Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 211739
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
1030 AM MST SUN DEC 21 2014

.SHORT TERM...(TODAY THROUGH TUESDAY)
ISSUED AT 420 AM MST SUN DEC 21 2014

QUITE A CHALLENGING FORECAST FOR THE NEXT 24 HOURS WITH SNOW...WIND
AND THE POTENTIAL FOR SIGNIFICANT IMPACTS TO PRE-HOLIDAY TRAVEL IN
AND AROUND THE CWA. LATEST WATER VAPOR LOOP SHOWS A FIREHOSE OF AN
UPPER JET CRASHING INTO THE PACIFIC NORTHWEST. AMPLE MID AND HIGH
LEVEL MOISTURE CONTINUES TO STREAM INTO THE MOUNTAINS OF SOUTHEAST
WYOMING VIA STRONG WESTERLY FLOW ALOFT...SETTING THE STAGE FOR AN
INCREASE IN MOUNTAIN SNOWS SHORTLY. RADAR HAS BEEN RELATIVELY QUIET
OVER THE PAST FEW HOURS...BUT SNOW SHOULD INCREASE IN BOTH COVERAGE
AND INTENSITY AFTER SUNRISE AS LARGE SCALE ASCENT RAPIDLY INCREASES
IN THE LEFT FRONT QUADRANT OF A 150 KNOT H25 JET. THIS WILL COMBINE
WITH EXCELLENT OROGRAPHICS ON THE NORTHERN EDGE OF A BELT OF MOIST
WEST-TO-NORTHWEST FLOW TO PRODUCE SEVERAL HOURS OF HEAVY SNOW. QPF
FROM VIRTUALLY EVERY MODEL SUGGESTS UPWARDS OF 1.5 INCHES OF LIQUID
FOR OUR HIGH PEAKS...WHICH TRANSLATES TO UPWARDS OF 2 TO 3 FEET IF
WE ASSUME A 20 TO 1 RATIO. THE WINTER STORM WARNING LOOKS GOOD FOR
THOSE AREAS. WILL CONTINUE WITH A 12Z TUE EXPIRATION AS THE ARRIVAL
OF MID LEVEL DRY AIR SHOULD CUT OFF PRECIPITATION FROM WEST TO EAST
ON TUE MORNING.

WINDS ARE THE OTHER MAJOR CONCERN IN THE NEAR TERM. WILL CONTINUE A
HIGH WIND WARNING FOR THE WIND PRONE LOCATIONS INCLUDING BRX AND THE
I80 SUMMIT BETWEEN CHEYENNE AND LARAMIE. BUFORD IN THE SOUTH LARAMIE
RANGE HAS ALREADY MANAGED TO ACHIEVE 60 MPH GUSTS...SO WE MIGHT NEED
TO BUMP UP THE START TIME BEFORE THE DAY CREW ARRIVES DEPENDING ON
TRENDS. EXPECT WINDS WILL CONTINUE TO INCREASE THROUGH THE DAY AS
THE NAM H85 CAG-CPR GRADIENT CLIMBS TO 80 METERS BY 18Z. PROGGED H7
FLOW IN EXCESS OF 70 KT OVER THE WIND CORRIDOR AND GOOD LOW LEVEL
MIXING ON AFTERNOON FORECAST SOUNDINGS SUGGEST AN EXCELLENT SETUP
FOR 65 TO 75 MPH GUSTS GIVEN GOOD MID LEVEL SUBSIDENCE SEEN IN THE
OMEGA FIELDS. UPGRADED NORTHERN CARBON COUNTY / FERRIS MOUNTAINS
AND SHIRLEY BASIN / TO A HIGH WIND WARNING AS GFS H75 FLOW APPEARS
TO BE WELL IN EXCESS OF 50 KT AROUND MID DAY. FALLING AND BLOWING
SNOW IS DEFINITELY A CONCERN HERE...BUT 700-300 MILLIBAR RH PROGS
SUGGEST THE BEST MOISTURE IS TO THE SOUTH. JUST NOT CONFIDENT SNOW
AMOUNTS WILL BE HIGH ENOUGH WITHOUT GAP ENHANCEMENTS TO CAUSE
WINTER STORM WARNING TYPE IMPACTS.

NOT SO MUCH THE CASE FOR ARLINGTON THOUGH...WHICH WILL BE EXTREMELY
WINDY FOR REASONS ALREADY STATED. PERIODIC LIGHT SNOW SHOULD DEVELOP
OVER THE NORTH SNOWY RANGE FOOTHILLS BY LATE MORNING. AREA FORECAST
SOUNDINGS ARE VERY CONCERNING WITH 60+ KT FLOW JUST OFF THE SURFACE
DURING THE AFTERNOON AND EVENING. WITH COLD ADVECTION ALOFT AIDING
IN MIXING AND POTENTIAL FOR CONVECTIVE SNOW SHOWERS GIVEN SLIGHTLY
NEGATIVE LIFTED INDICES...BELIEVE BLOWING SNOW COULD EASILY BE A
MAJOR ISSUE FOR INTERSTATE TRAVEL. EVEN IF ONLY AN INCH OR TWO OF
NEW SNOW FALLS...THE COMBINATION OF 70 MPH WIND GUSTS AND FALLING
AND/OR BLOWING SNOW HAS POTENTIAL TO RESULT IN VERY LOW VISIBILITIES
AND POSSIBLY WHITEOUT CONDITIONS. WE THOUGHT A WINTER STORM WARNING
WOULD BETTER HIGHLIGHT THESE IMPACTS...SO OPTED TO CHANGE FROM NPW
TO WSW HEADLINES AT ARL. WOULD NOT BE ENTIRELY SURPRISED IF THE DAY
SHIFT NEEDS TO CONSIDER BLIZZARD HEADLINES IF THESE CONCERNS ARE
REALIZED. WE ALSO INCLUDED SOUTHWEST CARBON COUNTY ON THE WEST SIDE
OF THE SIERRA MADRE WHERE UPSLOPE ENHANCEMENTS MAY YIELD LOCAL
AMOUNTS OF 6 TO 8 INCHES AROUND DIXON AND BAGGS. SHADOWING IN THE
UPPER NORTH PLATTE RIVER VALLEY AND LARAMIE VALLEY COULD LIMIT
SNOWFALL TO LESS THAN AN INCH...BUT UPGRADED THE WINTER STORM WATCH
TO AN ADVISORY FOR DANGEROUS TRAVEL. NOT TOO CONCERNED WITH THE
SOUTH LARAMIE RANGE THE ACCUMULATING SNOW SHOULD HOLD OFF UNTIL THE
OVERNIGHT...WHEN WINDS START TO COME DOWN. DO NOT EXPECT MUCH SNOW
AT ALL EAST OF THE LARAMIE RANGE WITH DRY DOWNSLOPING WINDS. A BIT
WARMER ON THE LEE SIDE TOO...SUPPORTING MORE RAIN THAN SNOW THROUGH
THE DURATION OF THE EVENT.

HIGH WINDS WILL END AFTER 06Z AS LOW-LEVEL GRADIENTS QUICKLY DROP
OFF OVERNIGHT...BUT BREEZY CONDITIONS WILL LIKELY PERSIST WITH AREAS
OF BLOWING SNOW AND REDUCED VISIBILITIES CONTINUING THROUGH MONDAY
MORNING OR EARLY AFTERNOON. SNOW AND WIND COULD BECOME AN ISSUE OVER
THE NORTHERN PANHANDLE ON MON WITH FAST NORTH/NORTHWEST FLOW ALOFT
AND STRONG LOW LEVEL CAA. DESPITE CLOUD COVER...GFS/NAM/ECMWF EACH
DEPICT A WELL-MIXED BOUNDARY LAYER WITH 50 TO 60 KT AVAILABLE TO
MIX AROUND 18Z MON. CONTEMPLATED SOME SORT OF WATCH HEADLINE WITH
INCREASED CONFIDENCE IN A HIGHER IMPACT EVENT...BUT PLAN TO FOLLOW
LBF/S LEAD WITH AN SPS LATER THIS MORNING TO AVOID ANY CONFUSION
WITH ONGOING HAZARDS. THE MODELS REALLY WRAP UP THE MID-LEVEL
SHORTWAVE AND ASSOCIATED SURFACE LOW AFTER 00Z TUE AS IT
TRANSLATES EASTWARD INTO THE CENTRAL PLAINS. COLD AIR ALOFT
REMAINS INTACT WITH H7 TEMPS AROUND -14C...SO INSTABILITY SNOW
SHOWERS OR FLURRIES WILL STILL BE POSSIBLE ALTHOUGH CONDITIONS
SHOULD GENERALLY IMPROVE.

.LONG TERM...(TUESDAY NIGHT THROUGH SATURDAY)
ISSUED AT 330 AM MST SUN DEC 21 2014

A PRONOUNCED UPPER TROUGH OVER GREAT PLAINS WILL PROGRESS EAST
TOWARD THE ATLANTIC/GULF COASTS THURSDAY. A RETURN TO ZONAL FLOW
ALOFT WILL BE SHORT-LIVED AS THE NEXT TROUGH MOVING INTO THE
PACIFIC NORTHWEST AMPLIFIES OVER THE ROCKY MOUNTAINS ON FRIDAY.
MODEL DIFFERENCES APPEAR SATURDAY...AS THE ECMWF IS MORE PRONOUNCED
WITH THE UPPER RIDGE OVER THE PACIFIC COAST STATES AND ITS DOWNSTREAM
TROUGH AXIS EXTENDING FURTHER WEST THAN THE GFS.

MOISTURE WILL DWINDLE OVER THE SOUTHERN WY MOUNTAINS TUESDAY NIGHT
WITH A SURFACE RIDGE OF HIGH PRESSURE EXTENDING FROM WESTERN CO
WEDNESDAY. SURFACE LOW PRESSURE DEVELOPS ALONG A COLD FRONT OVER
NORTHERN ID WEDNESDAY NIGHT...THEN PUSHES SOUTHEAST ACROSS WY AND
WESTERN NE CHRISTMAS DAY. THE ECMWF IS TRENDING FASTER AND PRODUCES
MORE WIDESPREAD QPF THAN THE GFS. GENERALLY FOLLOWED THE ECMWF
SOLUTION...WITH THE PRECIPITATION DEVELOPING WEST OF THE LARAMIE
RANGE WEDNESDAY NIGHT...SPREADING EAST ACROSS THE REMAINDER OF
THE CWA CHRISTMAS DAY...AND ENDING THURSDAY NIGHT. IT/S STILL
EARLY TO PREDICT SNOW ACCUMULATIONS WITH THIS SYSTEM. A BIT OF
MOISTURE AND WEAK UPSLOPE MAY TRIGGER LIGHT SNOW FOR THE SNOWY
AND SIERRA MADRES FRIDAY...OTHERWISE IT WILL BE DRY HEADING INTO
THE HOLIDAY WEEKEND.

HIGH TEMPERATURES WEDNESDAY WILL BE NEAR OR SLIGHTLY ABOVE SEASONAL
NORMALS WITH 30S TO LOWER 40S...FALLING TO BELOW NORMAL CHRISTMAS
DAY THROUGH SATURDAY. LOW TEMPERATURES TUESDAY/WEDNESDAY NIGHTS WILL
AVERAGE IN THE TEENS AND 20S...FALLING INTO THE SINGLE DIGITS AND
LOWER TEENS THURSDAY/FRIDAY NIGHTS. NOT EXPECTING ANY STRONG WINDS
WITH BREEZY TO OCCASIONALLY WINDY PERIODS.

&&

.AVIATION...(FOR THE 18Z TAFS THROUGH MONDAY MORNING) ISSUED AT 235
AM MST SUN DEC 21 2014

POTENT STORM SYSTEM WILL BRING A COMBINATION OF SNOW AND HIGH WINDS
TO AREAS ALONG AND WEST OF THE LARAMIE RANGE...PRODUCING PREVAILING
MVFR CONDITIONS WITH PERIODS OF LIFR CONDITIONS IN MODERATE
SNOWFALL. LIGHT RAIN AND SNOW IS POSSIBLE ACROSS THE EASTERN HIGH
PLAINS...BUT THE TIMING OF THIS PRECIP WILL BE DIFFICULT SO KEPT
VCSH IN THE FORECAST FOR NOW. MVFR CONDITIONS ARE POSSIBLE WITH THIS
ACTIVITY AS IT MOVES SOUTHEAST ACROSS THE AREA. WEST WINDS WILL GUST
IN EXCESS OF 45-50 KNOTS AT TIMES TODAY...MAINLY BETWEEN KRWL AND
KCYS. WINDS WILL NOT BE AS STRONG ACROSS FAR EASTERN WYOMING AND
WESTERN NEBRASKA. STRONG WINDS WILL SLOWLY DIMINISH LATE THIS
EVENING.

&&

.FIRE WEATHER...
ISSUED AT 315 AM MST SUN DEC 21 2014

NO FIRE WEATHER CONCERNS THROUGH THE MIDDLE OF THE WEEK. VERY WINDY
CONDITIONS ARE EXPECTED TODAY ALONG AND WEST OF THE LARAMIE RANGE
WITH GUSTS BETWEEN 60 AND 70 MPH LIKELY. INCREASING MOISTURE WILL
KEEP THE RISK FOR RAPID FIRE GROWTH LOW. SIGNIFICANT MOUNTAIN SNOWS
WILL PERSIST OVER THE NEXT 24 TO 48 HOURS WITH TOTAL ACCUMULATIONS
IN EXCESS OF 2 FEET POSSIBLE IN THE SNOWIES AND SIERRA MADRES BEFORE
ALL IS SAID AND DONE.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...WINTER STORM WARNING UNTIL 5 AM MST TUESDAY FOR WYZ112-114.

     HIGH WIND WARNING FROM 9 AM THIS MORNING TO MIDNIGHT MST TONIGHT
     FOR WYZ104-105-109-116-117.

     WINTER STORM WARNING UNTIL 11 AM MST MONDAY FOR WYZ110-111.

     HIGH WIND WARNING UNTIL MIDNIGHT MST TONIGHT FOR WYZ106.

     WINTER WEATHER ADVISORY FROM NOON TODAY TO 8 AM MST MONDAY FOR
     WYZ113-115.

NE...NONE.
&&

$$

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



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