Jim Orlando, Kevin Smalley, Sean Stelten

What causes a storm, severe parameters, tools, etc.

Thunderstorms need three things to develop:MoistureInstabilitySome sort of triggerFront, dryline, small scale pressure trough, gravity wave, etc.Severe storms need one more ingredient:Wind shear (both directional and speed)

Single cellPopcorn stormsUsually not severe and fairly short-lived

Multi-cellGroup of cells moving togetherSome severe potentialLow tornado potential

Squall linesNow called Quasi-Linear Convective Systems (QCLS)A line of stormsSevere potential mainly includes high winds and hail, but there is some tornado potential

SupercellStorm with a rotating updraftVERY high severe potentialMain tornado producing storm

Convective Available Potential EnergyThe ability that a parcel has to riseLarge CAPE usually means large vertical velocitiesDifferent typesSurface-basedMixed-layerMost unstable0-3 kmSkinny and Fat CAPESkinny = weaker updraftsFat = stronger updrafts

Fat CAPESkinny CAPE

Thresholds (in J/kg)1 1,000: Marginally unstable1,000 2,500: Moderately unstable2,500 3,500: Very unstable3,500+: Extremely unstableFor 0 3 km CAPE, 100+ J/kg is considered a favorable environment for tornadoes

Convective InhibitionNegative CAPEThe result of a capping inversion, which can prevent instability from being realizedWays to overcome CIN:Heating (daytime or warm air advection)Added moistureLift

Thresholds (in J/kg)0 50: Weak cap50 200: Moderate cap200+: Strong capThese values are negative on most model and mesoanalysis charts

Surface-basedMixed-layer

Change in wind direction and speed with height, all in one numberMost useful values are 0 1 km and EffectiveEffective accounts for storm depth (inflow base to equilibrium level)Thresholds0 1 km: 15 20+ kts supportive of tornadoesEffective: 25 40+ kts supportive of supercells

Effective01 km

Measure of the potential for a rotating updraft in a supercellHigher numbers are associated with a higher potential for tornadoesThree types:0 1 km, 0 3 km, EffectiveEffective 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 201124 May 2011

You can also get a good idea of helicity by looking at a chart called a hodographLooping/hook shaped hodographs are indicative of high helicity valuesGreensburg EF5Parkersburg EF5

Combination of CAPE and SRHTwo types: 0 1 km, 0 3 kmThresholds>2.5: Favorable for tornadoesStorms must develop in order for this to be usefulCan be skewed by high CAPE values

LCL HeightsLower heights (

LCL = 1500m

LCL = 750m

Lifted Index (LI)Another stability index (taken at 500mb)The lower the number (more negative), the more unstable the atmosphereSupercell CompositeSignificant Tornado ParameterFor these, lets play a game!

Not scary Chen math, unfortunately

These are best when used to pinpoint a region that severe weather could occur on a given dayThey should never be used to make any other decisions without actually analyzing the environment

Jet StreamIncreases shearSeparates downdraft from updraftCreates longer-lived stormsLow-level Jet (LLJ)Brings in moisture and warmer temperaturesIncreases shear and helicityUsually kicks in at sunsetUpper-level Jet StreaksIncreased shear and upward motionUpward motions in right entrance and left exit

Frontal boundariesCold/warm/stationary fronts, drylinesCan enhance shear and helicitySource of liftSurface observationsGives an idea of whats going on at this momentLook for mesoscale featuresSatellite imageryLook for clearing early and robust CU fields closer to initiation

Water vapor imageryShows drying and moistening in the mid upper levelsSee where fronts/shortwaves are locatedMoistureStorms wont form without itTd > 60F best700mb tempsGood indicator of cap strength< 12C in the spring w/o a strong lifting mechanismCan be slightly higher during the summer or with strong lift

Theta-EMeasure of heat and moisture in the atmosphereLook for theta-e advection/ridgesA sign of increased instability and convergenceCould be an area of convective developmentStorm MotionCalculated average direction and speed of stomrsGives an idea of how storms will move

SPC MesoanalysisTwisterdataCOD AnalysisAMS Model AnimatorWPCRAP Weather

How to chase safely and effectively

South/Southeast of the hook echo/storm is the safest place to beWatch for right turning!!Avoid core punchingUnless you have a death wish for all of your car windowsAvoid rain-wrapped messesPull off on side roads, not main roads.

To see a tornado on your first time outOr secondOr thirdGood chasers usually tend to see a tornado about once every seven chasesDont let this deter you from chasing, you cant wind the game if you dont play!

LONG car ridesSitting, waiting, wishing..Footballs and Frisbees are your friends!Lots of gas station/fast food mealsSketchy dirt roadsFew bathroom breaksAwesome tunesThe wonder of nature, baby!THE TIME OF YOUR LIFE! AND MEMORIES!

Wear REAL shoes or bootsStay away from sandals, especially flip flopsWatch the skies, not just the computerOnly your eyes can tell you whats actually happeningTry to learn something, not just watch

So were not held liable if you act stupid. Just kidding. But seriously.

CHASE WITH SOMEONE WHO HAS CHASED BEFOREChase with a partnerStay in the car if theres lightning nearbyHave an escape routeTry to head south if you feel that youre in dangerIf thats not possible, head east until you can go south

Dont tempt fate just to get that perfect shotSafety should ALWAYS be your number one concernGive strong/violent tornadoes (especially wedges) their spaceKeep an eye on the radar AND the tornadoWatch out for power poles!

Freshmen should NOT go chasing with other freshmen aloneDont get out of the car if theres hailDont chase at nightWhen stopping, pull ALL the way off the roadDont pull over on highwaysUse your flashersWatch out for those driversChaser convergence

Avoid minimum maintenance roadsPrevious rainfall could have washed them outNot all of these are marked.

Dont drive on flooded roadsTurn around, dont drownDont drive over power lines/debrisAvoid entering debris paths unless absolutely necessary, especially if there are people in your car that dont have long pants/closed shoesTry to avoid driving through squalls, especially at night

Watch out for animalsThey get spooked and like to run out in front of/into vehiclesWatch out for copsThey get spooked and like to run out in front of/into vehicles

jorlando@iastate.eduksmalley@iastate.edusstelten@iastate.edu