air maps, various weather discussion, and the convective weatheroutlook can all be obtained. The data service is crucial in determining if you chase or stay put. If your equipwith a modem and a portable computer, it is possible to obtain weather information during a chase from touchtone telephone. The limitation here is expending valuable time stopped at a phone booth and the real likelihoodof being in a town with NO touch tone telephones. Lastly, when the chase is over and your back in the motelroom you can find out what happened and why. I like to document all my chases and obtain weather informationfor my files.

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4.10 WEATHER DATA SOFTWARE

Having the raw meteorology data is one thing, but having the right software to analyze this data is another. Thebest software to analyze surface and upper air data is Weather Graphics and Digital Atmosphere developed byTim Vasquez, 900 Red Rock Road, Norman, OK 73026, phone: 405-573-0298. Surface METARS are plottedand analyzed quickly. I especially like the moisture convergence and streamlines analyses. The best software toplot and analyze soundings is RAOB by Environmental Research Services, 1134 Delaware Drive, Matamoros,PA 18336, phone: 717-491-4689. The program will draw hodographs and analyze multiple soundings in crosssection.

5. THUNDERSTORM DEVELOPMENT

Like snowflakes, each thunderstorm is different. However, storms have similar internal and external featureswhich have been documented by storm chasers over several years. Storms are either line shaped or isolated,having single or multiple cells. Here is a brief summary of the growth stages and general types of storms.

5.1 GROWTH STAGES OF STORMS

Doswell (1985) recognized that there are four stages of thunderstorm development: 1) cumulus, 2) toweringcumulus, 3) mature cumulonimbus, and 4) dissipating cumulonimbus. The average thunderstorm cell will gothrough the cycle of growing and dissipating in about thirty minutes provided there is enough instability in theatmosphere to break through the capping inversion. This CAPPING INVERSION is a warm layer of air thatusually exists between 5,000 and 15,000 feet above the ground. It is formed by radiational cooling of the airfrom the night before and/or sinking air in the atmosphere.

When the sun warms the earth, it does so unevenly. The warmer air becomes less dense than it’s surroundingsand rises in bubbles or parcels. These bubbles cool about five degrees Fahrenheit per 1,000 feet and becomevisible when the moisture condenses into cloud droplets. The CUMULUS STAGE is identified by thesecondensed UPDRAFTS. Usually, cumulus clouds are observed on warm, sunny days during the afternoon whenthe ground heating is maximum. Precipitation is absent below cloud base.

A variation of the cumulus cloud is STRATOCUMULUS. This low cloud (sometimes only a few hundred feetoff the ground) is usually observed in the morning especially when the atmosphere is quite humid. In manyinstances, these clouds are accompanied by drizzle or mist. Storms need moisture to survive and stratocumulusare an important indicator of deep moisture. Usually, stratocumulus will break up during late morning or earlyafternoon as the earth is heated or as you drive west to drier air.

Once the surface heating subsides in the late afternoon, most of the cumulus clouds will dissipate and only thelargest clouds remain to sustain themselves beyond the convective heating period. Cumulus dissipation occursbetween 4 and 6 p.m. during the spring as afternoon heating subsides and mixing of the lower atmosphere ceases.If the air mass is unstable enough to break through the capping inversion, cumulus clouds can grow upward andbecome cumulus congestus or TOWERING CUMULUS. The air in towering cumulus can rise several tens offeet per second and can give quite a bumpy plane ride when passing through them. Towering cumulus occurfrequently when air converges along a front or outflow boundary. Most towering cumulus evaporate since theirslightly warmer and moister air mixes

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with the surrounding air. This mixing lessens the temperature difference between the outside and inside of thecloud and thus, the cloud parcel ceases to rise. Other parcels of air can rise in the same area, thus, you’ll seetowering cumulus grow and evaporate repeatedly. Some times you’ll encounter a brief shower beneath suchtowering clouds. Refer to Figure 1.

Towering cumulus can have different angles of tilt to them depending on how strong the wind is in theatmosphere. If there are weak winds aloft, the towering cumulus will grow straight up. However, if theatmosphere contains stronger winds aloft, the cloud towers can be tilted over downwind. In general, thestronger the winds are aloft, the greater the tilt will be of the cloud tower. In situations of very strong windsaloft, the cloud towers may actually appear to develop horizontally. Only the strongest updrafts can stand up insuch a sheared environment.

When sufficient cloud mass is transported through the capping inversion to maintain a constant warmertemperature within the cloud, otherwise known as an undiluted core, the cloud continues to rise and can becomea CUMULONIMBUS or thunderstorm. The air in the cumulonimbus does not rise indefinitely since theenvironmental temperatures begin to warm from radiational effects. Eventually the parcel temperature will equalthe environmental temperature at the EQUILIBRIUM LEVEL.

As the rising air encounters the equilibrium level, the air spreads out horizontally forming the cumulonimbusANVIL. The anvil is the top part of the cumulonimbus cloud and appears flat like a blacksmiths anvil. The anvilis comprised mostly of ice crystals since the air is usually quite cold at this elevation. If the upper winds areweak, the anvil will appear rounded. Usually, stronger winds aloft blow the anvil downwind. Refer to Figure 2.

Prior to reaching the anvil, the moisture within the cloud cools to the point where cloud droplets grow into raindroplets. The process of making rain is enhanced by electricity. Soon, the amount of rain becomes to great tohold aloft and the rain begins to fall to the ground in a DOWNDRAFT. The MATURE STAGE of athunderstorm contains both updrafts and downdrafts. Not all the time does the precipitation reach the surface.In drier climates, the precipitation evaporates before reaching the surface, a phenomenon known as VIRGA. Ifthe environment is moist enough, precipitation can reach the ground. As the rain cooled air reaches the ground,the air spreads out horizontally creating a GUST FRONT. Sometimes dirt is kicked up from beneath the fallingprecipitation or various types of outflow clouds are created. The mature stage is the most active stage of athunderstorm when precipitation usually reaches the surface and cloud-to-ground lightning is most intense.

An important turning point in storm formation occurs during the mature stage. For most storms, the presence ofa downdraft marks the starting point for the storm's demise as precipitation falls, sometimes through the updraft.It is only when the atmosphere around the storm has just the right ingredients of instability, moisture, and windshear can the storm grow and intensify to the point it can become severe. It is these ingredients which make orbreak a storm. If the downdraft is too strong, the storm updraft will be undercut and die. Storms which canbalance the updraft air with the downdraft air have the best chance of lasting the longest. Thus, the mature stagecan last ten minutes or several hours. Updraft speeds can range from several tens of feet per second to over ahundred feet per second depending on the strength of the storm. Almost all tornadoes are produced in themature stage of a thunderstorm. Typically, the updraft air increases in velocity as a portion of the downdraftwraps around the base of the updraft and begins to constrict the warm air inflow. The tornado eventuallydissipates along with the updraft.

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The DISSIPATING STAGE of a thunderstorm contains mostly downdrafts. Whatever updrafts are left aredisorganized or elevated. Rain tapers off to drizzle or virga and lightning flashes will decrease in number duringthis stage. For the most part, the severe weather has ended by the time a storm has reached the dissipating stage.This stage can last from minutes to hours depending on the strength of the initial updraft.

5.1.1 THE THUNDERSTORM SPECTRUM

Byers and Braham (1949) recognized that storms contained one or more updrafts or "cells" in their work in theThunderstorm Project. They showed that some updrafts were more intense and lasted longer than others. Theywere the first to combine updraft type with updraft strength. From their work, four varieties of storms werefound: 1) single cell, weak updraft, 2) single cell, strong updraft, 3) multicell, weak updraft, or 4) multicell,strong updraft.

More recently Moller and Doswell (1985) have added the SUPERCELL to the list of storm types. Supercellshave a single, large, intense updraft which remains in a steady-state for several hours. Such storms are able tobalance their inflow and outflow long enough to produce a history of severe weather. Almost all intensetornadoes are from supercells, however, not all tornadoes are from supercells.

Moller and Doswell arranged the various storm types into a chart called the thunderstorm spectrum. Thespectrum divides storm types into their relative frequency of occurrence, relative frequency of threat (to thepublic), and relative updraft strength. Within the spectrum, single and multiple cell storms were shown as weakor strong, and were also broken into cluster or line types. The latter differentiated between clusters of stormsthat develop in air masses from lines of storms which typically develop along fronts or boundaries. Supercellstorms have the most intense updrafts. Fortunately for the public, supercells are rare, however, they pose thegreatest threat to life and property. Refer to Figure 3.

Supercells can evolve from stronger single cell or multicell storms and visa versa. Thus, the author has drawn adouble arrow across these categories to illustrate this point. It should also be noted that not every storm fits intoa nice, neat category. In fact, there are many hybrid storms which share characteristics of single cell andsupercell as well as multicell and supercell. Future observation of storm behavior and structure may yieldchanges in the current thunderstorm spectrum.

5.2 MULTICELL STORMS

Most thunderstorms contain several cells of updrafts and downdrafts and are called MULTICELL storms. Youcan distinguish a multicell storm from a single cell storm by counting the number of visible cloud towers(updrafts). A multicell has simply two or more updrafts with the heaviest precipitation falling beneath the highestcloud top. The line of towers extending from the storm is called a FLANKING LINE. It is the flanking linewhich sustains a thunderstorm by bringing in puffs of warm, rising air into the main cloud tower. When multicellstorms develop in an environment where wind shifts from southerly to westerly with height, new updraftdevelopment usually occurs along the upwind (Southwest) side with the older cells decaying toward thedownwind (northeast) side. Refer to Figure 4.

The isolated, non-severe, multicell storms usually last less than an hour, long enough to produce a gust front.These storms can bring a brief respite of cool weather accompanied with or without precipitation

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but that is about all. When underneath the cloud base, it is more difficult to discern if a storm is a multicell. Theupdrafts tend to share the same cloud base sometimes giving the impression there is only one large updraft.Multiple updrafts will produce multiple downdrafts. Thus, pulses of precipitation falling northeast of the cloudbase may be a clue that your watching a multicell.

The isolated, severe multicell storm may last several hours depending on how the storm can balance its inflowwith the outflow. Large hail and damaging winds are the main threat. Occasionally, the isolated, severe multicellstorms produce tornadoes. However, these tornadoes are usually brief in duration. This type of storm mayevolve into a supercell if the air is unstable enough.

Line oriented, non-severe, multicell storms can last up to several hours depending on your orientation to the lineand the line’s movement. Precipitation can be brief to heavy. Small hail and gusty winds may accompany thesestorms. Usually, such storms occur when the atmosphere is marginally unstable or when the gust front hasmoved a considerable distance ahead of the precipitation area. Passage of the gust front will result in a windshift and cooler air. Precipitation may lag the leading edge of the gust front by a few miles to a few tens of miles.

Severe, multicell storms can produce large hail, damaging winds, and frequent lightning. (A severe storm bydefinition is one that has 3/4 inch diameter hail or larger, 50 knot winds, funnels, or produces a tornado.) Asevere multicell storm can have one or all these weather features. Severe, multicell storms can last for severalhours especially when they are oriented in a line. Cold, outflow air generated by the storms helps form newupdrafts along or behind the cold air boundary. As a result, the line of storms is able to propagate further intothe warmer air. Lines of storms act like a large rotary plow which exchange warm air for cold air, leaving lots ofprecipitation in its wake.

5.2.1 SQUALL LINES

Thunderstorms oriented in a line are referred to as a SQUALL LINE. The squall line consists of several stormcells, so it would be classified as multicellular. Squall lines are common along cold fronts and can extend acrossseveral states. The width of a squall lines usually varies between 15 and 100 miles. Storm cells can beinterconnected (solid line) or separated (broken line). Passage of a squall line is accompanied by strong gustywinds, brief heavy rains, hail, and lightning. Typically, the most intense portion of the line is along the leadingedge and at the southern portion of the line.

The close spacing of storm cells in the line increases the competition of warm, moist air needed to sustain thecells. Thus, squall lines (especially solid lines) tend to be poor tornado producers. The lines are an efficient wayto overturn a large portion of the lower atmosphere. Thus, storm chasers should realize they will see far moresquall line storms than supercells. Squall lines are favored by mother nature, not storm chasers.

Squall lines are not confined solely to cold fronts. Other boundaries such as dry lines, and troughs can producesquall lines especially when the air east of the line is unstable. The squall line is usually associated with a strongupper wave or trough and a surface low pressure storm system. These systems overpower an unstable air massand frequently produce squall lines. Isolated storms will develop only if the atmosphere is suppressed longenough for the capping inversion to break in a few areas.

Tornadoes and large hail are more likely at the southern edge of the squall line where isolated convective towersare growing and becoming part of the line. With time, these storms grow and merge

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with the line sometimes causing the squall line to build southward. This backbuilding squall line occurs rapidlysuch that it would be unlikely to chase successfully. The author has observed squall line storms forming inOklahoma and building southward to near Del Rio, Texas within a few hours. The speed of the backbuilding linecan be over a hundred miles per hour. It is also important to recognize that although the squall line could bemoving east at 20 mph, the individual storm cells in the line could be moving northeastward at 40 mph!

Occasionally, an isolated storm will develop ahead of the squall line. This storm has the best chances forbecoming severe as it is not in direct competition with adjacent storms. A storm ahead of a squall line can reachsupercell status producing tornadoes and large hail. This seems to occur several times each year. However, it isextremely difficult to pinpoint where an isolated storm will form ahead of a line especially if there are no largescale boundaries to focus in on. Watch for the squall line cold air outflow which may surge a hundred milesahead of the storm, decelerate, and form new storms. In some instances, a dying squall line can regenerate newconvection especially if there is an upper level disturbance approaching the area. In some instances, isolatedstorms ahead of a dying squall line can merge and utilize the cold air boundary to enhance their updrafts.Tornadoes, hailstorms, and/or downbursts can be created by such storm mergers. 5.3 SUPERCELL STORMS

The most intense thunderstorm is the SUPERCELL. These storms are rare but they produce more tornadoesthan their multicell counter-parts which is the reason more chasers prefer to chase them. Supercell storms areusually isolated and contain a single rotating updraft and downdraft which can coexist for several hours. Thisenables the storm to build-up a great deal of energy from it's surroundings pulling in air from miles around.Updraft speeds can exceed 150 mph! Doswell, (1985) indicated that less than 20 percent of Oklahoma stormsreach supercell intensity and this drops to less than five percent by the time you reach the east coast.

Supercell storms tend to prolong the mature stage of the thunderstorm so there is more time for severe weatherto occur. The supercell can have enough time to produce more than one tornado. In fact, TORNADOFAMILIES describe where one tornado forms after another. Another term for the multiple production oftornadoes from the same storm is CYCLIC TORNADOGENESIS. Experienced storm chasers know thatwhen a supercell produces one tornado, there is a good probability of a second tornado a few miles to the east-northeast (to the right) of the old tornado.

There are three types of supercells currently recognized in the field. In the past, these have been namedCLASSIC, DRY LINE, and COLLAPSED supercells. More recently, the latter two storm types have beendefined by the amount of precipitation they produce. LOW PRECIPITATION SUPERCELLS, abbreviated as"LP" frequently develop on the high plains where surface base moisture is lower. In contrast, HIGHPRECIPITATION SUPERCELLS, abbreviated as "HP" are prevalent in areas where there is abundant lowlevel moisture. The most common supercell type appears to be HP. A word of caution is urged here in that notall supercell storms fit neatly into these three types. In fact, many hybrid types of supercells have been found,some which alternate between non-severe multicell and supercell. Refer to Figure 5.

5.3.1. THE CLASSIC SUPERCELL

The classic supercell is identified by a dominant precipitation area to the northeast of the main cloud

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tower. High altitude winds aloft (the JET STREAM) are responsible for shifting the bulk of the descendingprecipitation to the northeast. This must be done to insure the cloud base continues to draw in more warm,moist air. The cool downdraft northeast of the main cloud tower is called a FORWARD FLANKDOWNDRAFT, abbreviated FFD. The intensity of this downdraft must be just right to help sustain the storm.If the FFD is too strong, cool air can undercut the storm's updraft causing the demise of the storm. If the FFD istoo weak, not enough warm, moist air will be diverted into the storm and the storm will remain weak. Thus, theFFD must act as a formidable barrier to provide lift and channel the warm, moist inflow air toward the maincloud tower.

Presence of the FFD alone does not mean the storm will produce a tornado. Another very important downdraftis needed in back of the main cloud tower to help direct and constrict the inflow air. Air movingcounterclockwise around a rotating updraft will direct a portion of the FFD air around the back edge of thestorm. As this air descends, clouds west of the main cloud tower will begin to evaporate. The descending air onthe back side of the storm is called a REAR FLANK DOWNDRAFT, abbreviated RFD. The RFD presence atcloud base will appear as a cloud free zone or CLEAR SLOT, extending around and isolating the main cloudtower.

As the RFD wraps around the main cloud tower, warm inflow air flowing into the cloud base region becomesconstricted. With nearly the same amount of inflow air passing through a smaller area, the inflow air increases inspeed. The stage is now set for tornado formation. Then watch for a new cloud tower to develop on the inflowside (usually east) of the old one. Sometimes as the storm spins faster and faster, the flanking line becomesstriated looking like a large tail extending southwestward from the main cloud tower.

5.3.2. THE DRY LINE OR LP SUPERCELL

A hybrid of the classic supercell is the dry line or LP supercell. This storm usually forms in the higher terrain ofthe western plains. It differs from the classic supercell in that it has a smaller precipitation area (FFD) northeastof the main cloud tower. You may see a small rain shaft north of the cloud tower, or virga. This does not meanthe precipitation is small. Dry line supercells are known for their large hail. The "all of a sudden there was fist-size hail pounding my car" is a common statement made by chasers who get on the wrong side of a dry line typestorm.

The cloud tower of the dry line supercell also has a distinct appearance. It appears skeletal having spiral cloudbands and striations along the sides of the cloud tower appearing like a barber shop pole. Frequently, thesouthwest edge of the cloud tower tapers to a point and the base appears tiered. Occasional funnel cloudsappear along the sides of the cloud tower or in the virga area. Bluestein (1983) has written accounts of such LPtype storms.

5.3.3. THE COLLAPSED OR HP SUPERCELL

The High Precipitation supercell has been recognized by Moller et. al (1990). This storm has two largeprecipitation areas, one with the FFD and the other with the RFD. Heavy rain and hail usually accompany eachdowndraft. As the RFD spreads out at the bottom of the storm, it wraps and obscures the updraft withprecipitation. A portion of the updraft is pushed into the warm air and eventually, the HP updrafts appear theshape of a backwards letter “C” in plan view. The occluded portion of the updraft will be difficult to view unlessyou are in the updraft slot located upwind (in the path of) the cloud base. Being in the slot of an HP storm canbe dangerous as the wrapping precipitation curtains

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with strong winds can engulf you with little warning. Chances are that if you do see a tornado, it will be of poorcontrast. Thus, it is a good idea to keep your distance from such as storm. HP supercells have short, but intenseseverity. However, HP’s usually are not good multiple tornado producers as the outflow overtakes the stormand they become outflow dominant. Most supercell storms tend to be HP variety.

5.4 STORM-SCALE FEATURES

There are a variety of storm features storm chasers should recognize. The storm’s updraft can be divide intothree parts: 1) the anvil, 2) the main cloud tower, and 3) the rain free base. There are certain characteristicsabout these features which can tell the storm chaser or spotter how strong the storm is. Also, the storm’sdowndrafts (both RFD and FFD) can provide clues on storm strength. In this section, we will explore importantupdraft and downdraft features.

5.4.1 THE ANVIL

Many times a chaser will first see the top of a thunderstorm or ANVIL. The anvil is a cloud formed by waterand ice particles ejected from the updraft and spread downwind by the jet stream. The long dimension of theanvil will indicate the direction of the upper jet stream. Typically, thunderstorm anvils are between

30,000 and 50,000 feet above the ground and can extend a few miles to over 400 miles in length, depending onthe jet stream's intensity. A storm is said to be in SOUTHWESTERLY FLOW ALOFT when the upper windsblow from the southwest to the northeast. You can tell the direction of the upper winds as well as it's velocity bywatching the anvil. An anvil which spreads rapidly across the sky indicates strong upper winds, a key ingredientfor severe storms. Compare the direction of the anvil with the direction of the surface winds. The greater thedifference in wind direction, the greater the directional wind shear.

You can also judge the strength of the storm's updraft by watching the anvil. Thick and hard anvils usuallydenote a strong updraft whereas, thin and fuzzy anvils denote weaker updrafts. Another way to judge thestrength of the updraft is to determine how high the updraft extends above the anvil. That portion of the updraftabove the anvil level is called an OVERSHOOTING TOP or DOME. The higher the overshooting top, thestronger the updraft. If the updraft is quite strong, a portion of the anvil will be forced backwards into the jetstream. The anvil is said to be BACKSHEARED when this occurs. So ask yourself the following questionsabout the storm's anvil:

1. Did the anvil spread quickly across the sky? 2. Is the anvil thick? Is the sunlight blocked out behind it? 3. Is the anvil hard looking or cumuliform (especially on the south and west sides)? 4. Is the anvil backsheared? 5. Is there an overshooting top or splashing cirrus above the anvil level?

If you answered most of these questions yes, chances are good the updraft is strong and the potential for severeweather exists. Collapse of the overshooting top does not necessarily mean the storm is falling apart. Studieshave shown that the collapse of thunderstorm tops is sometimes followed by tornado activity at the surface.SPLASHING CIRRUS is the remnant cloud debris above the anvil after the

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overshooting top has collapsed. You may see pouch-like clouds hanging from the bottom of the anvil. These arecalled MAMMATUS and are formed as the air at the anvil level sinks. These clouds will not produce tornadoesand are not necessarily an indicator of storm severity. Refer to Figures 6 and 7.

5.4.2. THE UPDRAFT

As you approach the back edge of the storm, look for rising cloud towers indicating the updraft region. Forstorms which move east-northeastward, the best contrast of storm features occurs when you approach the stormfrom the east or southeast. Your visibility of the cloud towers will depend on many factors including the amountof haze, dust, rain and the sun angle. Usually the top of the largest cloud tower or CROWN will become visiblebefore the cloud base. Cumuliform crowns usually indicate an intense updraft capable of pushing most of theprecipitation downwind. In contrast, a mushy crown indicates a weaker updraft becoming shrouded in its ownprecipitation. When approaching the storm for position, keep in mind that cloud features will become moredefined if you pass into the area shaded by the anvil.

There are a number of features to look for at mid-levels on the main cloud tower for signs of updraft intensity.Check for cloud bands or STRIATIONS which may indicate the storm is rotating. These bands are mostpronounced on the east side of the main cloud tower and extend around to the north side. Also, check the northside for a vertical or STAIR STEPPED appearance extending from the cloud base up to the anvil. If the updraftcloud is tilted downwind or if the jet stream winds are unusually strong, a relatively precipitation free area orVAULT extends downwind giving the appearance of a large amphitheater overhead. On severe storms, thevault can contain very large hail so it is a good idea not to be in this area while watching a storm. Refer toFigure 8.

A RAIN FREE BASE (RFB) under the main storm tower is the main target for storm chasers. You can obtainthe greatest information about a storm by looking it's base. Large cloud bases tend to indicate a strongerupdraft. This is because warm, moist air in the center of a large updraft is less apt to mix with air outside it thanin a small updraft, allowing a greater difference in temperatures to occur between the large updraft and theatmosphere. The warmer the air, the faster it will rise (just like the old hot air balloon). So, storms having cloudbases that are small, ragged or ill-defined will usually not be as potent. See Figure 9.

There are certain features about the rain free base which can tell you if the storm has organized it's warm airinflow. First, warm, moist air must be blowing into the rain free base. Look for low level cloud bands such aslines of stratocumulus clouds streaming toward the storm from the southeast. These clouds are the fuel thatstorms fed upon. Also, look for cloud tails or INFLOW BANDS extending east-southeastward from the cloudbase. The largest inflow band usually forms along the boundary between the cool air of the FFD and the warm,inflow air on the strongest storms. This band typically extends eastward from the north side of rain free base andlooks like a BEAVER’S TAIL. Refer to Figure 10.

As the storm continues to organize, the rain free base will lower. Pieces of ragged cloud material called SCUD(which is short for stratocumulus under deck) may be seen in the precipitation areas or underneath the rain freebase. These cloud fragments are frequently mistaken for funnel clouds or tornadoes as they sometimes move andtake on various shapes, but they are not tornadic. These clouds do bear watching especially if the updraftremains in the warm air inflow.

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As the storm evolves and the precipitation area grows, small fragments of scud may move southward under therain free base and eventually attach to it. The extended lowering under the rain free base is called a WALLCLOUD. Wall clouds tend to form in the northern portion of the rain free base where there is interactionbetween the FFD air and the warm, inflow air. The wall cloud forms as rain cooled air from the FFD isindiscriminately pulled into the updraft as the updraft intensifies. Since rain cooled air is very humid, itcondenses quickly when lifted forming cloud material. Wall clouds take on a variety of sizes and shapes. Whenorganized, they can be several miles in diameter. Refer to Figure 11.

The presence of a wall cloud should be viewed with caution as most tornadoes do form from them. However,wall clouds are fairly common with heavy multicell storms as well as supercells and most wall clouds do notproduce tornadoes. Key features to judge the intensity of a wall cloud is it's persistence (longer than 10minutes), whether it has surface based inflow, or has signs of rotation. Occasionally, the base of the wall cloudtilts down-ward to the north pointing towards the main precipitation area. A TAIL CLOUD sometimes formson the northern edge as more rain-cooled air moves in. Pieces of scud may be seen along the tail or underneaththe wall cloud.

Ask yourself the following questions regarding the storm's updraft. If you answered yes to most of thesequestions, chances are the storm is intense and bears watching.

1. Is the storm isolated and have one main updraft? 2. Is the main cloud tower hard and crisp to anvil level? 3. Are there mid-level cloud striations? 4. Are there low-level clouds feeding into the storm from the east or southeast? 5. Are surface winds blowing into the storm (INFLOW vs. OUTFLOW)? 6. Does the rain free base have a "beaver tail" inflow band on it's north side? 7. Is there a distinct cloud lowering or wall cloud? 5.4.3. OUTFLOWS AND GUSTNADOES

The OUTFLOW is the rain cooled air from a thunderstorm's downdraft that descends and spreads out along theground. Cold air outflows exist with each thunderstorm. The size of the outflow is governed by the size of thedowndraft and ultimately the number, size, and duration of the updrafts. As the cold air spreads along theground, it displaces warmer air upward. If the warmer air is moist, a cloud shaped like an arc or shelf will formas the air rises, cools, and condenses. Thus, the terms arc or SHELF CLOUD is used to describe the cloudalong the leading edge of the thunderstorm cold air outflow. Refer to Figure 12.

The shelf cloud is a low stratus type cloud that sometime takes on the appearance as a stack of plates. Itfrequently appears dark and ragged with even darker skies behind it. Passage of the shelf cloud is accompaniedby a sudden windshift, cooler temperatures, and a sudden burst of precipitation. Rapid cloud motion along theleading edge of the shelf cloud can give the impression that the sky is boiling. The shelf cloud has a RAGGEDUNDERSIDE when passing overhead. You can actually see the cloud motions without time lapse and this issometimes mistaken for a wall cloud or funnels. Refer to Figure 13.

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The slope of the shelf cloud will change with time. A newly developed shelf cloud is rather steep, withheavy precipitation immediately behind the cloud. As the cool rain-laden boundary spreads out, so to does theshelf cloud. With time, the shelf cloud attains a shallower slope and decelerates as the precipitation area lagsfurther and further behind. Finally, the shelf cloud becomes nearly flat and the cold air outflow boundary maybecome stationary providing a boundary upon which new convection can be generated. Occasionally, SHEARFUNNELS are produced along the leading edge of the shelf cloud. These funnels are manifestations of turbulenteddies and rarely touch down.

A DOWNBURST is a strong downdraft that results in damaging winds at the ground. Downbursts are brokeninto two categories depending upon the size of the precipitation burst. Bursts less than 2.5 miles in diameter arecalled MICROBURSTS whereas those greater than 2.5 miles wide are called MACROBURSTS. There arewet and dry varieties of microbursts and macroburts depending on whether there is precipitation associated withit when the burst reaches the ground. Sometimes, a RAIN FOOT is an indication that a wet microburst andmacroburst is occurring. The rain is actually transported horizontally with the spreading out of the air. Imaginethese bursts resembling large water balloons which spread out when they impact the ground. Refer to Figure 14.

Dry microbursts or macrobursts are accompanied by very little precipitation at the ground. Such bursts arefrequent beneath virga or high based storms. Sometimes, plumes of BLOWING DUST move horizontallyaway from the storm beneath the descending air. In some instances, the forward or leading edge of the blowingdust can actually spread back up to cloud base. Many times, these plumes of blowing dust are mistaken fortornadoes. When in doubt, watch the dust for awhile. If it is being blown out away from the updraft, then youknow it is associated with downdraft air.

With the rapid outward acceleration of the cold air at the surface, small but intense vortices calledGUSTNADOES can develop along the leading edge of the outflow boundary. Gustnadoes are usually of shortduration lasting less than a minute, however, some gustnadoes can become quite strong tearing roofs off houses,or overturning a chase vehicle. Gustnadoes may be seen as dust whirls beneath the shelf cloud. Sometimes, asmall funnel or funnels can be observed extending from the shelf cloud. Though the formation of gustnadoes isstill being studied, it appears that some gustnadoes occur along areas where the gust front is acceleratinghorizontally. Further enhancement of gustnado formation appears the result of strong vertical accelerations ofwarm, moist air up over the gust front. It is interesting to note that not all gust fronts produce gustnadoes.Refer to Figure 15. Chasers should exercise care when around outflow boundaries where heavy rains, strong winds, and blowingdust can reduce your visibility. Remember strong straight-line winds can overturn your automobile, topplepower lines, or cause damage to buildings. Roads are often flooded. Keep an eye on the movement of the shelfcloud and the density of precipitation behind it. If you have to penetrate the line, look for a clearing where therain curtains have parted. Frequent lightning is a good indicator to an intense squall line. When there is only asquall line, it is better to just turn around and head for the motel to catch up on your sleep.

6. FIELD STRATEGY

To obtain the best view of a tornado, the key is to position yourself on the southeast flank of a supercellthunderstorm. This relative position has to be maintained while moving with the storm in order to provide thebest visibility and photographic contrast of a dark colored tornado against a bright, rain-free

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background. In contrast, a view southwest of the storm may yield a white tornado against a dark rain-ladenbackground or just a white precipitation shaft obscuring a tornado. The following is a list of favorable weatherphenomena to look for as you close in on your chase area.

6.1 LONG RANGE Look for:

a. A clearing west of the stratocumulus cloud cover. Skies going from cloudy to scattered will allow surfaceheating, especially if the cloud cover burns off early.

b. The presence of patchy altocumulus castellanus (ACCAS). This cloud type indicates lifting in the middletroposphere and signifies a destabilizing atmosphere. Watch for agitated areas in the convection.

b. An anvil dome or backsheared anvil. These features can give you an idea of the relative strength of theupdraft. In general, the higher the anvil dome, the stronger the updraft. Also, the greater the extent of

backshearing, the stronger the updraft especially when the winds aloft are high. Thick, chunky anvils thatblock out the sun can indicate a strong updraft in contrast to thin, fibrous anvils that you can see the sunthrough.

d. A sharp, knifing anvil that rapidly crosses the sky. This indicates strong winds aloft. Compare thedirection the anvil is pointing with the direction of the surface winds. A 90 degree difference or more indicates strong directional shearing. Storms can utilize the shear in the atmosphere to help generate or sustain updraft spin.

e. Mammatus clouds hanging from the anvil. These clouds tend to be more pronounced toward the latter partof the mature thunderstorm stage. You may be too late to get underneath the cloud base.

f. Isolated cells or towering cumulus which have little or no other competition for available low level moisture from other storms. Storms should be separated by about thirty miles, though can share the same

anvil. If a squall line is present, go to the southern most storm.

g. Striations at mid-levels of the storm indicating updraft rotation.

h. Rapid vertical growth of the main cloud tower. Are the updrafts large or small? Is the main cloud towercrisp or fuzzy? Large, crisp updrafts tend to be stronger than small, fuzzy ones.

i. A subsidence region or cloud free area in back of the storm. This feature may signify a rear flank downdraft, dry line, or short wave.

6.2 MEDIUM RANGE Look for:

a. An isolated storm. You’ll have to choose a target storm. Compare to the other storms around.

c. A flanking line of cumulus congestus extending southwestward from the main cloud tower. Look for aflanking line with congestus increasing in height toward the main tower. Those flanking lines that blow upand glaciate prior to reaching the main cloud tower likely indicate a backbuilding linear storm.

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c. A vertical wall or vault on the east or north sides of the main cloud tower extending to the anvil indicatingthe updraft can stand up in the presence of high vertical wind shear.

d. Cyclonic striations (like a barber shop pole) spiraling up the side of the main cloud tower. Such cloudstriations are the result of high speed air smearing the cloud features and can indicate rotation.

e. An extensive rain-free base with precipitation falling to the northeast. Visibility will depend on severalfactors including the amount of haze, trees, hills, and other obstructions.

f. A low cloud base or wall cloud. Wall clouds have initially been observed forming from scud south of theoutflow area that enlarges and attaches to the rain free base.

g. A flat inflow band attached to the east side of the rain free cloud base. This cloud feature mayresemble that of a large beaver tail. The band probably demarcates that warm, inflow air from cool outflowair. Other smaller inflow bands may be observed to the east and southeast of the rain free base.

6.3 CLOSE RANGE Look for:

a. A wall cloud below the rain free cloud base which is persistent (say more than 10 minutes or so). Determinethe direction of cloud tag motions (scud). Watch for signs of rotation. It has been observed that when yourunder the center of rotation, you may be unaware of the wall cloud over your head since the cloud baseheight blends in with the height of the ambient cloud base.

b. Cyclonic striations under the rain free base which spiral in toward the wall cloud (or common center).

c. A doughnut-type or concave structure to the wall cloud. An unsaturated hole in the cloud base. Chances are your too close at this stage.

d. Vigorous dust whirls rising from the ground beneath the rain free base or wall cloud. This is usually thestart of circulation reaching the ground. Be sure to denote differences between tornadic whirls and gustnadoes.

e. Precipitation falling in thin veils or curtains spiraling around the wall cloud. Be aware that a tornado can beimbedded in precipitation behind precipitation curtains.

f. Look for the large inflow tail (beaver tail) to intersect the northern portion of the rain free base. This featurepoints the way under cloud base. Watch for occasional hail in this area.

g. Golfball or greater hail to the north and west of the wall cloud. This may appear as a white or green shaftdepending on the angle of the sun and your position. Approaching the storm's updraft from the north quadrants(core punching) is not advised. h. Shelf clouds or roll clouds demarcating the leading edge of the gust front. Be advised that strong damagingwinds can occur. Gustnadoes are frequently observed on the gust front in strong outflow situations. Chancesare slim for tornado development when the gust front has undercut the updraft and pushed far out ahead of thestorm. However, new updraft development can occur along decaying outflow boundaries.

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I. The rear flank downdraft (RFD) forming a clear slot which occludes the main cloud tower. It has beennoted on several occasions that tornadogenesis was simultaneous with the RFD occlusion. The RFD alsopropagates the flanking line eastward. New wall clouds have been observed forming at the nose of theclear slot in multiple tornado outbreaks.

7. THE TORNADO

Tornadoes can develop as the updraft mesocyclone occludes. Reasons for tornadogenesis are still being studied.However, there are certain identifiable features in the lifetime of a tornado which the storm chaser should befamiliar with.

7.1 LIFE CYCLE

The initial stage of tornado formation is frequently referred to as the BEGINNING STAGE. Tornadoesusually develop from wall clouds, though not always. Regardless, a persistent wall cloud is recognized as anexcellent precursor for tornado formation. So much so, that weather services will issue a tornado warning whena persistent wall cloud is sighted. Emphasis is on the word persistent (longer than 10 minutes). Also, look forsigns of rotation either cloud tags circulating around the lowering, wrapping of precipitation curtains, etc.Sometimes it is difficult to see rotation in the wall cloud especially from far away.

During the beginning stage, DUST WHIRLS occur below the wall cloud and this may be the first indication thatan intense circulation has reached the ground. A visible connection of the funnel to the dust cloud is notnecessary to confirm it as a tornado. Thin, needle-like funnels may be observed under the wall cloud during thisstage. However, the circulation may or may not intensify beyond this stage. Remember the average tornado lasts3 minutes and travels less than a mile on the ground. Refer to Figure 16.

The tornado enters the ORGANIZING STAGE when the circulation intensifies to the point where the visiblefunnel extends towards the ground. Dust whirls intensify and dirt may be carried aloft obscuring the funnel.As the circulation continues to intensify, the tornado may reach the MATURE STAGE. This occurs when thetornado grows to its maximum size. In many cases, a CLEAR SLOT representing the rear flank downdraft willbe observed to south of the wall cloud. The clear slot will travel around the east side of the updraft. This cloudfree area will allow diffused sunlight to filter down giving a whiter appearance to the tornado, wall cloud, ormain cloud tower.

Next, the tornado will enter the ROPE STAGE. The tornado size shrinks and decreases during this time.Sometimes the visible funnel width decreases uniformly while other times the funnel appears to lift off from theground. Frequently, the top and bottom portion of the vortex become displaced and the tornado appears tilted.The wall cloud may appear lifted or not exist as it becomes eroded by the rear flank downdraft. Finally, thetornado retracts into what's left of the cloud base and circulation at the ground ceases. The tornado has “ropedout”. A few detached clouds may remain briefly. Dust may continue to linger where the tornado existed. Beaware of debris cascading out of the sky. Large sheets of plywood and metal may appear like small papersfloating overhead, quite misleading.

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7.2 TORNADO SCALE FEATURES

Tornadoes come in a variety of shapes and sizes. The most common type of vortex is the rope or needle shape.However, tornadoes which remain on the ground longer than average can appear column shaped, cone shaped,or wedge shaped. The sizes and shapes can change with time along the path. Some of the larger circulation’smay appear to have several smaller tornadoes or embedded SUB-VORTICES. Refer to Figure 17.

The number of sub-vortices have been known to vary from two to seven at a time with the most common numberas three. Sub-vortex formation usually begins on the south side of a cyclonic rotating tornado. Vortexcondensation seems to begin at the ground and moves rapidly upward toward the cloud base or wall cloud. Asthe vortex rotates around the east side of the parent circulation, it grows to its maximum size. (Now a newsubvortex can form where the first one did, and begins its life cycle.) As the sub-vortex continues around thenorth side of the circulation, it begins to decay. Moving south on the west side, the sub-vortex will rope out anddissipate.

There are three types of air motions in a tornado: tangential, radial, and vertical. Tangential flow is usuallystrongest and can be observed easily as air flow rotates around the circulation. Radial motion appears as inflowair at the surface which converges and accelerates toward the tornado center (sort of like spokes on a wheel).Vertical motions are the most difficult to recognize especially if the visible tornado surface is smooth. Lookingwest at a cyclonic tornado, upward motion will be on the north side, and downward motion will be on the southside. Perturbations or waves may be observed moving up or down the edge of the vortex throughout its life.Occasionally, you may see other features like horizontal vortices rolling up the sides of the tornado, or smallanticyclonic funnels at cloud base on the east side of the wall cloud or ambient cloud base.

7.3 CYCLIC TORNADOGENESIS

A rule of thumb to follow is: where there is one tornado, there is a good chance of a second one forming a fewmiles east and north east of the old tornado. Since updrafts grow and dissipate, so do the tornadoes that thereattached to. As the rear flank downdraft occludes the mesocyclone circulation, the flanking line is propagatedeastward and the cycle of updraft growth and occlusion can occur again and again. Each time the updraftoccludes, tornadoes can form. A series of tornadoes produced from the same storm complex is termed a"FAMILY". Multiple tornadoes associated with multiple storms is usually termed an "OUTBREAK".

7.4 TORNADO PERCEPTION

There are many stories, folk tales, and myths associated with tornadoes. The perception of the tornado, its visualappearance, and mystery of the unknown has led to several misconceptions, a few briefly discussed.

Tornadoes were once thought as giant vacuum cleaners in the sky which suck up and devour everything in theirpath. Buildings were once thought to explode because of the low air pressure. Recent engineering studies haveshown this not to be true as the air pressure drop in tornadoes is only a few percent of normal air pressure. Mostbuildings have enough ventilation to offset any sudden barometric changes. It has become apparent that tornadodamage is mostly caused by wind action with entrained debris. Structures are blasted by gravel, wood, concrete,timbers, cars, etc. in these windstorms.

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Opening windows in a tornado was once thought to help keep building damage to a minimum. Recent studies byMinor et. al (1977) suggest that opening windows could actually be detrimental to a structure by letting the windinside the building. Actually, there is more than enough flying debris around to break windows well in advanceof stronger tornadic winds.

The color of a tornado has been described in all hues from black to blue to red and orange. Actually, the visibleportion of the tornado is white from condensation (same as clouds). The color of a tornado is dependent on thesun angle, cloud height above you, and your angle of view. Though tornadoes can pick up soil and reflect theircolor, most remain a white color. For example, the author was looking east observing a black tornado against abright background while another chaser west of the tornado saw the true color as a white tornado against a bluebackground.

Be aware of all the tornado look-a-likes out in the field. Dark rain shafts, blowing dust from an outflow, smokeplumes, and lightning illuminated telephone poles at night can fool even the most experienced chaser.Occasionally, you may see other features like horizontal vortices rolling up the sides of the tornado, or smallanticyclonic funnels at cloud base on the east side of the wall cloud.

8. OBSERVATIONAL SUMMARY

Two recent theories have been put forth in the study of tornado formation or tornadogenesis. Lemon andDoswell (1979) indicate that tornado formation is related to the presence of a rear flank downdraft (RFD) whichoccludes the updraft. Another theory has been put forth by Snow (1974) and describes how updraft anddowndraft rotation are enhanced by the drawing-up of vortex "tubes" produced by wind shear.

8.1 REAR FLANK DOWNDRAFT OCCLUSION THEORY

The first stage of this theory already involves a strong rotating updraft. The updraft is located along thesouthwestern edge of the storm complex. The air is drawn into the base of the updraft from the southeastdirection, the location where the air is warm, and moist. Since the air in this region is buoyant, it ascends anddoes so in a helical manner finally exiting at the top of the updraft. Strong upper winds, move the precipitationdownstream to the northeast in the anvil. Eventually the air cools to the point it descends with the aid ofprecipitation drag and eventually reaches the surface. This is the Forward Flank Downdraft (FFD). As the raincooled air spreads out along the ground, an outflow boundary develops. The leading edge of this boundary is thegust front. Refer to Figure 18.

On the upwind side of the updraft, airflow is essentially blocked. Some of the air decelerates and is forced todescend. Frequently, this area has a veil of precipitation evaporating as it descends. The precipitation is forced tothe west and south around the updraft circulation. Air subsiding on the west side of the updraft causes a cloudfree area or clear slot and denotes the rear flank downdraft (RFD). There is still considerable debate on whetherthis air at the top of the storm is forced to the surface or whether successive layers of descending air cause this tooccur. There has also been some suggestions that the rotating updraft is responsible for the RFD wrappingaround it rather than the RFD causing the updraft to rotate.

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Near the center of the mesocyclone about mid-height of the storm, a vortex begins. As the divided mesocyclone(updraft and RFD) extends to the surface, so does the vortex. Eventually, the RFD spreads out as it reaches thesurface and a portion of the RFD air is wrapped into the overall mesocyclone circulation forcing the occlusion ofthe updraft at low levels. It is at this stage that the tornado can extend below cloud base. The divergence ofRFD air at the surface enhances convergence along the new gust front boundary. As a result, flanking linegrowth is enhanced and the RFD propagates the line eastward.

Finally, what was responsible for beginning tornadogenesis becomes instrumental in its demise. The RFDliterally shuts off the inflow air into the updraft, weakening it to the point where the mesocyclone circulationbreaks down and the tornado dissipates. Note the tornado is purely the outgrowth of the mesocyclone rotation.The rapid push of the RFD around the occluding updraft provides a boundary where a new updraft can developjust to the east. The formation of a new wall cloud and tornado can occur within minutes of the old updraftocclusion and occasionally two tornadoes are observed at the same time: a roping tornado with the old updraftand an organizing tornado with the new updraft. This cycle can continue to the extent that successive occlusionsof updrafts in the storm system can lead to multiple tornadoes.

8.2 HORIZONTAL VORTEX TUBE THEORY

The thrust of this theory relies on the premise that vertical wind shear produces a source of rotation along ahorizontal axis. Since wind speed typically increases and veers with height, a rotational component exists in thewind. Refer to Figure 19.

Invisible horizontal tubes of air are created in the wind flow. These tubes converge under the updraft channeledby the outflow boundary provided by the FFD. As the horizontal tubes are drawn into an updraft, they are tiltedinto the vertical. The tilted tubes are thought to enhance updraft rotation. With acceleration of air parcels in theupdraft, the tubes become stretched. Rotation of the updraft begins near mid-height concentrating to form amesocyclone. Just as with the RFD theory, this circulation eventually extends to the cloud base and can result intornado production.

9. PHOTOGRAPHY

Having a purpose for each picture will truly enhance the value of your film. To obtain good quality slides,prints, or video of thunderstorms and tornadoes takes a good quality camera and several years of personalexperience. Problems with blurred photos, over and under exposures, etc. can be minimized by the followingcertain procedures. Buy a camera that you can feel comfortable with. Elaborate cameras (those with auto-focus) don't necessarily take better pictures. Your basic SLR camera will cost around $100 and is well worth theprice. For video, Hi-8 or SVHS cameras provide the best quality more-so than regular VHS cameras. Thehigher quality is well worth the price averaging about $1,000. If money is no object, buy a digital video camera.Prices start around $3,500. Shop around for the best camera to fit your needs.

9.1 PROCEDURES

The following guidelines are to help you take better photographs of storms and tornadoes.

1. Take each photograph with a purpose in mind. Ask yourself, what do you want to show?

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2. Use a tripod or window clamp. This will help insure sharp, crisp photographs.

3. Make sure there is film in the camera. This sounds funny but its not when you have just photographed a tornado and find no film in the camera.

4. Be confident in your camera settings, such as ASA, F-stop, shutter speed and film speed.

5. Make sure the exposed film is identified and put in a secure place. Film canisters, boxes and old hamburgerwrappers are usually discarded on the floor. A roll of film can be easily lost and thrown out accidentallywhen the car is cleaned.

6. Be aware of the number of photographs you have taken. Typically, enroute to a storm you take several

pictures of it, then the wall cloud, and lightning. Having one photo left when the tornado forms can be quite frustrating. It is better to waste a few pictures at the end of a roll than to miss the tornado.

7. Try taking panoramic views of thunderstorms. These provide more information than single shots andcapture more beauty and visibility.

8. Try sequencing your photographs. Take still photos every 5 seconds during the tornadic stage so you cancapture cloud tag and debris motions.

9. Document photographs on your tape recorder. Record pictures by sequence, location, and direction ofview.

10. When possible use different lenses. Zoom lenses provide detail of tornado structure whereas wide anglelenses can present overall storm structure. the base of a tornado with a zoom lens is better. Also, it Is saferto approach the base of a tornado with the zoom lens than in person.

11. Marking the film site with a wooden stake will document the location of the film site. Later, youcan determine your relative position to the tornado. This also helps if the film is to be used for photogrammetric studies.

12. Try to frame your subject. Trees, ponds, and buildings can make the picture more interesting.

13. For overall storm shots and close up photos of the tornado base, make sure that some groundappears in the photo to add depth and scale. About the lowest quarter of the photograph should contain the background.

9.2 PHOTOGRAPHY PROBLEMS

Cameras need care and maintenance. Be aware of the following:

1. UNCLEAN LENSES- Fingerprints will smudge the glass and dust grit will impede zoom capabilities and can scratch the lens. A clear lens shield over the lens is much less expensive than the lens.

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2. HEAT- Try to keep all photographic equipment and film out of direct sunlight. An aluminum cameraon the dashboard can become too hot to handle in the glaring sun. Also, a locked car can get warmenough to heat damage film. Orange streaks on developed film usually indicate heat damage. It isundesirable to have an orange streak across a tornado.

3. OUTSIDE DEVELOPING- Sending your film away to be processed does involve some risk.Personally, the author has had two rolls of film destroyed in the last 8 years, one with a tornado sequence.This has happened when slide film is run through the color print processing. The result is purple negatives.A letter saying sorry and an unexposed roll of film is not the same. Be careful who you send your film too.Also be aware of scratches and glue residue on slides.

4. OUTDATED FILM- Check your supply of film carefully. Unexposed film from last year either off theshelf or in your chase gear can be outdated. Check the date of expiration on the roll of film carefully. Oldfilm can lead to color changes, and loss of exposure. Most manufacturers recommend immediateprocessing after exposure. Refrigeration can extend the life of your unprocessed film.

9.3 PHOTOGRAPHING LIGHTNING AND SPECIAL SHOTS

Taking good lightning photographs is relatively easy. All you need to have is a camera where you can controlthe shutter speed, a good tripod, and of course, an electrical storm. Cloud-to-cloud and cloud-to- groundlightning are make usually the best photographs. Sheet lightning can be useful to illuminate cloud features. Afew camera tips are presented:

1. Watch the storm for several minutes until you see a preference for lightning in a certain region of thestorm. This area is usually along the updraft/downdraft interface.

2. Position yourself away from city lights which can bleach out the photograph. In a severe electricalstorm with frequent lightning bolts, the city lights may not be a problem.

3. Secure the camera to a tripod. This helps prevent blurred photographs and really is a must.

3. Aim the lens at the target area and hold the shutter open until lightning occurs in the frame. This is ahit or miss way but you must realize that you may only get one good picture on a roll of film. The shuttercan be held open for several minutes if it is very dark (no city lights or moon light).

10. FORECASTING A CHASE

Accuracy in severe storm forecasting requires a combination of knowledge and experience in synopticclimatology, mesoanalysis, sounding analysis, interpretation, and luck. Each storm chaser usually has their owntechnique in forecasting severe weather. To my knowledge, there is no proven method which is better. Most ofthe forecast methods rely on some form of instability analysis. The forecast information presented herein is by nomeans complete. Some of the basic ingredients which are recognized in major severe weather events arepresented.

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The severe weather potential can usually be judged more than a day in advance. Analyzing weather servicemodel forecasts or watching the evening weather report on television can both inform you about the positions ofapproaching fronts and short waves. Storm systems on the west coast usually take 24 to 48 hours to cross theRocky Mountains. This gives most chasers enough time to prepare and head onto the plains to watch for stormdevelopment.

10.1 THE SEVERE WEATHER OUTLOOK

A helpful guide of severe weather potential is the convective outlook issued by the Storm Prediction Center(SPC) in Norman, Oklahoma. This information usually appears on morning weather programs like GoodMorning America, or The Weather Channel, or you can obtain it direct from the weather service or internet

The SEVERE WEATHER OUTLOOK is listed as slight, moderate, or high risk. The level of risk is based onforecasting the amount of area anticipated to be covered by severe weather. Slight risk means a few stormswhereas high risk means more widespread severe weather. This forecast product is for severe weather potential,not tornado potential. The severe thunderstorm definition is broader and even includes squall lines. Theseoutlooks are issued at 13Z, 1630Z, 20Z, and 01Z with times subject to change. Chasers should pay moreattention to the 1630Z outlook than the 13Z since the 13Z forecast is based on the previous nights upper airdata. Thus, significant changes between the 13Z and 1630Z forecasts do occur often. If severe weather doesoccur, it would most likely begin in the western portion of the outline area and spreads eastward throughout theevening. The actual statement of the severe weather outlook from the weather service has two parts. The first part liststhe coordinates outlining the risk area. Coordinates are given by station contractions so it is a good idea to befamiliar with them. The second part consists of a brief weather discussion which is very important as it gives anidea what the forecaster is thinking. Normally, the forecaster identifies themselves at the bottom of thestatement. Some times, two forecasters handle the outlook. One of the forecasters handles the severe weatherportion and the other handles the general thunderstorm discussion.

The severe weather outlook has a good track record. Most of the time severe thunderstorms will occur in thearea. It is for this reason that I'll usually chase when a moderate risk or better has been issued. However, therisk area is not a risk of tornadoes, but rather, a forecast of severe storm coverage. Many high risk days includesquall line situations. I've also encountered a number of "bust" chase days when a risk area was issued. Thesedays have usually been associated with an inactive dry line and/or a strong cap, or warm air advection aloft.

Chases should begin near noon for long hauls and at the drivers discretion for short hauls. Be monetarilyprepared to stay in a motel. The chase day decision usually follows the routine:

a. Look at model data and forecasts the night before b. Notify chase participants of chase potential (standby) c. Analyze or plot necessary weather maps during the morning of a potential chase day. d. Make the decision to chase or not, usually before noon. e. Notify chase participants of decision, where to meet, etc. f. Assemble equipment and personnel

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10.2 SURFACE MESOANALYSIS

Hourly analyses of surface weather data are most helpful in delineating the chase area. From experience,mesoanalysis has been very important in identifying and refining a small geographic area which has the highestpotential for severe thunderstorms. Hourly data between 9 am and noon are most crucial in forecasting shortterm severe weather potential. Station model maps should be plotted for each hour. Then, hourly changesshould be plotted on a second map. An example of a surface plot is shown in Figure 20. Though each chaserwill develop their own forecast routine, a few of the basic parameters are:

a. Plot altimeter or millibar pressure contours. Intervals are either .05 inches or 2 millibars. Thiswill help identify lows, highs, mesolows, and mesohighs. Look for rapid pressure falls. Severe weatherhas been associated in regions of sharply falling pressures, and within 200 miles northeast of meso-lows.

b. Locate all frontal boundaries, outflow boundaries, dry lines, and wind shift lines. Identify those stations where rain, fog, dust, or thunderstorms are occurring.

c. Draw temperature and dew point contours every 3 degrees when values are greater than 55 degrees. This will help identify the warm, moist sector. Locate the axes of maximum temperaturesand dew points. Severe weather is most likely from the intersection southward. Monitor hourly changesin temperature and dew point. Areas which are warming and moistening indicate destabilization.

d. Plot the wind direction and speed. Note changes of the wind vectors from hour to hour. Backing windsover a period of time may indicate increasing confluence and possible mesolow formation. e. Highlight regions of strong thermal gradients especially where the wind vectors are normal to the

gradient. A potential source of uplift results when warm air is forced over a cool boundary(isentropic lifting).

f. Indicate the cloud cover and layers (nephanalysis). Identify low clouds, middle clouds, high clouds; the cloud base elevations are less than 5000 feet, 6000 to 12000 feet and over 12000 feet, respectively. This analysis will give an idea of the edge of the low cloudiness, and extent of cirrus shields. Dense cloud cover can deter solar heating and thus, convection.

g. Analyze moisture convergence. For this you will need a computer and the right software (such as Tim Vasquezs’ Weather Graphics or Digital Atmosphere). Storms usually prefer areas where the deep surface

moisture is converging. Watch the moisture convergence patterns throughout the morning to see if itremains in place. A singular, concentric area of moisture convergence with strong gradients is better thanmany areas of weak convergence.

h. Plot surface stability values such as SWEAT or SPOT index. These indices give relative indications of themost unstable areas. Both indices have limited success with the highest values indicating the most unstableair. Usually, a computer with the right software is needed to calculate these values.

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10.3 UPPER AIR MAPS

Analysis of upper air maps will reveal the location of major features such as short wave TROUGHS comingover the Rocky Mountains. One of the shortcomings to upper air maps is that station density is sparse, thus, dataresolution is limited to the synoptic scale. We are usually dependent on maps provided from only two soundingsper day. Again, each chaser will develop their own forecast routine. I do the following:

10.3.1 850 MB MAP

a. Isolate the 850mb jet intersection with any surface stationary or warm fronts. The region west up to the surface low or dry line is a prime area for severe weather.

b. South winds with speeds greater than 20 knots are moderate and greater than 35 knots are strong for tornado potential. Identify the axis of the low level jet.

c. Locate the moist axis and 850mb low. A favorable situation for tornadoes in West Texas and Oklahoma occurs when an 850mb low is positioned in eastern Colorado during the morning.

d. Define regions where dew points are within four degrees of the temperature. Remember that thetop of the moist layer may actually lie below the 850mb level.

d. Define regions where the temperature is between 9 and 16 degrees. The chances for convection decreasewith higher temperatures and may indicate a strong inversion.

10.3.2 500 MB MAP

a. Locate all short waves and long waves. Look for a trough moving inland into southern California.This feature can enhance moisture convergence into West Texas in a day or two. Identify negative tilt troughs

(these have the main trough axis tilted southeast to northwest). Such troughs transport a great deal of energy and rapidly lift larger amounts of air. Open troughs are preferred over slow moving closed lows.

b. Define regions where wind directions are from the south-west at speeds greater than 30 knots for moderateand greater than 50 knots for strong tornado potential.

c. Locate the diffluent axis. Diffluence aloft can enhance vertical motion.

e. Locate regions of positive vorticity advection (PVA). Regions of PVA imply enhanced vertical motions andare usually found on the east side of a short wave trough. Caution is advised in relying solely upon PVAas strong lifting can lead to widespread convection. Severe storms can occur without PVA.

e. Locate 500mb temperatures less than -9 Celsius. The colder temperatures the better. Look for the possibility of cold air advection upstream (i.e. -9C at Oklahoma City and -15C at Albuquerque).

f. Note height fall trends. This will give an indication of trough movement or deepening. Negative numbersindicate a trough that is moving eastward and/or is intensifying.

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10.3.3 300 MB MAP

a. Locate the upper level jets: subtropical and polar. The left front or right rear quadrants are favorable forsevere weather.

b. Westerly winds greater than 40 knots are moderate and greater than 65 knots are strong for tornado potential.

10.3.4 SUMMARY

A quick general forecast area for severe weather is usually determined by the following:

a. Define the 850mb jet axis and shade the region on the cyclonic side. Remember the system will be movingthroughout the day.

b. Define the 500mb jet axis and shade the region on the anticyclonic side where winds are southwesterly greater than 20 knots.

c. Define the surface dry line and shade the region east of it's forecasted position.

10.4 SOUNDINGS AND STABILITY

Surface mesoanalysis is important to identify the boundaries, thermal axes, moist tongues, etc.. However, thebest looking surface conditions for severe weather sometimes does not materialize. The underlying reasons, mayseem to be too strong of a capping inversion, warm air advection aloft, or the premature pulling north of anupper low.

Plotting soundings and analyzing potential instability is crucial to determine the strength of the inversion, and toforecast the change of stability throughout the day. Plot all primary levels, and if possible, secondary levels forthe morning sounding. Refer to Figure 21. It is important to anticipate changes in the sounding profilethroughout the day (i.e. temperature increase in low levels, cooling aloft). Recent storm studies suggest thatlocal environments around storms differ from the non-convective environments a few miles away. Determine thefollowing parameters on each sounding: a. Locate the height and extent of the CAPPING INVERSION. Storms will not develop if the inversion is too

strong. If the inversion is weak, the atmosphere may overturn early in the day and squall lines may dominate.The best possibility for tornadoes exist in "explosive" thunderstorms which tend to be isolated. This is mostlikely to occur when the inversion is strong enough to hold until around 4 p.m. The inversion is erodedfrom below due to surface heating and air mixing. Thus, it is important that the target area remain essentiallycloud free to aid in the erosion process.

b. Determine the CONVECTIVE TEMPERATURE- Continue downward dry adiabatically from the mixingratio-temperature intersection to the ground (Surface pressure). Convective instability theoretically can't bereleased until the forecasted afternoon temperature reaches the convective temperature (CT). Then,compare the CT with the forecasted afternoon temperatures. If they are not too far apart, chances are thatconvection can occur. It is not a sure bet, however, as warm air advection aloft or drying near the groundcan cause the CT to increase. In contrast, mesoscale forcing, etc., can cause instability to be releasedbefore the CT is reached.

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c. Determine the amount of POSITIVE AREA and NEGATIVE AREA on the sounding. These areas are the difference between the environmental temperature profile and an assumed temperature of an ascending air parcel. Large positive areas indicate unstable air whereas large negative areas indicate stable air. The area is a function of energy on the Skew-T log-P diagram, but each grid box is not a constant energy value. Usually a computer is needed to equate the area to energy.

d. Note how much moisture is available at low levels of the atmosphere. Severe storm potential is moderateif the mean mixing ratio in the lowest 50 mb is 10 g/kg or greater, and strong is 12 g/kg

or greater. Storms need such moisture in order to survive.

d. Plot the wind structure with height (HODOGRAPH). Look especially at the wind structure in the lowest100 mb. Broad looped hodographs tend to provide environments for supercells whereas linear hodographshave environments which creates squall lines. If the hodograph trace looks like spaghetti, with windsgoing every which way, chances are that storms will not be severe if they develop. Wind vectors veeringwith height more than 40 degrees are moderate, and 70 degrees are strong for severe storm potential.Storms which move to the right of the hodograph trace enhance their storm relative inflow. The areabetween the tip of the (assumed or known) storm vector and the hodograph trace is known as STORMRELATIVE HELICITY. The most severe storms appear to have the greatest storm relativeinflow/helicity.

e. Determine the depth of the moist layer near the surface. If the moisture is too shallow, say less than 75 mb,there is usually insufficient moisture to support strong convection. Surface heating will mix dryer airdownward from aloft, winds may shift westerly, and the dry line passes. In contrast, if the moist layer isdeep, say over 200 mb, widespread convection is likely resulting in heavy rainfalls. From experience, a moistlayer of about 125 mb (plus or minus 50 mb) is most favorable for severe weather.

g. Note if there is a dry layer near or above the temperature inversion. Dry air aloft helps hold down atmosphere overturning being more stable and heavier than moist air at the same temperature.The typical BELL-SHAPED sounding as described by Miller (1972) represents a favorable unstable

environment for severe weather.

h. The LIFTED INDEX- Take the mean mixing ratio in the lowest 50 mb and follow the line to the intersection of the temperature sounding. Continue up moist adiabatically to 500 mb. The difference in temperature is the lifted index. LI's greater than -10 are moderate, -12 are strong for severe weather.

10.5 RADAR

Real time radar information is difficult to obtain in the field, as commercial radio and NOAA weather radio havevarious lag times. However, some local weather services allow ham radio personnel to set up a base stationduring severe weather. Hams broadcast to a spotter network the position, intensities, and movements of stormsin real time. You can listen in on the conversation with a receiver.

It is important to recognize that the radar sees precipitation size particles, not clouds. Smaller cloud droplets arenot "seen" by radar. DOPPLER RADAR’S pick up relative motions of objects that move toward or away fromthe radar. A single radar does not “see” objects moving parallel to the radar

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beam. There are a number of inherent biases with radar data, too numerous to mention here. Additionalinformation on radar, helpful to the storm chaser, can be found in Hiser (1970), and Lemon (1979).

10.5.1 RADAR REFLECTIVITY

Weather radar’s emit pulses of electromagnetic waves in a beam. The amount of energy reflected back to theradar from an object can be measured and segregated into different reflectivity levels. Many times these levelsare color coded on the radar display to give the viewer a better understanding of the storm intensity. Lowerreflectivity levels indicate smaller precipitation particles whereas higher reflectivity levels indicate largerprecipitation particles. Usually, the higher reflectivity levels are painted a red color to draw the viewersattention. Radar scopes can display the storm in plan view or in cross section. In plan view, the echo appears asif you are looking down on it from outer space with the radar located at the center of image. Distance to theradar echo can be obtained by counting range marks or using a cursor that is linked to a computer.

Severe thunderstorms are usually associated with higher reflectivity’s. Also, the shape of the precipitation echocan describe the type of storm and severity. In plan view, an isolated thunderstorm will appear as a roundedecho. The most intense storms have a sharp gradient of reflectivity along their inflow side with an ECHOOVERHANG above the inflow. The overhang is caused by rapid rising air that is condensing out large amountsof water and keeping it aloft. Echo reflectivity’s may appear V-shaped with each color scale. Such a V-shapedstorm echo is sometimes referred to as a FLYING EAGLE ECHO and is frequently associated with the mostsevere storms. Sometimes as the updraft rotates, it pulls some of the precipitation around it from the forwardflank downdraft (FFD). This results in an appendage or HOOK ECHO formation. Such an echo formationindicates the storm is rotating and there is about a 50/50 chance that the storm will be tornadic. Refer to Figure22.

Vertical cross sections of a storm also can be observed on the radar. Storm tops in access of 50,000 feet orpenetration of the equilibrium level by 5,000 feet stand a good chance of being severe. The most severe stormshold a lot of precipitation aloft. This precipitation eventually descends around the top of the updraft (like that ofa fountain). The height and size of the echo free vault is proportional to the strength of the updraft. Stormshaving BOUNDED WEAK ECHO REGIONS (BWERs) are usually associated with severe weather. Refer toFigure 23.

10.5.2 RADIAL VELOCITY

The beauty of Doppler Radar is its ability to measure the velocity of particles going away or coming toward theradar. Thus, the operator can infer air motion within a storm. Outbound velocities to the right of inboundvelocities indicate counterclockwise circulation. Computer algorithms can analyze such circulation’s and candetermine if the rotation is strong enough to be classified a MESOCYCLONE.

Most tornadic storms have a mesocyclone. Detection of the mesocyclone will depend on the distance of theradar to the storm and possible attenuation of the radar signal from other storms or objects. Radar “sees” stormsbest if the storm is between 40 and 60 miles away. The radar beam tends to undershoot storms near the radarsite and overshoot storms that are far away. Furthermore, the radar beam is narrow when close in to the radarand quite broad when far away. Radar algorithms are more apt to

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detect larger circulation’s than smaller ones. Many times, radar operators look for GATE-TO-GATE velocitieswhen judging the intensity of a mesocyclone. Well organized mesocyclones with high gate-to-gate wind shearscan trip the TORNADO VORTEX SIGNATURE algorithm or TVS.

10.6 SATELLITE

As surface data stations are closing due to funding cutbacks, the storm chaser becomes more dependent onsatellites to provide detail on the sub-synoptic scale. Towering cumulus along dry line waves or outflowboundaries can be observed. Areas of clearing, cloud boundaries, and cirrus streaks can be detected on thesatellite photograph. Excellent references to read for serious chasers are Siebers (1975), Miller (1978), andPurdom (1979).

Satellite photographs can be obtained at most weather stations. It is a good idea to plot cloud boundaries, cloudlines, and areas free of clouds on the surface mesoanalysis. Remember that the photos are about 40 minutes old.Thus, by the time you see a small cloud anvil on the photograph, chances are the storm is well along. There aretwo types of satellite photos: visible and infrared: Identify the following on the photograph:

a. POLAR (PJ) and SUBTROPICAL (SJ) JETS. These usually appear as a line of cirrus (cooler tops, thus widespread enhancement). Wind maxima occur along or to the north of the cirrus edge. From experience, it appears that the ST axis delineates the farthest southern extent of organized severe thunderstorms. The forward left or rear right quadrant of a wind maxima are favorable for severe weather.

b. JET STREAKS (identified by a patch of cirrus) rounding the base of a trough. Jet streaks indicatelocalized lifting in advance of a short wave. Severe storms can erupt when the jet streak coincides withmaximum daytime heating over an unstable atmosphere.

c. A COMMA CLOUD. This will appear as a large spiral area of cirrus type clouds which denote an upperlow system. Severe weather is most likely east and southeast of the comma cloud along

or just behind the tail. Experience indicates that comma cloud centers which pass over the Rocky Mountainssouth of Denver, Colorado in the morning, have excellent potential for producing severe weather in thesouthern and central plains by afternoon.

c. THE DIFFLUENT ZONE. This zone lies in the region of between the main polar jet and the mainsubtropical jet. It sometimes appears as a V-shaped cloud free area between the upper jets. Vertical motionin this area is enhanced. Storm anvils will widen in such an area.

e. Cumulus clouds. These clouds will appear as small white dots on the photo and are good indicators of the extent of the moist field. LINES OF CUMULUS usually indicate fronts, dryline, or outflow boundaries. Intersecting lines of cumulus are important focal points for convection initiation.Look for waves in the cumulus clouds which may indicate a meso-low or dry line bulge. Enhanced areas oftowering cumulus(may appear like popcorn) are likely areas for convection initiation.

f. Low clouds. Large fuzzy white areas can indicate stratus type clouds. These areas may have a difficult timein "burning off" during the day. Thus, convective potential can be restricted.

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g. Dust. To the rear of fronts and dry lines, dust streaks may appear as a grayish smudge. This will indicatestrong winds and direction.

h. Shadows. Shadows can give an idea of the heights of certain high clouds. At low sun angles, cloud textureis most apparent. Note thunderstorm anvils and overshooting tops.

i. DRY SLOT. This is the large clearing area behind comma cloud tails which indicate a region of subsidence,or sinking air behind a dry front. This stable area is where convection potential is low. However, the eastboundary adjacent to the moist field is quite favorable for convection as the dry slot can enhance dry linemovement and vertical forcing.

10.7 CLIMATOLOGY

The tornado season begins in February along the Gulf States and migrates northwestward into the Plain statesduring the spring. Strong dynamic storm systems tend to produce fast-moving thunderstorms in March and Aprilwhich can attain translational speeds over 50 mph to the north-east direction. Thus, it is quite difficult to chasethese early season storms. Not only do these storms move so fast, but most road networks run east-west ornorth-south which forces the chaser to "zigzag" toward the storm. The early spring storms may produce longtrack tornadoes, but the number of "outbreaks" are few and far between.

By early May, moist Gulf air penetrates deep into the U.S., storm systems slow down, and the length of daylightincreases making it easier to chase severe storms. Tornadoes are produced more in clusters than corridors. Lesstime is spent driving and more on watching the sky. From experience, it appears that a maximum in tornadofrequency occurs during the last three weeks in May throughout north and west Texas up into Kansas. Theannual climatology of tornado frequency also reflects this maximum.

About the last week in May, the upper air flow shifts from a southwest direction to a northwest direction as aridge of high pressure begins to build over the Rocky Mountains. The tornado season shifts from Texas andOklahoma into the mid-west states during June and July.

Experience has shown that several times each spring, a surface low pressure center will form on the lee of theRocky Mountains (cyclogenesis) and begin tracking in an east or northeastward direction usually in conjunctionwith an upper low. Severe weather frequently accompanies these storm systems. Grazulis (1984) presented aclimatology of low pressure tracks, associated with strong tornadoes. He showed an average storm track fromsoutheast Colorado, through central Kansas, and northeastward into Wisconsin.

Studying the formation and movement of low pressure systems has indicated that severe weather in the southernplains is most likely to occur within twenty-four hours following cyclogenesis in southeast Colorado.Furthermore, the track of the main surface low is crucial in identifying the area most likely targeted for severeweather. A study of surface low tracks by the author revealed that if the surface low pulls northward rapidly, thesevere weather threat for the southern plains is greatly reduced. In contrast, the best severe weather episodeshave been with open wave troughs (at 500mb) that track east or northeast at moderate speeds across the plainstates. The “foci” or center of such elliptical-shaped troughs track between San Francisco and Los Angeles,across the four-corners area of the United States, and into southeast Colorado. Lows too far south bringunidirectional flow across the area whereas lows that track too far north shift the severe weather threat into thenorthern plains.

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11. POST-STORM GROUND DAMAGE SURVEY

After a tornado event, the storm chaser should inform the National Weather Service of the location, and time ofthe tornado. The type of damage caused should also be described. A general ground survey of the damagewould be helpful.

11.1 F-SCALE RATING

The purpose of the ground survey is to identify the tornadoes direction, damage width and length. Gradations inthe damage path can be mapped using the scale developed by Fujita (1971). The F-scale is a rating of tornadodamage and intensity based on the appearance of structure damage. The scale ranges from 0 to 5 with 5 beingthe most intense damage. One of the shortcomings to the F-scale is that it does not consider the strength orinherent weaknesses of structures in the wind. Thus, it is emphasized that extreme caution should be used whenusing such a simplified scale on homes that vary greatly in construction. The damage correlated wind speedslisted below are approximations by the author.

F0- Some damage to chimneys and TV antennas. Roof shingles are displaced. Small branches are broken ontrees. Weaker trees and power lines are toppled. Wind speeds are around 60 mph.

F1- Roof decking is removed on permanent homes, carports are overturned, shallow rooted trees are uprooted,automobiles are overturned. Unanchored mobile homes slide. Wind speeds are 60 to 85 mph.

F2- Roofs are blown off of homes leaving stronger walls standing. Sheds and other outbuildings are demolished,unanchored mobile homes are overturned, metal roofs are peeled back. Small wood missiles are generated.Wind speeds are between 85 and 110 mph.

F3- Exterior walls and roofs blown off homes. Metal building collapse or severe damage. Forests flattened.Many non-reinforced masonry structures collapsed. Mobile homes destroyed. Wind speeds are between 110 and135 mph.

F4- Few walls are left standing in well built homes. Pile of debris remains on foundations. Large steel andconcrete missiles are thrown far distances. Wind speeds are between 135 and 170 mph.

F5- Homes on slab foundations are leveled with debris removed. Schools, motels and other marginallyengineered buildings have considerable damage with exterior walls and roofs gone. Top stories or buildings aredemolished. Winds exceed 170 mph.

Plot a map showing the F-scale rating of the damaged houses. Additional information can be shown such as thedirection of fallen trees and trajectories of missiles. Remember that straight-line appearing damage can occurfrom tornadoes and visa versa. Other information on damage analysis can be found Marshall (1985).

11.2 PHOTOGRAPHING DAMAGE

All windstorm damage looks awesome. But there are certain features in the damage which can sort out theintensity of the storm. The following features should be photographed:

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a. Homes varying in F-scale intensity damage. Take general views documenting the type structureand roof geometry.

b. Wall/foundation and roof/wall surfaces. Are there any connections present? Please identify type

and extent of connection damage (i.e. 2 x 4 bottom plates bolted to a concrete slab).

d. Missiles. From small wood splinters to large steel beams. Document the size and shape of such missileslaunched by the tornado.

d. General views of mobile homes. Take specific views of anchor straps and connections at ground.

11.3 EYEWITNESS INTERVIEWS

Eyewitnesses to the storm can provide valuable information to be used in documenting storm characteristics.However, it is important to recognize the every person has their own perspective about tornadoes. Separatingfacts from fiction will be necessary to obtain an accurate account of the storm. For example, a person observedseven tornadoes at once. When questioned further, it was apparent that he was seeing one large tornado withseven subvortices. Some of the important questions to ask are:

1. Address/Location of eyewitness? 2. Did you see the storm (tornado)? 3. Which way did the storm approach? What time? 4. Hail- a. What direction did the hail (rain) fall from? b. What was the largest size? What time? c. What shape was the hail? (round, spiked) d. Was the hail hard or soft? e. What direction was the wind before (after) the hail and/or tornado? 5. Tornadoes- a. How many tornadoes did you see? What time? b. What direction did they come from? c. Was there any damage? d. Do you know anyone who took pictures of the tornado?

12. SAFETY RULES

Storm chasing does not have to be dangerous. Being safety conscious is being smart. The author believes thatthe most dangerous part about storm chasing is in driving. No matter how defensive you drive, a car accidentcan occur suddenly and unexpectedly. In a car:

1. Make sure the driver keeps their eyes on the road. It is so tempting to gaze at a storm in the distance. Thiscan lead to driving off the road or into a head on collision.

2. Tips like using your turn signals when turning or changing lanes seem simple, but a lot of driverson the road never use them. Using the side and rear mirrors when changing lanes is not enough. A quick

glance backward can identify someone driving in your blind spot.

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3. Long distance travel for several days can lead to fatigue. Switch driving partners every few hours. After awhile, a symptom called "chasers vertigo" may appear. This is where a chaser always

observes towering cumulus on the distant horizon.

4. Avoid speeding and hydroplaning on wet roads. Be sure to stay on paved roads. Wet, unpavedroads can bring a storm chase vehicle to a halt.

5. Make sure the vehicle is reliable. A broken fan belt in the middle of Motley County, Texas couldleave you stranded for a long time.

When closing in on a thunderstorm, the hazards to the chaser increase. Besides driving, lightning becomes thebiggest hazard to the storm chaser. Lightning can have positive or negative polarity and can come from any partof the thunderstorm.

A developing thunderstorm is usually inactive with lightning. Lightning is first apparent within the storm (Cloud-to-cloud) at the anvil level. At night, the cloud-to-cloud flash can illuminate the underside of the anvil and revealstorm structure. Within the cloud, this flash will cause the clouds become illuminated over large distances.Thus, the term "sheet lightning" has been used to describe this type of lightning.

Cloud-to-ground flashes become more frequent as the storm enters the mature stage. The most common type ofcloud-to-ground lightning has negative polarity and stems from the precipitation region. This flash may appearto repeat several times and cause prolonged static on your car radio. Positive lightning is usually infrequent andcan strike without warning. This type of lightning can occur from the rain free base or anvil-to-ground.

The frequency of lightning can relate to storm severity. The author has observed higher flash rates in severestorms especially during the late mature stage and early dissipating stage. For added safety, storm chasers shouldknow CPR (cardiovascular pulmonary resuscitation). Classes are usually offered by the local Red Cross Agency.It is important that the storm chaser be aware of the lightning hazard and know the following safety rules:

1. When possible, stay in your vehicle. This will offer some protection to occupants from a direct lightningstrike. Though, the author has seen photographs where lightning had passed directly through the car.

2. Be aware of your height with respect to the surrounding terrain. Lightning prefers to strike taller objects. In West Texas, where there are few trees, a storm chaser on a hill with an aluminumcamera on a tripod would be the tallest lightning rod around.

3. Though lightning prefers higher obstructions, it is not uncommon for a bolt to strike an object in a valley,especially near a lake or pond.

4. Note the relative cone of protection that power lines have over the road. Position your vehiclenear lines that will offer some protection from lightning.

5. Lightning can strike outside the storm cloud and/or precipitation areas.

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Other hazards around thunderstorms can stem from the storm itself or it's effect on the environment. When nearor around thunderstorms:

1. Avoid heavy precipitation area. Visibility is reduced and driving becomes more difficult. Core-punching isnot recommended.

2. Watch for flooded roads. It is extremely difficult to judge the depth of water over the road. Rainwater

runoff can be a fast moving current which can easily push or float a chase vehicle off theroad. Avoid crossing flooded roads and bridges.

2. Avoid driving under wall clouds. Tornadoes can develop rapidly and without warning. Strong down-draftwinds can occur. Chasers should not become debris tags or kamikaze pilots just for photogrammetricpurposes.

3. Make sure you always have an alternate route, especially in a tornadic situation. Being caught on a dead endroad with a tornado bearing down can be an unwelcome experience.

REFERENCES

Bluestein, and C.R. Parks, 1983: A synoptic and photographic climatology of low-precipitation severethunderstorms in the southern plains. Mon. Wea. Rev., 111, 2034-2046.

Byers, H.R., and R.R. Braham Jr., 1949: The Thunderstorm, U.S. Government Printing Office, Washington DC,287pp.

Doswell III, C.A., 1985: The Operational Meteorology of Convective Weather, Part II: Storm Scale Analysis,NOAA Technical Memorandum ERL ESG-15, 240 pp. (Available from the National Technical InformationService, 5285 Port Royal Road, Springfield, VA 22161)

Fujita, T.T., 1971: Proposed characterization of tornadoes and hurricanes by area and intensity. SMRP Report,No. 91, Univ. of Chicago, 42 pp.

Grazulis, T.P., 1984: Violent Tornado Climatography 1880-1982, NUREG Report CR3670, 165 pp.

Hiser, H.W., 1970: Radar Meteorology. Radar Meteorological Laboratory, University of Miami, 306 pp.

Lemon, L.R., 1977: New severe thunderstorm radar identification techniques and warning criteria: A preliminaryreport. NOAA Tech. Memo., NWS-NSSFC-1, 60 pp.__________, and C.A. Doswell III, 1979: Severe thunderstorm evolution and mesocyclone structure as relatedto tornadogenesis. Mon. Wea. Rev., 107, 1184-1197.

Marshall, T.P., J.R. McDonald, and K.C. Mehta, 1983: Utilization of load and resistance statistics in a windspeed assessment. IDR Report #67, Texas Tech University, 91 pp.

Miller, R.C., 1972: Notes on analysis and severe storm forecasting procedures if the Air Force Global WeatherCentral, Air Weather Service.

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_________, and J.A. McGinley, 1978: Using satellite imagery to detect and track comma clouds and theapplication of the zone technique in forecasting severe storms. 94 pp.

Minor, J.E., J.R. McDonald, and K.C. Mehta, 1977: The tornado: An engineering-oriented perspective.NOAA Tech. Memo., ERL-NSSL-82, 196 pp.

Moller, A.R., and C. A. Doswell III, 1988: A Proposed Advanced Storm Spotter’s Training Program, Preprints,15th Conference on Severe Local Storms, Indianapolis, IN, p. 173-177. (Available from the AmericanMeteorological Society, 45 Beacon St., Boston, MA 02108)

__________, and C. A. Doswell III, and R. Przybylinski, 1990: High-Precipitation Supercells. A conceptualmodel and documentation. Preprints, 16th Conference on Severe Local Storms, Kananaskis Park, Alberta,Canada, p. 52-57. (Available from the American Meteorological Society, 45 Beacon St., Boston, MA 02108).

Purdom, J.F.W., 1979: The development and evolution of deep convection. 11th Conference on Severe LocalStorms, 143-150.

Siebers, J.O., F. Hidalgo, S.A. Tegtmeier, and M. Young, 1975: Guide for using GOES/SMS imagery in severeweather forecasting, USAF, AWS, unnumbered technical guide, 56 pp.

Snow, J.T., 1984: The Tornado. Scientific American, 250, 86-96.