OntheLackofSevereWeatherinTexasandOklahomaon18April2000J.H.FillinghamAOS453,MesoscaleMeteorologyG.TripoliandH.HassenzahlMay13,2007ABSTRACT TherewasdramaticpotentialforconvectiveactivityassociatedwithadrylinethroughTexasandOkalahomaon18and19April,2000.Thiscasestudyanalyzestheatmosphericconditionswhichsmotheredtheconvectivepotentialforcinganullevent.ThroughsubsidenceatmidlevelsontheeasternsideofadrylinethroughwesternTexasandOklahoma,thelowlevelinversion,separatingmoistsurfaceairandelevatedmixedlayer,wasnotbrokenbutreinforced.Thesubsidenceisattributedtoconvergenceassociatedwithalowleveljet,aswellaslowlevelfrontolosisassociatedwithwarmingthroughoutthedayonthe18th.Itwasfoundthattheconvergencewasagreaterforcingforsubsidencethanthefrontolisis.________1.Introduction Withdominantwesterlyflowoverthedesertsouthwest,incombinationwiththeluckyplacementoftheRockyMountainsandtheGulfofMexico,itisextremelycommontoseedramaticconvectionandthunderstormsovertheGreatPlainsintheearlyspring.TheGreatPlainsisknownastornadoalleybecauselowlevelmoistairisadvectednorthwardoutoftheGulfofMexicoandupagainsttheMountains.Withdesiccated,conditionallyunstableairbeingblownoffthemountaintopsaloadedgunsituationisdevelopedasthiswarm,dry,unstableairisforcedoverthemoistsurfacelayer. AlthoughseverweatheriscommonoverthiscorneroftheEarth,itisdoesnotalwaysformwhenconditionsareright.Thefinalpieceofthepuzzleisdynamicliftingovertheloadedgunregionorsurfaceheatingwhichforcesthemoistsurfaceairthroughacappinginversionandintothedrymixedlayerallowingforfreeconvection.Ifthereisnoliftingorheating,therearenostorms,andthereisdefinitelynosevereweather.Althoughitisakeyingredient,verticalmotioncanalsobethehardesttoforecastintheSouthwestandGreatPlains.WiththeRockyMountainsdisturbingthewesterlyflowoffthedesert,itiscommontoseeverticallypropagatinggravitywaves,althoughnotverystrongastheairisusuallyunstableduringaconvectiveevent.Evenso,thedisturbedflowcanaddortakeawayfromverticalforcingovertheregion.Verticalflowisoftenforcedbysynopticscalemeteorology.UpperleveltroughsdeepeningoverthewesternUSwillgeneratesurfacecyclonesintheleeoftheRockyMountains.Thissynopticsituationleadstoupperlevelwesterlywindsoverthemountains,andlowlevelsoutherlyflowofftheGulf,eastofthemountainsassociatedwiththesurfacecyclone.Whatiscommoninthesesituations,isverticalmotionsforcedbythedynamicsoftheupperlevelwave.Theseverticalmotionscontributegreatlytobreakingalowlevelinversioncapleadingtothefreeconvectiondescribedprevious.OnApril18,2000adramaticloadedgunset-upwasevidentoverthewesternGreatPlains:Nebraska,Kansas,Oklahoma,andTexas.OneexampleoftheconvectivepotentialisNormanOklahomaat0000zonthe19th,whereCAPEexceeded3000whileatthesametimeacrossthePanhandleofTexasintoOklahomatherewasatendegreedropintemperaturewitha30degreeincreaseindewpointtemperature.Thistypeofsurfaceandupperairset-upthreatenedtodevelopdramaticsevereweather,however,nothinghappened.Throughthedayonthe18thandintothe19th,therewasnosevereweather.Notonlywastherenosevereweather,buttherewashardlyanysensibleweatheratall.After0000zonthe19th,therewereafewsmallscalethunderstormsinwestcentralTexas,andsomelargerstormsinsouthwestNebraska,buttheentireregioncharacterizedbytheloadedgunverticalstructureexperiencednothingbutclearskies. Thiscasestudyintendstoshowthatthereasonforthelackofanyrealconvectiveactivitywaswidespreadsubsidenceoveralmosttheentireregionofmoistairatthesurface.Thissubsidenceisattributedtotwoatmosphericcharacteristics.Thefirstisamidleveljetstreakwhichpushedintotheregionsettingupalargeregionofconvergenceassociatedwithitsleftexitregion.Thesecondforcingwaslowlevelfrontolosisassociatedwithdaytimewarmingandlowlevelwinds.