Severe Weather and Storm Chasing

Joe Halvorson, Chris MacIntosh, Tim Marquis

April 10, 2012

Severe Weather Basics.

What causes a storm, severe parameters, tools.

The Basics Thunderstorms need three things to

develop: Moisture Instability Lift/Trigger

Severe storms need one more ingredient: Wind shear (directional, speed)

Storm Types Single cell: “popcorn” storms

Usually not severe, short-lived Multicell: group of cells moving together

Moderate severe, tornado potential Squall line: line of storms

Moderate severe, low tornado potential Supercell: storm with a rotating updraft

(mesocyclone) Very high severe potential Main tornado producer

Singe cell

Multicell

Multicell radar

4 cells

Quasi-Linear Convective System (QLCS)

Supercell Structure

Supercell Structure

Parameters CAPE/CIN(H) Bulk Shear Storm-Relative Helicity Energy Helicity Index Others

LCL Heights, LI, Lapse Rates. Supercell Composite, Significant Tornado

Parameter

CAPE Convective Available Potential Energy

Ability that a parcel has to rise Large CAPE usually means large vertical velocities

Different types: Surface-based Mixed-layer (Mean layer) Most unstable 0–3 km

“Skinny” and “Fat” CAPE Skinny: weaker updrafts Fat: stronger updrafts

“Fat” CAPE

“Skinny” CAPE

CAPE!

CAPE!

CAPE Thresholds (in J kg-1):

1–1,000 : Marginally unstable 1,000–2,500 : Moderately unstable 2,500–3,500 : Very unstable 3,500+ : Extremely unstable

For 0–3 km CAPE, 100+ J kg-1 best for tornadic environment

CIN (CINH) Convective Inhibition

“Cap,” can prevent instability from being realized

Negative CAPE Ways to overcome CIN:

Heating (daytime or warm air advection) Moisture Lift

CIN!

CIN (CINH) Thresholds (in J kg-1):

0–50 : Weak cap 50–200 : Moderate cap 200+ : Strong cap

These values are negative on most model and mesoanalysis charts.

CAPE/CIN: 24 May 2011Surface-based

Mixed-layer

Bulk Shear Change in wind direction and speed with

height put into a single number Most useful: 0–1 km, Effective

Effective accounts for storm depth (inflow base to equilibrium level)

Thresholds 0–1 km : 15-20+ kts supportive of tornadoes Effective : 25–40+ kts supportive of

supercells

Bulk Shear: 27 April 2011Effective

0–1 km

Storm-Relative Helicity (SRH) Measure of the potential for a rotating

updraft in a supercell Higher numbers associated with a

higher potential for tornadoes Three types:

0–1 km, 0–3 km, effective Effective is most useful in discriminating

between tornadic and nontornadic storms.

Storm-Relative Helicity (SRH) Thresholds (in m2 s-2)

0–1 km : 100+ 0–3 km : 250+ Effective : 100+

0–1 km SRH: Outbreaks27 April 2011

24 May 2011

HodographsGreensburg EF5

Parkersburg EF5

Outbreak Sounding

Energy Helicity Index (EHI) Combination of CAPE and SRH Two types: 0–1 km, 0–3 km Thresholds

>2.5 : Favorable for tornadoes Storms must develop for this to be

useful Can be skewed by high CAPE values

0–1 km EHI: Outbreaks27 April 2011

24 May 2011

Other Parameters LCL Heights

Lower heights (<1000 m) associated with tornadoes Higher moisture available, keeping RFD warm

Lifted Index (LI) Another stability index (taken at 500 mb) The lower the number, the more unstable the

atmosphere

Other Parameters Supercell Composite Significant Tornado Parameter

For these two, we shall play a game!

But first… MATH!

STP MLCAPE1500

*EBSsfc20

*ESRH150

*2000 MLLCL

1500*250MLCIN

200

SCP MUCAPE1000

*EBS20

*ESRH50

Not scary Chen math, but still a nuisance.

Game time!

Game time!

Where was the severe weather?

?

The point of STP & SCP These are best used to pinpoint a region

that severe weather could occur on a given day.

Should never be used to make any other decisions without actually analyzing the environment.

More things to look at! Jet Stream

Increases shear Separates downdraft from updraft

Longer-lived storms Low-level Jet (LLJ)

Brings in moisture and warmer temperatures, increases shear and helicity

Upper-Level Jet Streaks Increased shear, upward motion to increase

instability Upward regions: right entrance, left exit

500 mb Jet: 27 April 2011

LLJ: 9 April 2011

Upper Level Jet Streak

Left Entrance

Right Entrance

Left Exit Right

Exit

More things to look at! Frontal boundaries

Cold and warm fronts, dryline Can enhance shear, helicity Source of lift

Surface observations Gives an idea of what is going on at this

moment Look for mesoscale features

Satellite Imagery Look for clearing early, robust CU fields

More things to look at! Water Vapor Imagery

Shows drying, moistening in the mid- and upper-levels

See where fronts/shortwaves are located

More things to look at! Moisture

Storms won’t form without it Td > 60°F for best environment

700 mb temperatures Good indicator of cap strength < 12°C in the spring without a strong lifting

mechanism Can be slightly higher (<13°C) during

summer months or with strong lift

More things to look at! Theta-E

Measure of heat and moisture in the atmosphere

Look for theta-e advection/ridges Increased instability, convergence Could be area of convective development

Storm Motion Calculated average direction and speed of

storms Gives an idea of how storms will move

Where Do We Look At This Stuff? SPC Mesoanalysis TwisterData COD Analysis AMS Model Animator HPC RAP Weather

Storm Chasing Basics.

How to chase safely and effectively.

The Beginner’s Way S/SE is safest place to be

Watch for right-turning Avoid core-punching Avoid rain-wrapped messes Pull off on side roads, not main roads

What NOT to expect… To see a tornado on your first time out…

or second… or third…

Good chasers usually see a tornado about once in every seven chases.

Don’t let this deter you from chasing… Can’t win the game if you don’t play!

What to expect… LONG car rides Sitting, waiting, wishing… football and

frisbee! Lots of gas station/fast food meals Sketchy dirt roads

And in turn, few rest stops. The beauty of the Plains Awesome tunes! THE TIME OF YOUR LIFE! Memories!

Tips Wear real shoes or boots, not flip flops. Watch the skies, not just the computer

screen. Only your eyes can tell you what’s actually

happening around you. Try to learn as much as possible, not

just watch.

Storm Chasing Safety.

So we’re not held liable if you act stupid. Just kidding. But seriously.

Safety Tips CHASE WITH SOMEONE WHO HAS

CHASED BEFORE. Chase with a partner Stay in the car as much as possible if

there’s lightning nearby. Have an escape route

Try to head south if you feel that you’re in danger.

If that’s not possible, head east until you can go south.

Good Resource: Titan U. http://www.youtube.com/watch?v=d_Kh

AhitJE0&list=PL68D75C6534412169&index=4&feature=plpp_video

Don’t Get Close on Big Days! Going with experience and technology. Freshman shouldn’t go out with other

freshman alone. Watch out for power poles! Things NOT to do:

http://www.youtube.com/watch?v=h84yTNXCXPQ

http://www.youtube.com/watch?v=CaQ3L1jftxs

More Safety Tips Don’t get out of the car if there’s hail. Don’t chase at night. Make sure you pull off the road ALL THE

WAY when you stop to look at something. Don’t pull over on highways to look. Flashers. Watch out for “those” drivers.

More Safety Tips Avoid “minimum maintenance” roads

Previous rainfall could have washed them out.

Don’t drive on flooded roads. Turn around, don’t drown!

Don’t drive over power lines/debris. Try to avoid driving through squalls.

This is never possible in Iowa.

WATCH OUT FOR ANIMALS. They get spooked and like to run out in

front of/into the side of moving vehicles.

Questions?cwmac@iastate.edujrhalvy@iastate.edu

tmarquis@iastate.edu