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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

April 10, 2012

Joe Halvorson, Chris MacIntosh, Tim Marquis

Severe Weather and Storm Chasing

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

Multicell radar
Multicell radar

4 cells



  • 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

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

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.

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

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

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.

Safety tips
Safety Tips But seriously.


  • 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. But seriously.


Don t get close on big days
Don’t Get Close on Big Days! But seriously.

  • 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 But seriously.

  • 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 But seriously.

  • 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 But seriously.