Tornado Alley When Do Tornadoes Occur?

TornadoesTornadoes Photo: D. Zaras, NOAA Dr. Christopher M. GodfreyUniversity of North Carolina at AshevilleA tornado is a violently rotating column of air incontact with the groundTornadogenesis is the formation of a tornadoA visible condensation funnel is NOT necessary tohave a tornadoHowever, just a funnel without a circulation incontact with the ground is NOT a tornadoTornadoes may have wind speeds between 40 and300 m.p.h!On a local scale, the tornado is the most intense ofall atmospheric circulationsATMS 103ATMS 103Tornado AlleyWhen do tornadoes occur? ornadoes severe storms 75% of all tornadoes occur in the U.S.Most tornadoes occur in tornado alley (central Texasto Nebraska)Note: This is the totalnumber of tornadoes ina multi-year study. Do Iknow which years? No,but you get the idea.ATMS 103ATMS 103Tornado Frequency by Time of Day Most tornadoes occur from late afternoon throughearly evening (1 8 p.m.)ATMS 103Mainly during spring (April, May, June) in the U.S.Tornadoes can occur year-round!The Fujita Scale (Used prior to 2007) Relates tornado intensity indirectly to structuraland/or vegetative damageATMS 1031

Tornado DamageThe Enhanced Fujita ScaleEF-Scale in use by the NWS startingFebruary 1, 2007More complex than F-Scale Photo: Jim LaDueATMS 103ATMS 103Why the NWS created the EF-Scale Need more damage indicatorsTo recalibrate windsassociated with F-scale ratingsTo better correlate wind andratingTo account for constructionvariabilityFlexibility, extensibility,expandabilityThe framed house is one of onlya few F-scale damage indicators.EF-Scale Damage Indicators (DIs) 28 DIs identified initiallyEach DI has several Degrees of Damage(DOD)DIs and DODs can be added or modifiedFramed houseSingle wide mobile homeSmall Retail BuildingEvidence indicates that a wellconstructed house can be blownaway by winds much less than260 m.p.h. (F5 threshold).ATMS 103ATMS 10328 Damage IndicatorsDegrees of DamageSome consecutive DODs have larger overlap than othersDODDamage DescriptionEXPLBUB1Threshold of visible damage6353802Loss of roof covering material ( 20%), gutters and/or awning; loss of vinyl or metalsiding7963973Broken glass in doors and windows96791144Uplift of roof deck and loss of significant roof covering material ( 20%); collapse ofchimney; garage doors collapse inward or outward; failure of porch or carport97811165Entire house shifts off foundation121103141Schools6Large sections of roof structure removed; most walls remain standing122104142Professionalbuildings7exterior walls tures8Most walls collapsed except small interior rooms.Metalbuildings/canopies1521271789All walls collapsed170142198Towers/poles10Destruction of engineered and/or well constructed residence; slab swept clean162220VegetationATMS 103200Example DODs for a Framed House DI (FR12 or DI2)Expected, Lower Bound, and Upper Bound of windspeed (in m.p.h.) for each Degree of DamageATMS 1032

F-Scale Converted to EF-ScaleF ScaleWind SpeedEF-ScaleDOD to Wind Speed to EF-Scale questionWind 317EF5200-234Wind speeds in mph, 3-second gustDODDamage Description – Framed HouseEXPLBUB1Threshold of visible damage6353802Loss of roof covering material ( 20%), gutters and/or awning; loss ofvinyl or metal siding7963973Broken glass in doors and windows96791144Uplift of roof deck and loss of significant roof covering material ( 20%);collapse of chimney; garage doors collapse inward or outward; failure ofporch or carport97811165Entire house shifts off foundation1211031416Large sections of roof structure removed; most walls remain standing1221041427exterior walls collapsed1321131538Most walls collapsed except small interior rooms.1521271789All walls collapsed17014219810Destruction of engineered and/or well constructed residence; slabswept clean200162220Expected wind:97 mphWhat DOD best corresponds with this damage?ATMS 103ATMS 103EF-Scale answerTornado FormationEF-Scale CategoriesWind Speed 200EF5 200Tilting Vertical wind shear gets tilted horizontally to form the rotatingmesocycloneWind Speed in mph, 3-Second gustATMS 103ATMS 103Tornado FormationTornado FormationVortex stretching RFD cuts off warm airsource from low-levelupdraft circulation rises more slowly Upper-level updraftstretches the entirecirculation Tornado forms byconservation ofangular momentumVortex stretchingLet’s use some math to see how this works ATMS 103ATMS 1033

Tornado FormationTornado FormationDynamic pipe effect Constricted mid-level flow in stretched portion of vortex forceslower levels to constrict as well Process proceeds toward the groundBottom-up process Descending air behind gust front and ascending air ahead ofgust front creates rotation As gust front moves under updraft, low-level mesocyclonerotation stretches beneath mid-level mesocyclone to form atornadoATMS 103Tornado FormationATMS 103Multi-Vortex Tornado with Three Suction VorticesVortex breakdown Downdraft descends incentral core of existingtornado Strong shear betweenupdraft and centraldowndraft leads to smallersuction vortices Worst damage occurs fromsuction vortices The same process occursin a multiple vortex tornadoATMS 103Examples of ground marks left behind by suctionvortices within tornadoesATMS 103Multiple Vortex TornadoesDamage from Suction VorticesSource: Lewellen and Zimmerman, 2009: Bulletin of the American Meteorological SocietyATMS 103ATMS 1034

Tornado Wind Speeds Tornado Wind SpeedsWind speeds depend on bothrotational and translationalvelocitiesIf a tornado travels east, thenthe strongest winds are on thesouthern edge of the tornadoSuction vortices further increaseor decrease resulting windspeedATMS 103Features of a Tornadic Supercell Rotating updraftHook echo (most of the time )Rear flank downdraft (RFD)MesocycloneTornado A tornado must exist at some point during the lifeof the supercell for it to be a ‘tornadic’ supercellATMS 103Tornado Location in a SupercellATMS 103ATMS 103Rotating Updraft In tornadic supercells, the storms usually spinin only one direction (CCW)Because of the environmental shear thatsupercells form in, the updraft is enhanced onthe southern flank of the stormThe environment favors the CCW rotationtypically found on the southern flankRotation on the northern flank of the storm isusually weakATMS 1033 May 1999 – Radar ImageATMS 1035

Radar Reflectivity – Vertical Scan of aTornadic SupercellTornadic SupercellTornadoATMS 103Rapid-Scan DOW Observations:ATMS 103Tornado in Medford, OK: 1 May 2012Goshen County, WY, 5 June 2009Source: f2011-RapidScan-FINAL-B.pdfATMS 103ATMS 103Forecasting Tornadoes Tornado passage nearMcLean, TexasMesonet site on 28March 2007 (yes,2007, despite the datein the image.)Impossible to predict exact location of a tornado (so far )No operational forecast models resolve tornadoesExamine specific indices from forecast models to determinelocations favorable for supercell developmentSPC does this every day 5 times a day for today 2 times a day for tomorrow 1 time a day for two days from nowPeak Gust: 127 m.p.h.Pressure Drop: 9 mbATMS 103ATMS 1036

Other Small-Scale Vortices WaterspoutLandspout – a non-supercell tornado thatforms without a preexisting midlevelmesocyclone; source of circulation is nearthe groundGustnado – circulation spins up on leadingedge of gust frontMature or Spray-Vortex Stage Dark-spot StagePhoto: C. GodfreyAny tornado over waterUsually a non-supercell tornado over water(develops over open water in fair weather)Life cycle: dark-spot stage spiral pattern stage spray-ring stage mature or spray-vortex stage decay stageATMS 103ATMS 103Other Small-Scale Vortices Tornado MythsDust devil – A well-developed dust whirl,usually of short duration rendered visible bydust, sand, and debris Can cause damage up to F1 on Fujita scaleBest developed on a hot, calm afternoon withclear skies, in a dry region where intense surfaceheating causes a very steep lapse rateFAKEPICTURE!Source: 05 dust devil.htmlATMS 103Southwest Corner of Basement This myth dates back to 1887 in a book ontornadoes by John Park Finley.It reigned as popular wisdom for 80 yearsIn 1966, a University of Kansas professorstudied this question exactly – is thesouthwest corner safer?The answer was an emphatic NO!ATMS 103ATMS 103Open Windows to Equalize Air Pressure It’s a waste of time and puts you in the way offlying glass and debrisIt could actually help the wind to remove yourroof and will allow debris into the houseInside/outside pressure differences would beequalized by fresh gaping holes inwindows/doors/walls well before an explosivepressure drop could approach the houseATMS 1037

Open Windows to Equalize Air PressureRivers Protect Cities Dates back to Native American tribal legendsResidents thought that Emporia, KS was “protected”by the Cottonwood and Neosho rivers. In 1974, atornado killed six people and damaged 20 millionworth of property. Another tornado struck Emporia in1991.Tornadoes are so rare that one or two generationscould pass without a tornado hitting a particular areaATMS 103Hills Protect Cities Similar to the river-protection myth Topeka was thought to be safe because ofBurnett’s Mound until a tornado sweptthrough town.Again, tornadoes are rare and small towns inthe plains are mere needles in a haystack.ATMS 103Tornadoes Never Strike Big CitiesMiami, FLEnough said Atlanta, GAFort Worth, TXATMS 103ATMS 103Take Shelter Under an Overpass Modern day mythDates back to 1991 and the Andover, KStornadoFilm crew for TV station sought protectionduring a tornado from an overpass and thefilm was distributed widelyThe tornado was weak and missed them!Winds move faster under an overpassATMS 103ATMS 1038

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Tornado Wind Speeds ATMS 103 Tornado Wind Speeds ATMS 103 Rotating updraft Hook echo (most of the time ) Rear flank downdraft (RFD) Mesocyclone Tornado A tornado must exist at some point during the life of the supercell for it to be a ‘tornadic’ supercell F