FXUS65 KCYS 231212
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
512 AM MST WED DEC 23 2009

.SHORT TERM...TODAY THROUGH THURSDAY...COMPLICATED WEATHER
SITUATION SETTING UP OVER THE AREA THROUGH CHRISTMAS EVE. FOR
NOW...CURRENT KCYS RADAR SHOWS A BAND OF LIGHT TO MODERATE SNOW
REMAINING OVER XTRM SOUTHEAST WYOMING DUE TO EASTERLY UPSLOPE 
FLOW...BASICALLY ALONG THE SRN LARAMIE RANGE EASTWARD INTO THE
HIGH PLAINS ALONG I80. OFFICIALLY...3 INCHES OF SNOW WAS OBSERVED
AT THE OFFICE IN CHEYENNE WITH LIGHT SNOW CONTINUING AT 4 AM. RADAR
SHOWS THE BACK EDGE OF THE SNOWBAND PUSHING INTO WYOMING FROM THE
EAST...WITH SNOW GRADUALLY TAPERING OFF TO FLURRIES AROUND DAWN.
THE ONLY EXCEPTION MAY BE THE SRN LARAMIE RANGE...WITH MODEL
SHOWING THE LIGHT SNOW LINGERING THROUGH THE MORNING. TEMPS ARE
EXPECTED TO REMAIN STEADY IN THE MID 20S THIS MORNING WITH CLOUDY
SKIES. SOME FREEZING FOG WILL PERSIST ACROSS THE NEBRASKA
PANHANDLE EARLY THIS MORNING...BUT EXPECT FOG TO GRADUALLY LIFT AS
WINDS BACK FROM THE NORTH.

THE STORM SYSTEM RESPONSIBLE FOR THE SNOW THIS MORNING WILL
CONTINUE TO PUSH EASTWARD THROUGH NEW MEXICO AND INTO TEXAS TODAY.
MODELS SHOW A SECONDARY UPPER LEVEL TROUGH AND ASSOCIATED ARCTIC
COLD FRONT DIGGING SOUTHWARD INTO MONTANA AND WYOMING BY THIS
AFTERNOON. THIS SYSTEM WILL NOT ONLY BRING IN SOME COLD ARCTIC
AIR...BUT MORE SNOW AND INCREASING WINDS AS WELL. AFTER A BRIEF BREAK
FROM THE SNOWFALL THIS MORNING AND EARLY THIS AFTERNOON...EXPECT
SNOW TO REDEVELOP BY EARLY THIS EVENING...STARTING ACROSS EAST
CNTRL WYOMING AND THEN SOUTHWARD INTO SOUTHEAST WYOMING AND THE
NEBRASKA PANHANDLE BY LATE THIS EVENING. WITH MORE FAVORABLE UPPER
LEVEL DYNAMICS OVER THE FORECAST AREA AS THE NRN BRANCH OF THE JET
DIVES SOUTH ACROSS WYOMING...COMBINED WITH SOME UPSLOPE AND WEAK
LLVL FRONTOGENESIS...EXPECT ANOTHER 2 TO 5 INCHES OF SNOW
ACCUMULATION BY LATE TONIGHT AND INTO THURSDAY MORNING. KEPT
WINTER WEATHER HEADLINES AS IS WITH SOME MINOR CHANGES IN WORDING
AND TIMING. NOT VERY CONFIDENT WITH THE WINTER STORM WARNING
ACROSS WRN NEBRASKA WITH THOSE LOCATIONS MISSING OUT ON MOST OF
THE SNOW THIS MORNING...WILL HAVE TO WAIT AND SEE IF MODELS COME
INTO BETTER AGREEMENT WITH PRECIP VALUES THROUGH TONIGHT. MAY NEED
TO DOWNGRADE THE WSW IF CURRENT ACCUMULATION FORECASTS ARE NOT
EXPECTED. OTHERWISE...INCLUDED THE SIERRA MADRE RANGE IN THE
WINTER WEATHER ADV WITH 5 TO 10 INCHES OF SNOW EXPECTED DUE TO
FAVORABLE UPSLOPE FLOW. EXPECT HIGH TEMPS THIS MORNING WITH TEMPS
REMAINING IN THE LOW TO MID 20S. TEMPS WILL GRADUALLY LOWER INTO
THE TEENS BY THIS EVENING AS THE STRONG ARCTIC FRONT PUSHES
THROUGH THE AREA...WITH LOWS IN THE SINGLE DIGITS TONIGHT AND
PROBABLY BELOW ZERO IN A FEW LOCATIONS. IN ADDITION...NORTH TO
NORTHWEST WINDS WILL INCREASE BY THIS AFTERNOON AND TONIGHT
RESULTING IN BLOWING AND DRIFTING SNOW THROUGH THURSDAY. MAY NEED
TO EXTEND THE CURRENT ADVISORIES THROUGH THURSDAY...OR REISSUE FOR
BLOWING SNOW FOR AREAS ALONG AND EAST OF THE LARAMIE RANGE WITH
SUSTAINED WINDS BETWEEN 15 TO 30 MPH AND GUSTS UP TO 45 MPH
POSSIBLE THURSDAY AFTERNOON. EXPECT ANOTHER BRIEF LULL IN THE
SNOWFALL THURSDAY AS THE MAIN ARCTIC VORTEX DIGS SOUTH OVER THE AREA.
HOWEVER...MODELS NOW SHOW JET ENERGY AND SOME LLVL UPSLOPE FLOW
ACROSS THE NEBRASKA PANHANDLE AND ERN WYOMING THURSDAY AFTERNOON
INTO THURSDAY NIGHT AS THE VORTEX MOVES SOUTHEAST OF THE 
REGION...WITH ANOTHER ROUND OF LIGHT SNOW POSSIBLE. INCREASED POP
ON THURSDAY...TO AROUND 55 PCT ACROSS SRN NE AND BETWEEN 30 TO 40
PCT ACROSS SOUTHEAST WYOMING. STRONG WINDS WILL CONTINUE INTO
THURSDAY EVENING WITH BLOWING SNOW AND VERY COLD TEMPS.

.LONG TERM...THURSDAY NIGHT THROUGH TUESDAY...
THE MAIN FORECAST PROBLEM OF LONG TERM PERTAINS TO HOW THE SECONDARY 
UPPER LOW EXITS THURSDAY NIGHT AND FRIDAY. AT 00Z FRIDAY...THE 
OPERATIONAL AND ENSEMBLE GFS HAVE THE UPPER LOW LOCATED NR 
SCOTTSBLUFF...THE ECMWF NR RAPID CITY...THE NAM NR GILLETTE...AND 
THE GEM EAST OF ALLIANCE.  SO CLEARLY...00Z AND 06Z DETERMINISTIC 
MODEL RUNS ARE STRUGGLING ON PATTERN EVOLUTION AND HOW QUICKLY THIS 
UPPER LOW DROPS SOUTH ALONG THE FRONT RANGE AND EVENTUALLY PHASES 
WITH AND REPLACES THE INITIAL UPPER LOW OVER THE CENTRAL PLAINS. 
THE EXACT TRACK THAT THIS UPPER LOW TAKES WILL DICTATE HOW MUCH 
WRAPAROUND MOISTURE AND RESIDUAL SNOW REMAINS ACROSS THE CWA 
THURSDAY NIGHT AND FRIDAY. GIVEN THIS UNCERTAINTY WILL CONTINUE WITH 
POPS FOR THURSDAY NIGHT AND FRIDAY...DECREASING THEM FROM WEST TO 
EAST ON FRIDAY. ALTHOUGH SIGNIFICANT ACCUMULATIONS SHOULD BE OVER BY 
THIS TIMEFRAME...HEAVIER LOCALIZED SNOW BANDS MAY SET UP... 
ESPECIALLY EAST OF THE MOUNTAINS POSSIBLY PRODUCING A QUICK COUPLE 
OF INCHES. ANOTHER CONCERN DURING THIS PERIOD IS THE NORTHWEST WINDS 
OF 20 TO 30 MPH GUSTING UP TO 40 MPH AT TIMES. THIS WILL CONTINUE TO 
CREATE SIGNIFICANT AREAS OF BLOWING AND DRIFTING SNOW ACROSS THE 
AREA INTO CHRISTMAS DAY. AS A RESULT...HOLIDAY TRAVEL WILL CONTINUE 
TO BE IMPACTED THURSDAY NIGHT AND FRIDAY. THIS TIMEFRAME WILL NEED 
TO BE MONITORED FOR A POSSIBLE EXTENSION OF CURRENT HILITES...OR 
PERHAPS A BLOWING SNOW ADVISORY...ALTHOUGH WILL WAIT FOR MORE 
CONSISTENT MODEL OUTPUT. UPPER LOW FINDS THE MID-MISSISSIPPI RIVER 
VALLEY BY 00Z SATURDAY...WITH CYCLONIC NORTHERLY FLOW ENTRENCHED 
OVER THE NRN ROCKIES...BRINGING AN END TO SNOW CHANCES AND A GRADUAL 
DECREASE TO WINDS. A DRIER AIR MASS WILL SETTLE IN AS AN AREA OF 
HIGH PRESSURE DEVELOPS OVER THE NRN ROCKIES INTO EARLY NEXT WEEK. 

TEMPERATURES WILL REMAIN WELL BELOW NORMAL FOR CHRISTMAS DAY...WITH 
HIGHS ONLY REACHING THE TEENS. THESE TEMPERATURES COMBINED WITH THE 
STRONG NORTHERLY WINDS...WILL DROP WIND CHILLS INTO THE -10 TO -20F 
RANGE FOR CHRISTMAS MORNING. TEMPERATURES WILL SLOWLY MODERATE FOR 
THE WEEKEND THROUGH EARLY NEXT WEEK...WITH HIGHS RETURNING TO THE 
20S TO NEAR 30 EAST. 
&&

.AVIATION...A STRONG UPPER LEVEL STORM SYSTEM WILL CONTINUE TO BRING POOR 
CONDITIONS TO AREA TERMINALS TODAY AND TONIGHT. INITIAL BAND OF SNOW 
THAT IMPACTED KCYS AND TO A LESSER EXTENT KLAR IS EXPECTED TO WEAKEN 
A BIT BY LATE MORNING...PROVIDING A LULL IN THE ACTION. EVEN 
SO...CONDITIONS WILL REMAIN MVFR/IFR IN LOW CIGS DURING THIS BRIEF 
RESPITE. SNOW WILL DEVELOP ONCE AGAIN DURING THE AFTERNOON HOURS 
IMPACTING ALL TERMINALS WITH IFR/LIFR CIGS/VSBYS IN LIGHT TO 
MODERATE SNOW. SNOW WILL DECREASE SOME LATE TONIGHT ALLOWING 
CONDITIONS TO IMPROVE SOMEWHAT...HOWEVER STRONG NORTH WINDS WILL 
CREATE SIGNIFICANT AREAS OF BLOWING AND DRIFTING SNOW ACROSS THE 
REGION. STRONG WINDS AND OFF AND ON SNOW SHOWERS WILL CONTINUE FOR 
THURSDAY AND FRIDAY AS WELL.
&&

.FIRE WEATHER...A WINTER STORM WILL IMPACT THE AREA POSSIBLY
THROUGH CHRISTMAS DAY WITH STRONG WINDS...SNOW AND BLOWING SNOW.
NO FIRE WEATHER CONCERNS AT THIS TIME.
&&

.CYS WATCHES/WARNINGS/ADVISORIES...

WY...WINTER WEATHER ADVISORY UNTIL 6 AM THURSDAY FOR WYZ101-WYZ102-
     WYZ103-WYZ106-WYZ107-WYZ108-WYZ116-WYZ117-WYZ118-WYZ119.

     WINTER WEATHER ADVISORY UNTIL MIDNIGHT THURSDAY FOR WYZ110-
     WYZ112-WYZ114-WYZ115.

NE...WINTER STORM WARNING FROM 3 PM WEDNESDAY TO 6 PM THURSDAY FOR 
     NEZ002-NEZ003-NEZ019-NEZ020-NEZ021-NEZ054-NEZ055-NEZ095-
     NEZ096.

&&

$$

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