NIDISWeeklyClimate,WaterandDroughtAssessmentSummary

UpperColoradoRiverBasinJuly12,2011

PrecipitaEonandSnowpack

Forthewateryearto‐date,mostoftheUpperColoradoRiverBasin(UCRB)hasseenaboveaverageprecipitaEon(Fig.1).SomeareasoftheUpperandLowerGreenRiverbasinsinnortheastUtahandsouthwestWyominghavereceivedinexcessof300%oftheiraveragewater‐yearprecipitaEon.ThevalleyareasandtheFourCornersregionhavebeenthedriestpartsoftheUCRB,receivingbetween50%and90%ofaverageprecipitaEon.EastoftheUCRB,northeastColoradohasseenneartoaboveaverageprecipitaEon,whilesoutheastCOandtheSanLuisValleyhavebeenverydry,receivinglessthan50%oftheiraverages.

ForthebeginningofJuly,precipitaEonwaswidespreadacrossthenorthernporEonoftheUCRBandtheeasternplains(Fig.2).AreasofnortheastUTandnortheastCOhavereceivedover2inchesofmoisturemonth‐to‐date.SoutheastCOhasreceivedbetweenaquarterinchtoaninchofmoisture,helpingtopreventfurtherdeterioraEngdroughtcondiEons.TheFourCornersregionandtheSanLuisValleyhaveremainedrelaEvelydry,withmanyareasreceivinglessthanatenthofaninchofmoisture.

Fig.1:October–JuneprecipitaEonasapercentofaverage. Fig.2:Julymonth‐to‐dateprecipitaEonininches.

StreamflowandWaterSupplyAsofJuly11th,about98%oftheUSGSstreamgagesintheUCRBrecordednormal(25th–75thpercenEle)orabovenormal7‐dayaveragestreamflowswith64%ofthegagesrecordingflowsabovethe75thpercenEleand30%reporEnghighflows(Fig.3).KeygagesontheColoradoRiverneartheCO‐UTstatelineandtheGreenRiveratGreenRiver,UThaveabovenormal7‐dayaveragestreamflowatthe93rdand97thpercenEles,respecEvely(Fig.4).StreamflowontheSanJuanRivernearBluff,UTisatthe44thpercenEle.

MostofthemajorreservoirsintheUCRBhaveconEnuedtoseelargestoragevolumeincreasessincethebeginningofJuly.StoragevolumesatMcPheeandNavajoreservoirshavebegundecreasing.AllofthemajorreservoirsaboveLakePowellarecurrentlyabovetheiraverageJulylevels.LakePowell’sstoragehasincreased3.7%month‐to‐dateandiscurrentlyat86%ofaverage.

Fig.3:7‐dayaveragedischargecomparedtohistoricaldischargeforJuly11th.

Fig.4:USGS7‐dayaveragedischargeoverEmeattheCO‐UTstateline(top),GreenRiver,UT(middle)andBluff,UT(bofom).

WaterDemandThismonthsofar,aboveaveragetemperatureshavebeenseenacrossmostoftheUCRBandeasternplainsofCO.Muchwarmertemperatures(4°to8°aboveaverage)wereobservedovertheFourCorners,theSanLuisValleyandsoutheastCO.ThewarmertemperatureshavecontributedtohigherpotenEalevapotranspiraEon(PET)indroughtstrickenareas.IntheFourCorners,PETiscurrentlyjustaboveaverage,ontrackwiththedrieryears.IntheSanLuisValleyandintheArkansasbasin,PETiscurrentlytrackingabovethehighestETyear,duringthedroughtof2002(Fig.5).ThismeansthatanyprecipitaEonthatdoesfallintheareawillquicklybelostagaintotheatmosphereandwillbeofliflebenefitinalleviaEngcurrentdroughtimpacts.

SoilmoisturecondiEonsremainpoorfortheSanLuisValley.SoilmoistureisaboveaveragealongtheWasatchrangeinUT,inthenorthernCOmountains,andinnortheastCO.SatelliteimageryofvegetaEoncondiEonsshowverydryvegetaEonwithliflegrowtharoundtheFourCorners,theSanLuisValley,andsoutheastCO(Fig.6).VegetaEonscondiEonsaremoistforthenorthernporEonoftheUCRBandarenearaveragefornortheastCO.

PrecipitaEonForecastAdeepmonsoonalmoistureplumewillbegintoshiieastwardtodaywithsignificantlydrierairmovinginfromCalifornia.AsaresultareasinthesouthwesternpartoftheUCRBwillbegintoseelessthunderstormcoveragebeginningtoday,withdrycondiEonsspreadingnorthandeastwardthroughtheendoftheweek.Stormsthatdodevelopshouldalsobemovingabitfasterthanseeninpreviousdays,buttheriskofflashfloodsandmudslideswillremainhighduetothealreadysaturatedsoils.QuanEtaEvePrecipitaEonForecastshows0.5inchesofprecipitaEonacrosstheenErebasinthroughtheendoftheweek,withthewesternslopeoftheColoradomountainspickingupoveraninch.ByFridaymostoftheUCRBwillexperiencelifleifanyconvecEonwithsomebreezycondiEonsdevelopingoverthehigherterrainandtemperaturesreturningtonearorslightlyabovenormal.GenerallydrycondiEonswillpersistoverthebasinthroughthisweekendbeforethenextsurgeofmonsoonalmoisturemovesintotheareabyearlynextweek.Expectscaferedthundershowerstoreturn,aswellasslightlycooleraiernoonhightemperatures.

Fig.5:ReferenceevapotranspiraEonsinceApril1statCenter,COintheSanLuisValley.

Fig.6:July10thVegDRImap,basedonsatellite‐derivedobservaEonsofvegetaEon.

DroughtandWaterDiscussion

StatusquoisrecommendedintheUCRBforthecurrentU.S.DroughtMonitor(USDM)map(Fig.7).WithtwoweeksofconEnuousaiernoonshowersacrossthenorthernFrontRange,condiEonshavegreatlyimproved.ItisrecommendedthatD0beretractedsouthward(removedabovetheblackline,Fig.7).

LocalexpertshaveadvisedusthatsouthwestCheyenneCountyisworsethanwhatiscurrentlydepicted.Inthisdatasparseregion,wearedeferringtotheirobservaEonsandrecommendingthatthegradientbeEghtenedinthesouthwestporEonofthecountywithanexpansionoftheD2line.

Finally,anexpansionoftheD4intheSanLuisValleyisrecommended—aslightwestwardexpansionintoRioGrandeCountyandsouthintoCosEllaandConejoscounEestocovermoreofthevalleyfloorwouldbemorerepresentaEveofcondiEonsthere.

DroughtcategoriesandtheirassociatedpercenEles

Fig.7:July5threleaseofU.S.DroughtMonitorfortheUCRB