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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
  • Bulk Shear
  • Storm-Relative Helicity
  • Energy Helicity Index
  • Others
    • LCL Heights, LI, Lapse Rates.
    • Supercell Composite, Significant Tornado Parameter
  • 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


  • 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
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!
  • 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
  • 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.