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MET4300
Lecture 19Blizzards(CH15)
• Winds>30kt (35mph)• Visibility<¼miinblowingorfallingsnow• Last>3h• Cancausehypothermia andfrostbite;economiclosscanreachmillions• MainlyhappenintheGreatPlainsofthenorthernUSandsouthernCanada
duetotheuniquegeography:N-Smountainmassifisolatesthelowertroposhere ontheGreatPlainsfromthewarmairoverthePacific.
• ColdairmassoverCanadianArcticduetolongArcticnights;Leecyclonesdriveblizzards;MoistureisprovidedbytheGulfofMexico.
Blizzards
High probability (>50%) over North & SouthDakota, Minnesota, Nebraska, northern Iowa, and eastern Colorado
Average about 1-2 blizzardsPer year over ND, SD, and MN
ThreeIngredientsnecessaryforablizzardtoform
• Extremelycoldair:originatesontheplainsofwest-centralCanadainwinter,snow-coveredsurface,infraredradiativecooling,solarrefelction,longwinternights,underclearskiesà stronghighpressuresystem(increasethepressuregradient)
• BlizzardsoccurwithinLeecyclones,whichprovidewindandsnow.TheworstblizzardstypicallyoccurwithColoradocyclones,butblizzardsassociatedwithAlbertaClippersnormallyhavethecoldesttemperatures.
Where Do US Frontal Cyclones Form?• Lee of the Rocky
Mountains – Alberta Clippers– Colorado Cyclones
• Gulf Coast – TX-LA boarder– Off Georgia-Carolinas
Coast• East Cost• Bering Sea & Gulf of
Alaska in the Pacific Ocean (Pacific Cyclones)
ThreeTypesofBlizzardintheUS
• ColoradoCycloneBlizzard• AlbertaClipperBlizzard• GroundBlizzard
ColoradoCyclonesandBlizzards:DevelopmentStage
• AwaveinthepolarjetstreammovesacrosstheColorado/NewMexicoRockies
• Curvatureandjetstreakeffectstriggertheformationofsurfacelowpressure.
Upper level (300mb)
• Atsurface,forblizzardconditionstodevelop,bittercoldairandassociatedhighpressuremustalreadybeinplaceovertheCanadianPlainsà behindthearcticfront
• AsurfacecyclonedevelopseasternColorado• MidwintercooltemperaturesarealreadyintheUS,dueto
previouscoldairintrusion(southernmostcoldfrontbelow).
Surface
ColoradoCyclonesandBlizzards:DevelopmentStage
• Thearcticairmass movessouthwardasthecyclonemoveseastward.
• Thearcticfrontapproachesthecyclonecenter,wrapsaroundthewestsideofthecyclone.
• Asthecycloneprogressnortheastwardandintensifies,thestrongpressuregradientdrivesthebitterlycoldairsouthwardwestofthecyclonecenter,creatingblizzardconditions
MaturePhase
Warm air east of the cyclone center wraps aloft around the NW side, producing a band of clouds and precipitation à Trowal (a trough of warm air aloft)
TROWAL provides moisture to feed the snowfall
1997 blizzard buried ND: the clouds deform and narrow in 12-hs.
1997NorthDakotaBlizzard
A snow swath in ~500km wide.
Snow falls from very cold arctic air, so it remains fluffy and dry, creating whiteouts and huge snow drifts at surface, even after the trowal airstream passes to the east.
Drifts can be as high as 10 to 20 feet.
2002Blizzard
• LowterraininMississippiRiverValley(500feetabovesealevel),highterrainineasternColorado(5000feetabovesealevel)
• TherisebetweentheMississippiandRockiescausetheair(flowingwestwardnorthofacyclone)climbingup
• Theliftingandadiabaticcoolingcausescloudformationandenhancesnowfallrates
TopographyEffect
InClassActivity
• ReadCH15&CH16
ForNextTime
• Ex.14.1&14.2
• AlbertaclipperstypicallydevelopaftercoldairisentrenchedcentralUSandJetstreamoverCanadaisorientedfromNWtoSE
• Acorrespondingwave(trough)triggersthedevelopmentoftheClippercyclone
• ThewaveisfarthernorththaninthecaseofColoradocyclone
AlbertaClippersandBlizzards
300 MB
• PreviouscoolairoverUSbroughtbyapreviouscyclone
• AnewClipperformswestofthecoldarcticairmass
AlbertaClippersandBlizzards
Surface
• TheClipperformsandtraversesSE-wardintotheGreatLakeregion
• Anewcoldarcticfrontformsandmovessouthwardtothewestofthecyclonecenter
• Temperaturecouldbeaslowas-34~-40Cbehindthiscoldfront.
• ProducelesssnowthanColoradocyclones(2-5inches)
• Drifting,whiteouts,bittercoldandstrongwindcanbeadisastrous(windchilltemperature-50to-60F)
AlbertaClippersandBlizzards
Mature Phase (Surface)
Winterof1996-1997:9BlizzardsintheDakotasandMontana:--1st twoeverpresidentialdisasterdeclarationsinND.-- producedbybothAlbertaCilppersandColoradocyclones.-- FargoNDreceived117inchesofsnowintotal.-- Averageoneblizzardsperweekbetween12/15/96-01/24/97
FinalblizzardintheWinterof1996-1997intheDakotasandMontana:ExtremelystrongpressuregradientoverND.Winds>58mph;16-24inchesofsnow;$21.5millionindamagetofarmstructuresandmilkwasting;RedRiverfloodinginundatingthecitiesofGrandForksandFargo
GroundBlizzard:blizzardscausedsolelybyblowingsnow
GroundBlizzards• Theskyisclear,butextremelycoldwindsblowacrosssnow-coveredground
• FrequentontheGreatPlainsinthewakeofsnowstorms
• Mainlyinsmallregionspronetodriftingandhighwinds
• Shallow(2-10m)layerofblowing---previouslyfallen---snowwhenitisverycold.
• Badroadconditionsincludingblackice• Verydangerousbecausetheyareunexpected,occurringafterthestormhaspassed.
SnowRollers:aninterestingphenomenablizzardsleavebehind
Afive-poundsnowroller
BlizzardSafety• Badroadconditioncausedisasters
• Carstrandedonroadside• Peopledieofhypothermiawhileseekinghelporcarbonmonoxidepoisoningwhilerunningthecar’sengineforwarmth
• Whensnowisblowing,youcanfeelalotcolderthanthewindchilltemperature(whichdoesn’ttakeintoaccountheatlossthroughmeltingandevaporation)
Stayinyourcar
Don’tdiefromhypothermiaorCOpoisoning
Asinglecandlecankeepyouwarm
Travelersshouldbring:Survivalkit: Sleepingbag; flashlight&batteries,afirstaidkit,aknife,Food&Water;extradryclothing,Pailforwaste;asmall-canformeltingsnowforadditionaldrinkingwater; Cellphone;Candles;water-proofmatchesWinterAutomobileSafetyitems:asackofsandorcarlitter,ashovel,awindshield-scraper,andbrush,atoolkit,atowrope,boostercables,andaccidentflares
BlizzardSafety
Summary• Blizzard:Galeforcewinds(>35kt)andvisibilityinblowingsnowlessthan¼mi(500m)andlast>3h
• Common(>1peryear)inGreatPlainsofUS&Canada,Siberia…
• Arcticfrontpushessouthbehindapolarfrontthatisalreadyfarsouth.– Leecyclogenesis(AlbertaClipper)andarctichighinsouthernCanada
– TROWALprovidesmoistureforsnow• Survival:Beabletowaititout