Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 021636
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
936 AM MST MON MAR 2 2015

.UPDATE...
ISSUED AT 929 AM MST MON MAR 2 2015

STARTED THE WINTER STORM WARNING FOR ALL OF CARBON COUNTY
EFFECTIVE IMMEDIATELY. SNOW IS ALREADY ACCUMULATING ON THE ROAD
SURFACES ACROSS MUCH OF THE COUNTY WITH 1 TO 2 INCHES OF SNOW ALREADY
ON THE GROUND IN RAWLINS. LATEST GFS SHOWING HIGH SNOW FALL RATES
AND BASED ON MOSAIC RADAR OVER WESTERN COLORADO...THERE IS NOT
MUCH DOUBT SNOW WILL CONTINUE TO INTENSIFY THIS MORNING WITH SNOW
HEAVY AT TIMES THROUGH THIS AFTERNOON. THE REST OF THE WINTER
STORM PRODUCTS LOOK FINE. SOME LIGHT SNOW ACCUMULATIONS WILL
OCCUR AROUND LARAMIE BUT NOT MAIN PRECIPITATION AREA WILL SHIFT
SLIGHTLY WEST THIS AFTERNOON BEFORE SPREADING EAST TONIGHT WITH
THE MAIN UPPER LEVEL FEATURE.

&&

.SHORT TERM...(TODAY THROUGH WEDNESDAY)
ISSUED AT 430 AM MST MON MAR 2 2015

A BUSY FORECAST PERIOD CONTINUES WITH A MAJOR WINTER STORM STILL
ON TRACK FOR MUCH OF THE CWA. MAIN CHALLENGES REVOLVE AROUND THE
TIMING/PLACEMENT OF HEAVIEST SNOWFALL AND WHAT IMPACTS THIS WILL
HAVE TO TRAVEL. DANGEROUS WIND CHILLS WILL UNDOUBTEDLY BECOME AN
ISSUE BY TUE NIGHT AND WED...AND TEMPERATURES WILL HINGE ON CLOUD
COVER AND SNOW DEPTHS HEADING INTO MIDWEEK. MORNING WV SAT IMAGERY
SHOWS DEEP TROUGHING OVER THE WESTERN CONUS. A SEPARATE AND MORE
VIGOROUS PIECE OF ENERGY WAS LOCATED OVER SOUTHERN CAL WITH THE
MUCH ANTICIPATED KICKER SYSTEM DIVING SOUTH ALONG THE WESTERN US
COASTLINE. EXCELLENT MOISTURE FETCH WAS OBSERVED DOWNSTREAM OVER
THE FOUR CORNERS...AS WELL AS GOOD INSTABILITY WITH A FEW RECENT
LIGHTNING STRIKES OVER CA/NV BENEATH THE UPPER LOW. THAT SAID...
CONFIDENCE IS HIGH FOR A HEAVY SNOWFALL EVENT IN OUR CWA.

WE DECIDED TO UPGRADE THE WINTER STORM WATCH TO A WARNING FOR THE
REST OF CARBON COUNTY...KEEPING START/END TIMES THE SAME. WE HAVE
SEEN SOLID RUN-TO-RUN AND MODEL-TO-MODEL CONSISTENCY AMONG BOTH
AMERICAN AND EUROPEAN GUIDANCE OVER THE LAST DAY OR SO. CONSENSUS
IS FOR THE SOUTHERN WAVE TO LIFT NORTHEAST INTO WESTERN CO BY THIS
AFTN WITH AN IMPRESSIVE 160+ KT H25 JET NOSING INTO SOUTHEAST WY
BY 00Z TUE. THIS PLACES OUR WESTERN ZONES IN THE FAVORED LEFT EXIT
REGION FOR A FEW HOURS THIS AFTERNOON...SO A PERIOD OF HEAVY SNOW
BANDING IS LIKELY WITH A SURFACE WARM FRONTAL BOUNDARY ORIENTED
FROM SW TO NE ACROSS THE CWA. NAM AND GFS MUCAPES BETWEEN 100 AND
200 J/KG SUGGESTS SOME CONVECTIVE ELEMENTS...SO RATES COULD EXCEED
AN INCH PER HOUR AT TIMES. SNOWIES AND SIERRA MADRES ARE STILL IN
GOOD SHAPE FOR AROUND 2 FEET OF TOTAL ACCUMULATIONS.

THE MORE WIDESPREAD EVENT HOLDS OFF UNTIL LATER THIS EVENING AND
OVERNIGHT WITH THE COLD FROPA. GFS/NAM/ECMWF ARE ALL IN EXCELLENT
AGREEMENT WITH THE ARCTIC FRONT SURGING SOUTH BETWEEN 06Z AND 12Z
TUE. SIGNIFICANT LLVL COLD ADVECTION SHOULD OCCUR WITH GFS 700 MB
TEMPS OVER CYS FALLING FROM +1C AT 00Z TO -13C AT 12Z TUE. FRONTAL
PASSAGE WILL RESULT IN ENHANCED LLVL CONVERGENCE AND FRONTOGENESIS
ALONG THE LARAMIE RANGE...AND UPSLOPE FLOW MOST PRONOUNCED OVER
THE NORTHERN MOUNTAINS. LATEST RUNS HAVE BEEN HITTING THE NORTH
LARAMIE RANGE HARD WITH AN AVERAGE OF AROUND 0.7 INCH QPF...WHICH
IS NOT SURPRISING WITH STEEPENING LAPSE RATES ALOFT. BEST SUPPORT
WILL BE FAST MOVING...BUT RATES SHOULD BE HIGH ENOUGH TO GET CLOSE
TO A FOOT OF SNOW OVER NORTH FACING SLOPES. WINDS SHOULD COME INTO
PLAY AS WELL AS COLD ADVECTION YIELDS STRONG MIXING OVERNIGHT WITH
30 TO 40 KTS AVAILABLE TO MIX. WENT AHEAD AND ADDED THE NORTHERN
LARAMIE RANGE AND LOWER ELEVATIONS OF CONVERSE TO THE WARNING EVEN
THOUGH AMOUNTS COULD BE MARGINAL.

A BAND OF SNOW SHOULD EXTEND FARTHER EAST ONTO THE PLAINS TOWARD
THE WESTERN NEBRASKA PANHANDLE. MOST LIKELY AREA FOR THIS TO OCCUR
IS ACROSS THE NORTHERN ZONES WITH THE INITIAL FRONTAL SURGE. BIG
QUESTIONS FOR THE INTERSTATE 80 CORRIDOR AS H7-H3 RH PROGS SHOW A
SUBSTANTIAL DRY SLOT SNEAKING INTO OUR EXTREME SOUTHEAST CWA. WE
BELIEVE LIGHT ACCUMULATIONS AND INCREASING WINDS TONIGHT WILL BE
ENOUGH TO CAUSE SIGNIFICANT TRAVEL DIFFICULTY...ESPECIALLY ALONG
INTERSTATE 80. DECIDED ON A WINTER WEATHER ADVISORY FROM THE SOUTH
LARAMIE RANGE NORTHEAST INTO THE NORTHERN PANHANDLE. SNOW SHOULD
SLOWLY END ON TUE NIGHT AND WED AS THE SYSTEM DEPARTS...BUT A FEW
SHOWERS WILL PERSIST AS A SECONDARY SHORTWAVE SWINGS AROUND THE
BACK SIDE OF THE LARGER SCALE TROUGH. DESPITE PLENTY OF CLOUDS
PREVENTING MAJOR RADIATIONAL COOLING...H7 TEMPS AS COLD AS -20 C
AT 12Z WED SUGGESTS SUB ZERO READINGS. EXPECT THAT SOME SORT OF
WIND CHILL HEADLINES WILL BE NECESSARY ON THE HEELS OF ONGOING
WINTER WEATHER PRODUCTS. KEPT HIGHS BELOW GUIDANCE FOR BOTH TUE
AND WED WITH WIDESPREAD SNOW AND COLD ADVECTION ALOFT.

.LONG TERM...(WEDNESDAY NIGHT THROUGH SUNDAY)
ISSUED AT 400 AM MST MON MAR 2 2015

00Z MODELS INDICATE NORTHWEST FLOW ALOFT BEHIND EXITING TROUGH
AND UPPER RIDGE EXPANDING EAST TOWARD THE GREAT BASIN. A WEAK
SHORTWAVE TROUGH MOVES SOUTHEAST ACROSS THE CWA THURSDAY MORNING
WITH LITTLE OR NO ASSOCIATED MOISTURE. 700MB WINDS INCREASE TO
50 KT ALONG THE I-80 CORRIDOR BETWEEN ARLINGTON AND CHEYENNE
WEDNESDAY NIGHT/THURSDAY MORNING. OVER THE NORTH SNOWY RANGE
FOOTHILLS/CENTRAL LARAMIE RANGE. THE CENTER OF UPPER RIDGE MOVES
INTO CA/NV FRIDAY WITH LIGHTER NORTHWEST FLOW. THERE ARE MODEL
DIFFERENCES THIS WEEKEND AS THE ECMWF SHOWS A POTENT SHORTWAVE
TROUGH TRACKING SOUTHEAST FROM ID/WESTERN MT ACROSS WY/CO
SATURDAY...PRODUCING LIGHT PRECIPITATION. THE GFS DOES NOT SHOW
THIS FEATURE BUT AMPLIFIES THE NORTHWEST FLOW ALOFT AS SHORTWAVE
ENERGY MOVES ACROSS THE NORTHERN U.S. FOR NOW...LEFT POPS BELOW
15 PERCENT BUT THEY MAY BE INCREASED LATER IF THE GFS TRENDS
TOWARD THE ECMWF. A COLD FRONT WILL MOVE THROUGH THE REGION
SATURDAY. IT DOES NOT LOOK AS COLD BEHIND THE FRONT COMPARED TO
EARLIER MODEL RUNS. HIGHS THURSDAY WILL BE ABOUT 20 DEGREES WARMER
THAN WEDNESDAY AS DOWNSLOPE WEST-NORTHWEST WINDS DEVELOP LEE OF
LARAMIE RANGE. TEMPERATURES FINALLY WARM TO NEAR SEASONAL NORMALS
FRIDAY WITH HIGHS IN THE 30S AND 40S. IT WILL BE A TAD COOLER FOR
THE PLAINS SATURDAY. UPPER RIDGE BUILDS CLOSER WITH DOWNSLOPE
NORTHWEST FLOW KEEPING TEMPERATURES SEASONABLE.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z TUESDAY MORNING)
ISSUED AT 300 AM MST MON MAR 2 2015

BRIEF MVFR CIGS/VSBYS ALONG THE KCYS-KSNY ROUTE EARLY THIS MORNING.
OTHERWISE...VFR WILL PREVAIL EAST OF THE LARAMIE RANGE TODAY.
DETERIORATING CONDITIONS WILL DEVELOP WEST OF THE LARAMIE RANGE
THIS MORNING WITH IFR CIGS/VSBYS IN SNOW. THE MOUNTAINS WILL BE
OBSCURED THROUGHOUT THE PERIOD. MVFR TO IFR CIGS/VSBYS THEN DEVELOP
EAST OF THE LARAMIE RANGE THIS AFTERNOON AND EVENING AS SNOW BECOMES
MORE WIDESPREAD. SOUTHERLY WINDS OF 10-15 KT WILL PREVAIL...WITH
PERHAPS SOME GUSTS ABOVE 20 KT POSSIBLE FOR THE PLAINS. WINDS WILL
SHIFT TO NORTHERLY AND BECOME RATHER GUSTY WITH THE PASSAGE OF A
STRONG COLD FRONT LATE TONIGHT.


&&

.FIRE WEATHER...
ISSUED AT 430 AM MST MON MAR 2 2015

NO FIRE WEATHER CONCERNS. SNOW IS EXPECTED TO DEVELOP TODAY AND
BECOME WIDESPREAD TONIGHT AND TUESDAY. SIGNIFICANT ACCUMULATIONS
ARE POSSIBLE...ESPECIALLY IN THE MOUNTAINS AND LOWER ELEVATION
AREAS WEST OF THE LARAMIE RANGE. THE SNOW WILL GRADUALLY DIMINISH
BY TUESDAY NIGHT. AN ARCTIC COLD FRONT WILL BRING BITTERLY COLD
TEMPERATURES AND DANGEROUS WIND CHILLS FROM TUESDAY NIGHT THROUGH
WEDNESDAY. MODERATING TEMPERATURES ARE EXPECTED LATE IN THE WEEK.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...WINTER WEATHER ADVISORY FROM 8 PM THIS EVENING TO 6 PM MST
     TUESDAY FOR WYZ102-106>108-115>117.

     WINTER STORM WARNING UNTIL 6 PM MST TUESDAY FOR WYZ104-105-
     109>114.

     WINTER STORM WARNING FROM 3 PM THIS AFTERNOON TO 6 PM MST
     TUESDAY FOR WYZ101-103.

NE...WINTER WEATHER ADVISORY FROM 8 PM THIS EVENING TO 6 PM MST
     TUESDAY FOR NEZ002-003-095-096.

&&

$$

UPDATE...SML
SHORT TERM...CLH
LONG TERM...MJ
AVIATION...MJ
FIRE WEATHER...CLH



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