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