Forecasting Workshop 2009

Ryan Alliss, Andy Mair, Matt Hoffman

WxChallenge Rules and Information

• All rules found at WxChallenge.com– Forecasts due at 0z, forecasting for 6z to 6z the

following day• Example for today’s forecast

– Verification of forecasts by ASOS and Climatology information

– Forecast Monday-Thursday– You can forecast for three days in advance if busy

Awards and Tournament

• Win a trophy if you finish first in your Category for a given period– Fresh/Soph-Cat 4, Junior/Senior- Cat 3, etc..

• At the end of the year if you are in the top 64 nationally you will participate in a tournament to crown the top forecaster.

ISU WxChallenge Page

• Site made for ISU students regarding WxChallenge– http://www.meteor.iastate.edu/~ralliss/wxchallen

ge/

• Links can be altered for any asos site through input of station

• Results for ISU will be posted in results section• All necessary information can be found here.

Submitting Forecasts

• Found at ISU WxChallenge page on left– “Enter Forecast”

• Or go directly to WxChallenge.com– Go to submit forecast tab

• Enter Username, Password, and School (ISU)• Enter Forecast for Day 1, Hit Submit – If busy the following days you can forecast ahead

for Day 2 and 3

Basic Meteorology

• Forecasting Red Book– Required for Mt111 and Mt311– Contains all necessary info for forecasting

• Green Book– Optional

• We will sum up key concepts

UTC Time

• Greenwich Mean Time• 0z is 7pm local time• 12z is 7am local time• When daylight savings time begins 0z will be

6pm and 12z will be 6am• Forecasts for wxchallenge due at 0z, therefore

you will have to send your forecasts in by 6pm when daylight savings time begins.

Pressure, units in millibars (mb)

• Pressure decreases with height– Surface around 1000mb – 30,000 ft around 500mb– Weight of the column of air above

• Pressure also varies horizontally– Surface lows and highs– Main contributor to wind, horizontal pressure

gradients– Areas of Low pressure cause disturbed weather

Plotting at different levels

• Surface plots plot varying pressure• Upper level plots use isobaric level and then

plot varying heights– 500, 850, 700, 500 mb charts– Lower heights associated with lower pressure and

thus more disturbed weather and vice versa for higher heights

• Temperature, humidity, wind speed, and direction also are plotted anywhere.

Surface METARS

• Plot basic information of the weather at a surface station

Clouds and Humidity

• Dewpoint– Measurement of amount of water vapor in the air– When dewpoint=temperature air is saturated

• Relative Humidity– Complicated equation, not used much outside of

tv forecasts

• Clouds– Occur when a level is near saturation, at least

RH>70%

Fronts

• Are defined by the line between air masses• Formed off of areas of low pressure• Converging winds cause upward motion, cloud

cover, rain/snow• Four main types of fronts– Cold Front– Warm Front– Occluded Front– Stationary Front

Satellite Imagery

• Visible– Reflection of light to satellite, what you would see if you

were in space• Infrared– Measures temperature of clouds– Clouds higher up are colder– Use at night

• Water Vapor– Measures water vapor in UPPER atmosphere– Used for seeing upper level circulations

Skew-ts

• Plot temperature, dewpoint, wind speed and direction with height

• Used to see where clouds are at– Dewpoint close to temp

• Severe Weather Forecasting applications– Instability, wind shear

• You will see these again

Low and High Pressure

• Low Pressure– Try to make equilibrium so more mass goes to low– Upward motion at surface yields disturbed

weather– Winds circulate counter-clockwise and in

• High Pressure– Downward motion yields clear skies and calm

winds– Winds circulate clockwise and out

Troughs and Ridges

• Waves exist in the atmosphere caused by pressure and temperature gradients

• Look at 500mb chart to find troughs and ridges• East side of trough– Upward motion, location of surface low

• East side of ridge– Downward motion, location of surface high

• Trough=bad weather, Ridge=good weather

How to Forecast

• What are we forecasting for?– Max and Min Temperature, Max Sustained Wind

Speed, and Liquid Precip Equivalent.

• Iowa Forecast Contest gives you more– Free Contest– Visibility, Ceiling, forecasting out past day 1…– Contact Jon Hobbs to join

Resources for Forecasting

• Models – Sager Model Animator- Images of Model Output– MOS (model output statistics)

• Human Guidance– NWS

• Point Forecast and Hourly Weather Graph

• Persistence Forecasting– Past observations that are similar to current situation

• All of these can be accessed at ISU WxChallenge Page

Models

• Use complex equations to “model” atmospheric behavior

• Outputted in the form of data that is converted to imagery

• Two main models used most often– GFS, Global Forecasting System• Longer range model, poor resolution

– NAM, North American Model• Shorter range model, higher resolution

Maximum Temperature

• Things that affect Max Temp– Cloud Cover– Rain cooled air– Temp Advection • Example- Winds blowing warmer air into area

– Wind Speed– Adiabatic Motions• Air flowing over mountain

– More to come in later Meteorology Classes

– Frontal Passage

Maximum Temperature cont.

• Start with persistence forecast then edit according to any of the effects that are now occurring– Example: Today’s High: 76, it was Clear with winds

out of the south with high pressure dominating. Tomorrow’s most likely weather: Clouds moving in with winds shifting to the north, cold front sweeps through.

– What should you expect the High to trend to?

I want numbers though!

• Once you know how the high will compare to tomorrow from today go to the models and NWS to compare.– MOS Max and Min Temps in Row “N/X”, 3 hr temps

are in Row “TMP”– Point Forecast Provides Max and Min temps for NWS

• Over time you will be able to quantify in your head how much difference between days the High will be from changing conditions, without looking at the models and relying on them.

Maximum Temperature Strategy

• Go to 850 mb chart, find nearest station, take the temperature at 12z, add 13 degrees C to it and that is an estimation for the high tomorrow. Remember to convert to f– Example– Will not work if front passes

• I’m not a huge fan of this method.

Minimum Temperature

• Things that affect min temp– Cloud Cover– Wind Speed– Frontal Passage– Rain Cooled Air– Snow Cover– Temp Advections

• Exact same effects right?

Minimum Temperature Strat.

• Look at that dewpoint!– If it is clear and calm winds, good chance the temp

will reach the dewpoint overnight– Sometimes forecasting the min temp involves

forecasting the dewpoint as well• What do you look at to forecast dewpoint?

– MOS, Hourly Weather Graph, Trends

• Look at both ends of period, back door lows occur

Max Sustained Wind Speed

• What causes wind?– Pressure Gradient Force

• What lowers “ideal” wind speed?– Friction– Curvature in flow

• How to forecast– Look at forecast pressure gradients

• Surface MSLP contours

– Use MOS, Hourly Weather Graph, and USL Model

MSLP Plot, Pressure Gradients!

Precipitation

• Forecasting Liquid Water Equivalent in inches.– Rain amount as is– For snow you use liquid water equivalent after it

melts• Around 1” water for 10” snow (1/10 ratio)

– Heavier, wet snow yields more of a 1/8 ratio– Dry, fluffy snow yields more of a 1/12 ratio

• Forecasting exact amount– Mteor Classes will forecast by categories rather

than exact amounts

Precip Cont.

• How to forecast– Find general area of forcing

• East side of Trough• PVA- Positive Vorticity Advection• Fronts/convergent regions

– Model Output• 24 hour qpf forecast• MOS

– Uses Categories » Cat 0=0”, Cat 1=Trace-.05”, Cat 2=.05-.25”, Cat 3=.25-.5”,

Cat 4=.5-1.0”, Cat 5=>1”

Precip Cont.

– Human Forecasts• HPC QPF• Hourly Weather Graph

– All can be accessed from ISU WxChallenge Page– Models tend to overestimate

• Be weary of bullseyes or ribbons of precip

– More points gained at smaller amounts• You forecast 0”, and it verifies to .08” is more points

gained than forecasting 1” and it verifies to 1.08”

– Precip can be very tough or very easy

Forecasting Snowfall

• 1000-500mb Thickness 540 line– If you are below 5400 m in thickness your layer is

suitable for snowfall– Soundings• Majority of layer must be below 32 degrees

• Human Forecasts– Look at what NWS has to say

• LIQUID WATER EQUIVALENT!

Forcing, Troughs, PVA

Same time, Precip, MSLP

24 hr Precip

Let’s Forecast!

• Go to the following page:– http://www.meteor.iastate.edu/~ralliss/casestudy

• Come up with the four things we forecast for– Max Temp– Min Temp– Max Sustained Wind Speed– Precip Amount

How did we do?

• What actually happened in Boston?• Important to look at verification to see if you

did good and if not why– Look at 72 hr obs• Was there cloud cover which you did not take into

account or some other effect?

• This is the key to becoming a good, experienced forecaster

Questions?