FXUS65 KCYS 252010
AFDCYS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
110 PM MST FRI DEC 25 2009

.UPDATE...BASED ON CALLS TO LUSK AND CHADRON...OUR TEAM HAS UPGRADED
THE WINTER STORM WARNING TO A BLIZZARD WARNING THROUGH 700 PM THIS
EVENING FOR NIOBRARA...NORTHERN SIOUX AND DAWES COUNTIES.  RUBIN

&&

.PREV DISCUSSION... /ISSUED 1039 AM MST FRI DEC 25 2009/ 

UPDATE...GRIDDED FORECASTS...ADVISORIES AND WINTER STORM WARNING
IN DECENT SHAPE. MADE MODIFICATIONS TO TEMPERATURES...
DEWPOINTS...WIND SPEEDS AND SNOW COVERAGE FOR CHRISTMAS DAY BASED
ON TRENDS AND LATEST MODEL GUIDANCE AND PROGNOSIS. RUBIN

PREV DISCUSSION... /ISSUED 342 AM MST FRI DEC 25 2009/ 

SHORT TERM...TODAY THROUGH SATURDAY NIGHT...COMPLICATED WEATHER
SITUATION CURRENTLY DEVELOPING OVER THE REGION AND GREAT PLAINS THIS
MORNING...WITH STRONG WINDS...BLOWING SNOW...ACCUMULATING SNOW AND
LOW WIND CHILLS EXPECTED INTO THIS WEEKEND. CURRENT IR SATELLITE
LOOP SHOWS CLOUD TOP ENHANCEMENT DEVELOPING OVER WRN NEBRASKA AND
SOUTHEAST WYOMING AS THE UPPER LEVEL ARCTIC LOW DIVES SOUTH INTO
THE GREAT PLAINS THIS MORNING. THIS LOW WILL MERGE WITH THE UPPER
LEVEL LOW CURRENTLY EJECTING NORTH OVER THE MISSISSIPPI VALLEY...ALONG
WITH A STRONG 988 MB SURFACE LOW. AS THE ARCTIC LOW DIGS SOUTH AND
EAST...MODELS CONTINUE TO SHOW THE STRONG SURFACE LOW RETROGRADE
WESTWARD INTO ERN NEBRASKA AND IOWA AND SNOW DEVELOPING OVER THE
NRN PLAINS...INTO WRN NEBRASKA AND MOST OF SOUTHEAST WYOMING
STARTING THIS MORNING...AND CONTINUING INTO SATURDAY.

WIND...MODELS HAVE CONTINUED TO SHOW STRONG 700MB WINDS IN EXCESS
OF 60 KTS WITH 850 MB WINDS NEAR 40 KTS TODAY IN ADDITION TO THE
STRONG SURFACE PRESSURE GRADIENT BETWEEN THE ARCTIC HIGH IN
MONTANA AND WRN WYOMING AND THE DEEP LOW ACROSS THE GREAT PLAINS.
WINDS HAVE ALREADY INCREASED ACROSS MOST AREAS WITH SUSTAINED
WINDS BETWEEN 20 TO 35 MPH AND GUSTS AS HIGH AS 50 MPH ALREADY.
EXPECT WINDS BETWEEN 25 TO 40 MPH WITH GUSTS UP TO 60 MPH TODAY.
HOWEVER...SINCE BLOWING SNOW...SNOW...AND REDUCED VISIBILITIES
ARE THE PRIMARY TRAVEL CONCERN...DECIDED TO CANCEL THE WIND
ADVISORY OVER THE PANHANDLE OF NEBRASKA...AND ADD THE STRONG WIND  
WORDING TO THE WINTER WEATHER ADVISORY.

WIND CHILL...WILL CONTINUE THE WIND CHILL ADV THIS MORNING SINCE
OBS SHOW WIND CHILLS BETWEEN -20 TO -30 DEGREES. WILL ADD THE
WORDING ONCE THE WINTER WEATHER ADVISORY AND WINTER STORM WARNING GO
INTO EFFECT.

SNOW ACCUMULATIONS...MODELS CONTINUE TO SHOW GOOD ISENTROPIC LIFT
OVER WRN NEBRASKA AND SOUTHEAST WYOMING IN NORTH TO NORTHEAST FLOW
IN THE MIDLEVELS. GOOD UPSLOPE FLOW ALSO ALONG THE PINE RIDGE FROM
DOUGLAS TO CHADRON...NRN LARAMIE RANGE...AND THE CHEYENNE RIDGE
TODAY AND TONIGHT. MODELS INDICATE DECENT OMEGA VALUES AROUND 10
MICROBARS ALONG THE NORTH FACE OF THESE LOCATIONS WITH NEARLY A
HALF AN INCH OF QPF ALONG THE PINE RIDGE...THE NRN LARAMIE
RANGE...AND THE CHEYENNE RIDGE BETWEEN SCOTTSBLUFF AND CHEYENNE.
EXPECT 1 TO 3 INCHES OF SNOW ALONG I80 FROM SIDNEY TO
RAWLINS...WITH LOCALLY HIGHER ACCUMULATIONS ACROSS THE HIGHER
ELEVATIONS OF THE SRN LARAMIE RANGE AND NEAR THE SNOWY RANGE.
EXPECT 2 TO 5 INCHES IN THE NORTH PLATTE RIVER VALLEY WITH LOCALLY
HIGHER AMOUNTS NEAR 7 INCHES JUST SOUTH OF TORRINGTON AND
SCOTTSBLUFF. THE JACKPOT IS EXPECTED TO BE THE PINE RIDGE OF NRN
NEBRASKA PANHANDLE WESTWARD INTO NIOBRARA COUNTY...AND THE NRN
LARAMIE RANGE WITH FAVORABLE UPSLOPE FLOW. EXPECT 5 TO 10 INCHES
OF SNOW ACROSS DAWES...SIOUX...NIOBRARA COUNTIES WITH SNOW
ACCUMULATIONS AROUND A FOOT ACROSS THE NRN LARAMIE RANGE IN
CONVERSE COUNTY. THEREFORE...UPGRADED THE WINTER STORM WATCH TO A
WARNING IN THESE LOCATIONS EXCEPT FOR THE NRN LARAMIE RANGE WHICH
WILL STILL REACH HIGHER ELEVATION ADVISORY CRITERIA. MAY NEED TO
ADD NORTHERN CONVERSE COUNTY AS WELL. KEPT ALL OTHER WINTER
WEATHER ADVISORIES FOR SNOW AND BLOWING SNOW...BUT ADDED THE
SOUTHERN NEBRASKA PANHANDLE AND GOSHEN COUNTY IN WYOMING TO THE
ADVISORY AS WELL. EXPECT HAZARDOUS TRAVEL CONDITIONS...WITH NEAR
BLIZZARD CONDITIONS AT TIMES OVER THE NRN NEBRASKA PANHANDLE. WILL
HAVE TO KEEP A CLOSE EYE ON THIS TODAY.

ACTIVITY EXPECTED TO SLOWLY IMPROVE BY SATURDAY AFTERNOON AND
SATURDAY EVENING...WITH MODELS SHOWING THE SURFACE LOW OVER IOWA
FILLING IN AS THE STORM SYSTEM BECOMES VERTICALLY STACKED.
GRADUALLY LOWERED POP FROM WEST TO EAST BY SATURDAY EVENING WITH
SUBSIDING WINDS AND DECREASED SNOWFALL RATES. HOWEVER...LIGHT SNOW
MAY CONTINUE TO FALL ALONG THE PINE RIDGE THROUGH SATURDAY NIGHT
DUE TO FAVORABLE LLVL NORTHERLY FLOW. 

LONG TERM...SUNDAY THROUGH THURSDAY...GFS AND ECMWF ARE SIMILAR 
IN THE LONG TERM...BUT LEANED TOWARD THE ECMWF AT THE END OF THE
PERIOD...WHICH HAS CYCLONIC FLOW ALOFT IN RESPONSE TO AN ALBERTA
CLIPPER MOVING ACROSS THE NORTH CENTRAL US. DOWNSLOPE WINDS AND
DRY CONDITIONS WILL OCCUR THROUGH THE EXTENDED PERIOD OUTSIDE OF
THE MOUNTAINS. EAST TO NORTHEAST FLOW WILL BE ALOFT ON SUN WITH AN
UPPER HIGH TO THE NORTH OF THE FORECAST AREA AND THE UPPER LOW
OVER THE GREAT LAKES REGION. THE UPPER HIGH WILL MOVE OVER THE
AREA SUN NIGHT. LOWERED TEMPS SUN NIGHT DUE TO MOSTLY CLEAR
SKIES...LIGHT WINDS AND SURFACE HIGH PRESSURE OVER THE CWA. ON
MON...A WEAK UPPER TROUGH WILL WILL MOVE INTO THE NORTHERN AND
CENTRAL ROCKIES. THE UPPER TROUGH WILL DEEPEN SOMEWHAT MON NIGHT
WITH THE AXIS OVER THE WESTERN PORTION OF THE CWA BY 12Z TUES.
THIS WILL RESULT IN A CHANCE OF SNOW OVER THE MOUNTAINS. THE UPPER
FLOW WILL BECOME NW ON TUES AND WILL CONTINUE THROUGH THURS...AS
THE UPPER TROUGH AXIS MOVES EAST OF THE FORECAST AREA AND AN UPPER
RIDGE BUILDS OFF THE WEST COAST. INTRODUCED A CHANCE OF LIGHT SNOW
ACROSS THE WY PORTION OF THE CWA ON THUR AS A DISTURBANCE IN
CYCLONIC FLOW ALOFT BEGINS TO MOVE ACROSS THE AREA. HIGH
TEMPERATURES WILL BE FROM 5 TO 10 DEGREES BELOW NORMAL SUN THROUGH
WED...AND AROUND NORMAL FOR THUR.

AVIATION...CONDITIONS WILL CONTINUE TO DETERIORATE THIS MORNING
WITH STRONG GUSTY WINDS...SNOW...AND BLOWING SNOW THROUGH THE DAY
RESULTING IN VISIBILITY AROUND 1 MILE AND CIGS BELOW 3K FEET. STRONG
NORTHWEST WINDS BETWEEN 25 TO 40 MPH ARE EXPECTED WITH GUSTS UP TO
60 MPH POSSIBLE. IFR CONDITIONS ARE EXPECTED TO PREVAIL FROM KLAR 
EASTWARD INTO THE NEBRASKA PANHANDLE WITH LOW IFR CONDITIONS
PROBABLE AT TIMES. CONDITIONS ARE NOT EXPECTED TO IMPROVE UNTIL
LATE SATURDAY. 

FIRE WEATHER...A STRONG WINTER STORM OVER THE GREAT PLAINS WILL
CONTINUE TO IMPACT THE AREA THROUGH EARLY THIS WEEKEND WITH STRONG
WINDS...SNOW AND BLOWING SNOW. DRIER WEATHER IS EXPECTED NEXT WEEK
WITH COLDER THAN NORMAL TEMPERATURES EXPECTED TO CONTINUE. NO
FIRE WEATHER CONCERNS AT THIS TIME.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...BLIZZARD WARNING UNTIL 7 PM FRIDAY FOR WYZ102.
     WINTER STORM WARNING FROM 7 PM FRIDAY TO NOON SATURDAY FOR 
     WYZ102.
     WINTER WEATHER ADVISORY UNTIL NOON SATURDAY FOR WYZ108.
     WINTER WEATHER ADVISORY UNTIL 5 AM SATURDAY FOR WYZ101-WYZ103-
     WYZ106-WYZ107-WYZ112-WYZ114-WYZ116-WYZ117-WYZ118-WYZ119.
NE...BLIZZARD WARNING UNTIL 7 PM FRIDAY FOR NEZ002-NEZ095.
     WINTER STORM WARNING FROM 7 PM FRIDAY TO NOON SATURDAY FOR 
     NEZ002-NEZ095.
     WINTER WEATHER ADVISORY UNTIL NOON SATURDAY FOR NEZ003-NEZ019-
     NEZ020-NEZ021-NEZ054-NEZ055-NEZ096.

&&

$$

SHORT TERM/AVIATION...TJT
LONG TERM...KMD