FXUS65 KCYS 221150
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
450 AM MST TUE DEC 22 2009

.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT...
AN ACTIVE HOLIDAY WEEK IN STORE...WITH AREAS OF FOG IN THE NRN
PANHANDLE THIS MORNING...AND A BOUT WITH SNOW ACROSS THE REGION
BEGINNING TONIGHT AND CONTINUING THROUGH WEDNESDAY NIGHT. FIRST
THINGS FIRST...A WEAK BOUNDARY SLIPPED THROUGH THE NRN PANHANDLE
YESTERDAY...INCREASING BL MOISTURE IN ITS WAKE. AS A
RESULT...LOCALLY DENSE FOG DEVELOPED LAST EVENING AND WILL PERSIST
IN AREAS GENERALLY NORTH OF THE PINE RIDGE THIS MORNING.
ELSEWHERE...SATELLITE PIX SHOW INCREASING CLOUDS ACROSS MUCH OF
THE INTERMOUNTAIN WEST AS THE AREA AWAITS THE MAIN WEATHER MAKER
OF THE SHORT TERM SET TO ARRIVE TONIGHT AND CONTINUE THROUGH MID-
WEEK. AN AMPLIFYING UPPER RIDGE CONTINUES TO CAUSE THE DOWNSTREAM
LONGWAVE TROUGH TO DEEPEN AS IT MOVES ONTO THE PACIFIC COAST. AN
INTERESTING DOUBLE BARREL UPPER LOW STRUCTURE WILL EXIST WITH THE
SOUTHERNMOST LOW DIGGING INTO THE GREAT BASIN THIS EVENING.
MEANWHILE...THE SECONDARY UPPER LOW WILL DIVE SOUTH OUT OF
ALBERTA DURING THE SAME TIME. STRONG DIFFLUENT SOUTHERLY FLOW WILL
SET THE STAGE FOR BROAD ISENTROPIC ASCENT BEGINNING AROUND SUNSET
ALONG THE FRONT RANGE. MEANWHILE...AS THIS LONGWAVE TROUGH MOVES
EAST...THE REFLECTION AT THE SFC WILL INDUCE PRESSURE FALLS AND
LOW LEVEL CONVERGENCE ALONG THE TERRAIN. SO...BOUNDARY LAYER FLOW
WILL TURN SOUTHEASTERLY EARLY THIS EVENING...AND EVENTUALLY
EASTERLY BY AROUND MIDNIGHT. WINDS WILL SWING AROUND TO NORTHERLY
ON WEDNESDAY AS THE SFC LOW DEEPENS ACROSS THE TX/OK PANHANDLES.
THE SECONDARY UPPER LOW WILL DROP INTO WESTERN MONTANA DURING THE
DAY ON MONDAY...FURTHER AIDING SFC PRESSURE FALLS IN THE LEE AND
OROGRAPHIC ASCENT. SO...EXPECT SNOW TO BEGIN TO FALL ACROSS MUCH
OF THE CWA THIS EVENING...AND INTENSIFY OVERNIGHT AND DURING THE
DAY ON WEDNESDAY. SNOW ACCUMULATIONS LOOKS FAIRLY SIMILAR TO
EARLIER RUNS...WITH A GENERAL 4 TO 8 INCHES ANTICIPATED ACROSS
LOWER ELEVATIONS. THE HIGHEST VALUES WILL BE CONFINED TO THE
NEBRASKA PORTION OF THE CWA AND THE MOUNTAINS. WINDS WILL ALSO BE
A BIT STRONGER ACROSS THE PANHANDLE WHICH SHOULD CREATE MORE
IMPACT WITH BLOWING AND DRIFTING. AS SUCH...HAVE UPGRADED THE
NEBRASKA ZONES TO A WINTER STORM WARNING WITH THIS PACKAGE. THE
WYOMING PLAINS WERE A BIT TRICKIER AS AMOUNTS SHOULD BE A BIT LESS
SINCE ITS FURTHER FROM BEST SYNOPTIC LIFT...BUT STILL COULD COME
CLOSE TO WARNING CRITERIA. FOR NOW...HAVE OPTED TO ISSUE A WINTER
WEATHER ADVISORY FOR THE WYOMING ZONES...WITH GENERALLY 3 TO 7
INCHES EXPECTED. WINDS SHOULD BE LESS ACROSS WYOMING AS WELL. THE
SFC PRESSURE GRADIENT TIGHTENS LATE WEDNESDAY NIGHT...INCREASING
WINDS SUBSTANTIALLY FOR AREAS IN EXTREME ERN WYOMING AND THE WRN
NEBRASKA PANHANDLE. IN FACT...SOME GUIDANCE SHOWS SUSTAINED WINDS
NEAR 30 KTS DURING THIS TIME FRAME. FOR NOW...HAVE INTRODUCED
BLOWING SNOW TO THIS PORTION OF THE FORECAST. NORTHERLY FLOW AND
STRONG COLD AIR ADVECTION WILL RESULT IN HIGH TEMPERATURES BEING
10 TO 15 DEGREES COOLER FOR WEDNESDAY.

.LONG TERM...THURSDAY THROUGH NEXT MONDAY NIGHT...
MAIN FORECAST CONCERN LATE THIS WEEK IS THE EXIT OF THE STRONG
STORM SYSTEM OVER THE GREAT PLAINS STATES. MODELS HAVING
DIFFICULTY WITH THE EVENTUAL DEPARTURE OF THIS STORM...WITH THE
MAIN DIFFERENCE BEING THE MOVEMENT OF THE ARCTIC AIR MASS DIGGING
SOUTHWARD OUT OF CANADA ON THURSDAY. THE GEM AND 00Z AND 06Z GFS
SHOWS THE FIRST UPPER LEVEL LOW EJECTING NE INTO THE CNTRL PLAINS
EARLY THURSDAY...JUST EAST OF THE FORECAST AREA...WITH A SECONDARY
UPPER LEVEL LOW AND ASSOCIATED ARCTIC COLD FRONT DIGGING SOUTH OUT
OF CANADA. BOTH MODELS SHOW THIS FEATURE BECOMING THE PRIMARY LOW
JUST SOUTHEAST OF WYOMING BY LATE THURSDAY AS VERY COLD AIR PUSHES
ACROSS WYOMING AND WRN NEBRASKA...AND SNOW NOT COMPLETELY ENDING
UNTIL CHRISTMAS AFTERNOON. THE ECMWF SHOWS BOTH UPPER LEVEL LOWS
FURTHER TO THE EAST COMPARED TO THE GEM/GFS SOLUTION...WITH THE
MAIN LOW FORMING OVER ERN NEBRASKA AND NW MISSOURI BY LATE
THURSDAY...RESULTING IN THE SNOWFALL ENDING ON THURSDAY MORNING.
ECMWF ALSO SHOWS LESS WIND...AS THE SURFACE HIGH MOVES OVER THE
AREA INSTEAD OF OVER WRN WYOMING AS THE GFS DEPICTS. HENCE...THERE
IS SOME UNCERTAINTY WHEN THE SNOWFALL AND CLOUD COVER WILL
DIMINISH...ESPECIALLY OVER XTRM ERN WYOMING AND THE PANHANDLE OF
NEBRASKA. WITH THE ENSEMBLES IN BETTER AGREEMENT WITH THE
OPERATIONAL GFS AND GEM...DECIDED TO TREND TOWARDS THE GFS AND
GEM. ALTHOUGH LIGHTER SNOW IS EXPECTED THURSDAY AND THURSDAY
NIGHT...NORTH TO NORTHWEST WINDS ARE EXPECTED TO INCREASE TO
AROUND 30 KTS OVER THE HIGH PLAINS AS A 1040 MB SETTLES OVER WRN
WYOMING...RESULTING IN BLOWING AND DRIFTING SNOW AS WELL AS WIND
CHILLS BELOW -10 DEGS THROUGH CHRISTMAS DAY. KEPT POP ABOVE 15 PCT
THROUGH CHRISTMAS MORNING AND ADDED BLOWING SNOW TO THE FORECAST.
TEMPS WILL BE VERY COLD CHRISTMAS EVE AND CHRISTMAS DAY WITH HIGHS
IN THE TEENS AND LOW 20S...AND LOWS A FEW DEGS ABOVE OR BELOW
ZERO.

FOR NEXT WEEKEND AND EARLY NEXT WEEK...MODELS GENERALLY SHOW SPLIT 
FLOW AS THE SRN BRANCH OF THE JET STREAM DIGS SOUTH INTO NRN MEXICO 
AND THE SRN PLAINS...AND THE NRN BRANCH OF THE JET RETREATS 
NORTHWARD NEAR THE CANADA/U.S. BORDER. WITH NORTH TO NORTHWEST 
FLOW...TEMPS WILL REMAIN BELOW NORMAL. HOWEVER...TEMPS ARE EXPECTED 
TO WARM SLIGHTLY WITH MODELS SHOWING INCREASING THICKNESSES INTO 
SUNDAY. KEPT POP BELOW 15 PCT THROUGH THE WEEKEND WITH A WEAK RIDGE 
OF HIGH PRESSURE WEST OF THE REGION.  

&&

.AVIATION...
VFR CONDITIONS WILL CONTINUE THROUGH EARLY THIS AFTERNOON BEFORE
CONDITIONS DETERIORATE THIS EVENING AS A STORM SYSTEM IMPACTS
SOUTHEAST WYOMING AND THE PANHANDLE OF NEBRASKA THROUGH THURSDAY
MORNING. SNOW IS EXPECTED TO DEVELOP AROUND SUNSET ALONG I80 IN
WYOMING...AND THEN GRADUALLY SPREAD NORTH AND EAST INTO NEBRASKA
AFTER MIDNIGHT TONIGHT. WIDESPREAD IFR CONDITIONS ARE EXPECTED
STARTING LATE THIS EVENING...AND CONTINUING INTO WEDNESDAY WITH
VIS BELOW 1 MILE AND CIGS BELOW 1K FEET AGL. NORTH TO NORTHEAST
WINDS ARE ALSO EXPECTED TO INCREASE TO 20 TO 30 KTS BY WEDNESDAY
EVENING RESULTING IN AREAS OF BLOWING AND DRIFTING SNOW.

&&

.FIRE WEATHER...
NO FIRE WEATHER CONCERNS THROUGH THE END OF THE WEEK. A STRONG
UPPER LEVEL STORM SYSTEM WILL BRING SNOW TO THE DISTRICT BEGINNING
TONIGHT AND CONTINUING THROUGH WEDNESDAY NIGHT. SNOWFALL
ACCUMULATIONS OF 2 TO 5 INCHES CAN BE EXPECTED IN THE VALLEYS WEST
OF THE MOUNTAINS...WITH 4 TO 8 INCHES ACROSS LOWER ELEVATIONS EAST
OF THE LARAMIE RANGE. MOUNTAIN LOCATIONS CAN EXPECT TO RECEIVE UP
TO A FOOT OF NEW SNOW FROM THIS STORM SYSTEM. TEMPERATURES WILL BE
10 TO 15 DEGREES COOLER ON WEDNESDAY COMPARED TO TODAY. NORTH
WINDS WILL CREATE AREAS OF BLOWING AND DRIFTING SNOW FOR WEDNESDAY
NIGHT AND THURSDAY. DRIER CONDITIONS EXPECTED BY THE WEEKEND.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...WINTER WEATHER ADVISORY FROM 6 PM TUESDAY TO 6 AM THURSDAY FOR 
     WYZ101-WYZ102-WYZ103-WYZ106-WYZ107-WYZ108-WYZ116-WYZ117-
     WYZ118-WYZ119.
     WINTER WEATHER ADVISORY FROM 6 PM TUESDAY TO MIDNIGHT THURSDAY 
     FOR WYZ110-WYZ114-WYZ115.
NE...WINTER STORM WARNING FROM 6 PM TUESDAY TO 6 AM THURSDAY FOR 
     NEZ002-NEZ003-NEZ019-NEZ020-NEZ021-NEZ054-NEZ055-NEZ095-
     NEZ096.

&&

$$

SHORT TERM/FIRE WEATHER...HAHN
LONG TERM/AVIATION...TJT