April 10, 2012
Joe Halvorson, Chris MacIntosh, Tim Marquis
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
CAPE!
“Skinny” 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 2011
Surface-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 2011
Effective
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: Outbreaks
27 April 2011
24 May 2011
Hodographs
Greensburg 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: Outbreaks
27 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!
MLCAPE EBS sfc ESRH 2000  MLLCL  250  MLCIN 
STP 
*
*
*
*
1500
20
150
1500
200
MUCAPE EBS ESRH
SCP 
*
*
1000
20
50
 scary Chen math, but still a nuisance.
Not
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
Exit
Right
Exit
Left Entrance
Right Entrance
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
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.
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&in
dex=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=h84yTN
XCXPQ
 http://www.youtube.com/watch?v=CaQ3L1j
ftxs
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.
cwmac@iastate.edu
jrhalvy@iastate.edu
tmarquis@iastate.edu