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April 10, 2012. Joe Halvorson, Chris MacIntosh, Tim Marquis. Severe Weather and Storm Chasing. What causes a storm, severe parameters, tools. Severe Weather Basics. The Basics. Thunderstorms need three things to develop: Moisture Instability Lift/Trigger

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Severe Weather and Storm Chasing

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Severe weather and storm chasing

April 10, 2012

Joe Halvorson, Chris MacIntosh, Tim Marquis

Severe Weather and Storm Chasing

Severe weather basics

What causes a storm, severe parameters, tools.

Severe Weather Basics.

The basics

The Basics

  • Thunderstorms need three things to develop:

    • Moisture

    • Instability

    • Lift/Trigger

  • Severe storms need one more ingredient:

    • Wind shear (directional, speed)

Storm types

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

Singe cell



Multicell radar

Multicell radar

4 cells

Quasi linear convective system qlcs

Quasi-Linear Convective System (QLCS)

Supercell structure

Supercell Structure

Supercell structure1

Supercell Structure




  • Bulk Shear

  • Storm-Relative Helicity

  • Energy Helicity Index

  • Others

    • LCL Heights, LI, Lapse Rates.

    • Supercell Composite, Significant Tornado Parameter

Severe weather and storm chasing


  • 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

Severe weather and storm chasing

“Fat” CAPE


“Skinny” CAPE


Severe weather and storm chasing


  • 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

Severe weather and storm chasing


Cin cinh1


  • 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 2011

CAPE/CIN: 24 May 2011



Bulk shear

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 2011

Bulk Shear: 27 April 2011


0–1 km

Storm relative helicity srh

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 srh1

Storm-Relative Helicity (SRH)

  • Thresholds (in m2 s-2)

    • 0–1 km : 100+

    • 0–3 km : 250+

    • Effective : 100+

0 1 km srh outbreaks

0–1 km SRH: Outbreaks

27 April 2011

24 May 2011



Greensburg EF5

Parkersburg EF5

Outbreak sounding

Outbreak Sounding

Energy helicity index ehi

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 outbreaks

0–1 km EHI: Outbreaks

27 April 2011

24 May 2011

Other parameters

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 parameters1

Other Parameters

  • Supercell Composite

  • Significant Tornado Parameter

    • For these two, we shall play a game!

But first math

But first… MATH!

Not scary Chen math, but still a nuisance.

Game time

Game time!

Game time1

Game time!

Where was the severe weather

Where was the severe weather?


The point of stp scp

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

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

500 mb Jet: 27 April 2011

Llj 9 april 2011

LLJ: 9 April 2011

Upper level jet streak

Upper Level Jet Streak

Left Exit


Left Entrance

Right Entrance

More things to look at1

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 at2

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 at3

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 at4

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

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.

Storm Chasing Basics.

The beginner s way

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

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

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!

Severe weather and storm chasing


  • 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.

Storm Chasing Safety.

Safety tips

Safety Tips


  • 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

Good Resource: Titan U.


Don t get close on big days

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:



More safety tips

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 tips1

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.



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