Jim Orlando, Kevin Smalley, Sean Stelten
What causes a storm, severe parameters,
tools, etc.
Thunderstorms need three things to develop:
Moisture
Instability
Some sort of trigger
Front, dryline, small scale pressure trough, gravity wave,
etc.
Severe storms need one more ingredient:
Wind shear (both directional and speed)
Single cell
“Popcorn” storms
Usually not severe and fairly short-lived
Multi-cell
Group of cells moving together
Some severe potential
Low tornado potential
4 cells
Squall lines
Now called Quasi-Linear Convective Systems (QCLS)
A line of storms
Severe potential mainly includes high winds and
hail, but there is some tornado potential
Supercell
Storm with a rotating updraft
VERY high severe potential
Main tornado producing storm
Convective Available Potential Energy
The ability that a parcel has to rise
Large CAPE usually means large vertical velocities
Different types
Surface-based
Mixed-layer
Most unstable
0-3 km
“Skinny” and “Fat” CAPE
Skinny = weaker updrafts
Fat = stronger updrafts
“Fat” CAPE
CAPE!
“Skinny” CAPE
CAPE!
Thresholds (in J/kg)
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 is considered a
favorable environment for tornadoes
Convective Inhibition
“Negative CAPE”
The result of a “capping inversion”, which can
prevent instability from being realized
Ways to overcome CIN:
Heating (daytime or warm air advection)
Added moisture
Lift
CIN!
Thresholds (in J/kg)
0 – 50: Weak cap
50 – 200: Moderate cap
200+: Strong cap
These values are negative on most model and
mesoanalysis charts
Surface-based
Mixed-layer
Change in wind direction and speed with height,
all in one number
Most useful values are 0 – 1 km and “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
Effective
0–1 km
Measure of the potential for a rotating updraft
in a supercell
Higher numbers are associated with a higher
potential for tornadoes
Three types:
0 – 1 km, 0 – 3 km, Effective
Effective is the most useful in discriminating
between tornadic and non-tornadic storms
Thresholds (in m2/s2)
0 – 1 km: 100+
0 – 3 km: 250+
Effective: 100+
27 April 2011
24 May 2011
You can also get a good idea of helicity by
looking at a chart called a hodograph
Looping/hook shaped hodographs are indicative of
high helicity values
Greensburg EF5
Parkersburg EF5
Combination of CAPE and SRH
Two types: 0 – 1 km, 0 – 3 km
Thresholds
>2.5: Favorable for tornadoes
Storms must develop in order for this to be
useful
Can be skewed by high CAPE values
LCL Heights
Lower heights (<1 km) help with tornado
formation, but tornadoes can still form with higher
LCLs
LCL = 1500m
LCL = 750m
Lifted Index (LI)
Another stability index (taken at 500mb)
The lower the number (more negative), the more
unstable the atmosphere
Supercell Composite
Significant Tornado Parameter
For these, let’s play a game!
Not scary Chen math, unfortunately……
MLCAPE EBS sfc ESRH 2000  MLLCL  250  MLCIN 
STP 
*
*
*
*
1500
20
150
1500
200
MUCAPE EBS ESRH
SCP 
*
*
1000
20
50
These are best when used to pinpoint a region
that severe weather could occur on a given day
They should never be used to make any other
decisions without actually analyzing the
environment
Jet Stream
Increases shear
Separates downdraft from updraft
Creates longer-lived storms
Low-level Jet (LLJ)
Brings in moisture and warmer temperatures
Increases shear and helicity
Usually kicks in at sunset
Upper-level Jet Streaks
Increased shear and upward motion
Upward motions in right entrance and left exit
Left
Exit
Right Exit
Left Entrance
Right Entrance
Frontal boundaries
Cold/warm/stationary fronts, drylines
Can enhance shear and helicity
Source of lift
Surface observations
Gives an idea of what’s going on at this moment
Look for mesoscale features
Satellite imagery
Look for clearing early and robust CU fields closer
to initiation
Water vapor imagery
Shows drying and moistening in the mid – upper
levels
See where fronts/shortwaves are located
Moisture
Storms won’t form without it
Td > 60F best
700mb temps
Good indicator of cap strength
< 12C in the spring w/o a strong lifting mechanism
Can be slightly higher during the summer or with
strong lift
Theta-E
Measure of heat and moisture in the atmosphere
Look for theta-e advection/ridges
A sign of increased instability and convergence
Could be an area of convective development
Storm Motion
Calculated average direction and speed of stomrs
Gives an idea of how storms will move
SPC Mesoanalysis
Twisterdata
COD Analysis
AMS Model Animator
WPC
RAP Weather
How to chase safely and effectively
South/Southeast of the “hook echo”/storm is
the safest place to be
Watch for right turning!!
Avoid core punching
Unless you have a death wish for all of your car
windows
Avoid rain-wrapped messes
Pull off on side roads, not main roads.
To see a tornado on your first time out
Or second
Or third
Good chasers usually tend to see a tornado
about once every seven chases
Don’t let this deter you from chasing, you can’t
wind the game if you don’t play!
LONG car rides
Sitting, waiting, wishing…..
Footballs and Frisbees are your friends!
Lots of gas station/fast food meals
Sketchy dirt roads
Few bathroom breaks
Awesome tunes
The wonder of nature, baby!
THE TIME OF YOUR LIFE! AND MEMORIES!
Wear REAL shoes or boots
Stay away from sandals, especially flip flops
Watch the skies, not just the computer
Only your eyes can tell you what’s actually
happening
Try to learn something, not just watch
So we’re not held liable if you act stupid.
Just kidding. But seriously.
CHASE WITH SOMEONE WHO HAS CHASED
BEFORE
Chase with a partner
Stay in the car 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
Don’t tempt fate just to get that perfect shot
Safety should ALWAYS be your number one concern
Give strong/violent tornadoes (especially
wedges) their space
Keep an eye on the radar AND the tornado
Watch out for power poles!
Freshmen should NOT go chasing with other
freshmen alone
Don’t get out of the car if there’s hail
Don’t chase at night
When stopping, pull ALL the way off the road
Don’t pull over on highways
Use your flashers
Watch out for “those” drivers
“Chaser convergence”
Avoid “minimum maintenance” roads
Previous rainfall could have washed them out
Not all of these are marked….
Don’t drive on flooded roads
Turn around, don’t drown
Don’t drive over power lines/debris
Avoid entering debris paths unless absolutely
necessary, especially if there are people in your
car that don’t have long pants/closed shoes
Try to avoid driving through squalls, especially
at night
Watch out for animals
They get spooked and like to run out in front
of/into vehicles
Watch out for cops
They get spooked and like to run out in front
of/into vehicles
jorlando@iastate.edu
ksmalley@iastate.edu
sstelten@iastate.edu