Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 201208
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
508 AM MST SAT DEC 20 2014

.SHORT TERM...(TODAY THROUGH MONDAY)
ISSUED AT 330 AM MST SAT DEC 20 2014

THE PREDAWN WATER VAPOR IMAGERY LOOP REVEALED A NEAR ZONAL FLOW
ALOFT ACROSS THE FORECAST AREA...WITH A VIGOROUS MOIST ONSHORE
FLOW RIDING ON 140+KT UPPER JET FROM THE EASTERN PACIFIC INTO MUCH
OF WA...OR AND CA. SOME MID/HIGH LEVEL CLOUDINESS WAS SPREADING
EAST ACROSS THE COWBOY STATE. BREEZY TO LOCALLY WINDY CONDITIONS
PREVAILED ALONG AND WEST OF THE LARAMIE RANGE. TEMPERATURES WERE
MILDER THAN RECENT NIGHTS WITH MID 20S TO MID 30S OBSERVED.

FORECAST CHALLENGES WILL FOCUS ON THE POTENTIAL FOR HIGH WINDS
AND SIGNIFICANT SNOWFALL FOR THE SOUTHERN MOUNTAINS THE SECOND
HALF OF THE WEEKEND INTO EARLY NEXT WEEK.

CURRENT DETERMINISTIC MODEL GUIDANCE REMAINS IN GOOD AGREEMENT
WITH THE LARGE SCALE PATTERN. NEAR ZONAL FLOW ALOFT TODAY WILL
TRANSITION TO A MORE PRONOUNCED NORTHWEST FLOW TONIGHT AND SUNDAY.
SHORTWAVE ENERGY WITHIN THE FLOW WILL AID IN THE AMPLIFICATION OF
A MEAN TROUGH OVER THE CENTRAL CONUS SUNDAY THROUGH MONDAY. AS THE
AXIS OF THE VERY FAST UPPER JET DIVES SOUTHEAST WEST OF THE FRONT
RANGE...Q-G LIFT IN LEFT FRONT REGION COMBINED WITH MOIST MID
LEVEL WESTERLY FLOW WILL GREATLY ENHANCE OROGRAPHIC SNOW PRODUCTION
OVER THE SNOWY AND SIERRA MADRE RANGES BEGINNING SUNDAY AND LASTING
THROUGH MONDAY MORNING. SNOWFALL RATES MAY EXCEED AN INCH PER HOUR
WITH THE POTENTIAL FOR A FEW FEET OF SNOW BY MONDAY NIGHT. STRONG
WEST WINDS UP TO 50 MPH WILL RESULT IN CONSIDERABLE BLOWING AND
DRIFTING SNOW CREATING VERY LOW VISIBILITY AND TREACHEROUS TRAVEL
THROUGH THE MOUNTAINS. THE WINTER STORM WATCH WAS UPGRADED TO A
WARNING. GFS TIME-HEIGHT CROSS SECTIONS INDICATE WINDS INCREASING
TO 60 KT JUST ABOVE THE INVERSION LATE TONIGHT. THE CRAIG TO CASPER
700/850 MB GRADIENT PEAKS BETWEEN 60 AND 80 METERS FROM 09Z SUNDAY
TO 06Z MONDAY. EXPECT HIGH WIND CRITERIA TO BE MET LATE TONIGHT
INITIALLY FOR THE ARLINGTON AND BORDEAUX AREAS...THEN SPREADING
SOUTH ALONG THE LARAMIE RANGE SUNDAY MORNING. HIGH WIND WATCHES
WERE UPGRADED TO WARNINGS.

HIGH TEMPERATURES TODAY AND SUNDAY WILL MODERATE INTO THE 30S
WEST AND 40S EAST. LOWS TONIGHT AND SUNDAY NIGHT WILL RANGE FROM
THE UPPER TEENS TO AROUND 30. A COLD FRONT WILL SWEEP SOUTHEAST
THROUGH THE FORECAST AREA EARLY MONDAY...WITH WINDY CONDITIONS
SPREADING OUT ONTO THE PLAINS. GFS 700MB WINDS PROGGED AROUND 50
KT DURING THE DAY MONDAY. NORTHWEST WINDS OF 25 TO 35 MPH WITH
GUSTS TO 45 MPH WILL BE POSSIBLE. LATER SHIFTS MAY HAVE TO CONSIDER
A WIND ADVISORY FOR THE WESTERN NE PANHANDLE. MONDAY WILL BE A
COOLER DAY WITH HIGHS IN THE 30S TO AROUND 40.

.LONG TERM...(MONDAY NIGHT THROUGH FRIDAY)
ISSUED AT 237 AM MST SAT DEC 20 2014

MAIN CHALLENGE THIS PERIOD REVOLVES AROUND A POTENTIAL WINTER STORM
AND ITS IMPACT ON HOLIDAY TRAVEL ACROSS THE AREA. MODELS CONTINUE TO
ADVERTISE A GENERALLY ACTIVE WEATHER PATTERN WITH PLENTY OF MOUNTAIN
SNOWFALL AND HIGH WINDS IN THE WIND PRONE LOCALES. TUESDAY WILL BE A
BIT OF A TRANSITION DAY AS THE ELONGATED UPPER TROUGH SHIFTS EAST OF
THE CWA. COOL NORTHERLY MID LEVEL FLOW SHOULD REMAIN INTACT AHEAD OF
BROAD UPPER-LEVEL RIDGING OVER THE WESTERN CONUS. KEPT HIGHS ON THE
COOLER SIDE OF GUIDANCE GIVEN POTENTIAL FOR MID AND HIGH LEVEL CLOUD
COVER...BUT WE MAY BE UNDERCUTTING THINGS SLIGHTLY AS THE GFS TRIES
TO DEVELOP SOME MODEST DOWNSLOPE FLOW IN THE LOWER LEVELS. BOTH THE
GFS AND ECMWF ESTABLISH THIS PATTERN BY WEDNESDAY...SO THAT WOULD BE
THE WARMEST DAY OF THE PERIOD IN EITHER CASE WITH H7 TEMPS AS WARM
AS +2 DEG C EAST OF THE SOUTH LARAMIE RANGE. A HIGH WIND EVENT LOOKS
PROBABLE ON WED/THU AS A BELT OF 50-60 KNOT H7 FLOW DEVELOPS ALONG
WITH A 70 METER H85 CAG-CPR GRADIENT PER THE GFS. POTENTIAL IS STILL
THERE FOR A WINTER STORM OVER THE CHRISTMAS HOLIDAY...HOWEVER THERE
IS STILL A CONSIDERABLE AMOUNT OF UNCERTAINTY. THE 20/00Z RUN OF THE
ECMWF IS STILL FAVORABLE FOR WIDESPREAD MODERATE/HEAVY SNOWFALL ON
CHRISTMAS DAY WITH A POTENT SHORTWAVE DIGGING INTO THE FOUR CORNERS
REGION. THE LATEST RUN OF THE GFS SHOWS A MUCH LOWER AMPLITUDE WAVE
TRAVERSING THE FLOW. THE NET RESULT IS A NORTHERN TRACK...BASICALLY
RIGHT OVER THE CWA WITH DRY DOWNSLOPE FLOW OCCURRING POST-FROPA.
THE GEM IS MORE IN LINE WITH THE ECMWF AT THE MOMENT...SO WILL HANG
ON TO POPS AS-IS WITH MINIMAL CHANGES TO THE PREVIOUS GRIDS. STILL
TOO EARLY TO BITE THE BAIT ON ANY ONE SOLUTION. STAY TUNED.

&&

.AVIATION...(FOR THE 12Z TAFS THROUGH 12Z SUNDAY MORNING)
ISSUED AT 508 AM MST SAT DEC 20 2014

VFR CONDITIONS SHOULD PREVAIL ACROSS MOST TERMINALS. MAIN HAZARD TO
AVIATION INTERESTS OVER THE NEXT 24 HOURS WILL BE HIGH WINDS IN THE
TYPICAL WIND PRONE LOCATIONS OF SOUTHEAST WYOMING. MOUNTAINS WILL BE
OBSCURED WITH OROGRAPHIC SNOW DEVELOPING LATER TODAY. MVFR CIGS ARE
FORECAST FOR KRWL AFTER 17Z SAT WITH INCREASING MOISTURE IN THE LOW
LEVELS. OTHERWISE...LOOK FOR GENERALLY BREEZY CONDITIONS DURING THE
MORNING AND AFTERNOON WITH GUSTS AROUND 30 KTS POSSIBLE OVER THE WY
SITES.

&&

.FIRE WEATHER...
ISSUED AT 300 AM MST SAT DEC 20 2014

NO FIRE WEATHER ISSUES EXPECTED THIS WEEKEND. BREEZY AND MAINLY DRY
CONDITIONS WILL CONTINUE ACROSS THE DISTRICTS TODAY. HIGH TEMPERATURES
WILL MODERATE INTO THE 30S AND 40S. STRONG WESTERLY WINDS AND SIGNIFICANT
MOUNTAIN SNOWFALL WILL RETURN SATURDAY NIGHT AND CONTINUE THROUGH MONDAY...
WITH WINTER STORM WARNINGS IN EFFECT FOR THE SNOWY AND SIERRA MADRE
RANGES FOR ONE TO TWO FEET OF SNOW. HIGH WIND WARNINGS ARE IN EFFECT
FOR THE WIND PRONE AREAS OF SOUTHEAST WYOMING FROM LATE TONIGHT THROUGH
SUNDAY EVENING.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...WINTER STORM WARNING FROM MIDNIGHT TONIGHT TO 6 PM MST MONDAY
     FOR WYZ112-114.

     HIGH WIND WARNING FROM MIDNIGHT TONIGHT TO MIDNIGHT MST SUNDAY
     NIGHT FOR WYZ106-110.

     HIGH WIND WARNING FROM 9 AM SUNDAY TO MIDNIGHT MST SUNDAY NIGHT
     FOR WYZ116-117.

NE...NONE.
&&

$$

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



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