FXUS65 KCYS 222131
AFDCYS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
231 PM MST TUE DEC 22 2009
.SHORT TERM...TONIGHT THROUGH THURSDAY...
UPSLOPE WINDS STILL ON TRACK TO DEVELOP TONIGHT AS SURFACE WINDS
SHIFT FROM SOUTHEAST TO NORTHEAST LATE THIS EVENING. OVERALL...THE
INHERITED FORECAST CAPTURED THE IDEA OF THE IMPENDING STORM
SYSTEM...BUT THE LATEST MODEL RUNS HAVE BEEN TRENDING TOWARD
SEPARATING THE TWO STORM SYSTEMS. THE FIRST TONIGHT WILL BE
ASSOCIATED WITH THE TROUGH AXIS EJECTING EAST ACROSS NORTHERN
ARIZONA LATE THIS AFTERNOON. INSTABILITY HAS BEEN SUFFICIENT IN
THE BASE OF THIS TROUGH TO PRODUCE LIGHTNING STRIKES ACROSS
SOUTHERN CALIFORNIA THIS MORNING AND EAST INTO CENTRAL ARIZONA
THIS AFTERNOON. THE FORECAST AREA WILL BE ON THE NORTHERN EDGE OF
THE IMPACTS ASSOCIATED WITH THIS SOUTHERN STORM SYSTEM TONIGHT AND
TOMORROW. LOW CLOUDS AND AT LEAST LIGHT SNOW WILL BE WIDESPREAD
ACROSS THE AREA...BUT THE ACCUMULATIONS WARRANTING THE WINTER
WEATHER ADVISORY BEGINNING AT 6 PM MST WILL BE CONFINED TO THE
SNOWY RANGE...LARAMIE VALLEY...LARAMIE RANGE AND AREAS JUST EAST
OF THE LARAMIE RANGE TONIGHT AND WEDNESDAY. ISENTROPIC LIFT
FORECAST IN THE LATEST MODEL RUNS CONTINUES TO BE BEST IN THESE
AREAS. CONVERSE...NIOBRARA AND PLATTE COUNTIES ARE NOT EXPECTED TO
SEE SIGNIFICANT SNOWFALL FROM THIS FIRST COMPONENT OF THE
STORM...AS LATEST MODEL RUNS HAVE BEEN TRENDING THE DYNAMICS AND
ISENTROPIC LIFT FARTHER SOUTH THAN PREVIOUS RUNS. THEREFORE HAVE
DELAYED THE START OF THE ADVISORY IN THE NORTHERN ZONES...AND THE
WINTER STORM WARNING IN THE WESTERN NEBRASKA PANHANDLE UNTIL 11 AM
MST AND 3 PM MST...RESPECTIVELY.
MODELS HAVE ALSO TRENDED LOWER WITH PRECIPITATION AMOUNTS
THROUGHOUT THE STORM SYSTEM...WITH THE CANADIAN THE LAST TO HAVE
MORE SIGNIFICANT LIQUID AMOUNTS IN THE 00Z RUN YESTERDAY. THE 12Z
CANADIAN JUMPED MORE IN LINE WITH THE CONSENSUS FROM THE GFS...NAM
AND EUROPEAN MODELS...AND HAVE TRENDED TOWARD SLIGHTLY LOWER
PRECIPITATION AMOUNTS. ALTHOUGH THE LOWERED SNOW AMOUNTS IN THE
PANHANDLE ARE MOSTLY JUST BELOW HEAVY SNOW CRITERIA...HAVE DECIDED
TO KEEP THE WINTER STORM WARNING IN PLACE AS IMPACTS ASSOCIATED
WITH MODERATE ACCUMULATIONS AND THE EXPECTED STRONG WINDS WILL
CERTAINLY CREATE CONDITIONS WORTHY OF A WINTER STORM WARNING. EVEN
NEAR BLIZZARD CONDITIONS COULD DEVELOP IN THE PANHANDLE WEDNESDAY
NIGHT AND EARLY THURSDAY...AND UPCOMING SHIFTS WILL NEED TO
MONITOR THIS POTENTIAL. WITH THE INCREASED WINDS ALSO EXPECTED TO
CREATE BLOWING AND DRIFTING SNOW CONCERNS IN SOUTHEAST
WYOMING...HAVE ADJUSTED THE ADVISORY TO INCLUDE BOTH SNOW AND
BLOWING SNOW.
THE MAIN STORM SYSTEM WILL BEGIN TO SHIFT EAST OF THE AREA
THURSDAY WITH BLOWING AND DRIFTING SNOW CONTINUING TO CREATE
HAZARDOUS CHRISTMAS EVE TRAVEL CONDITIONS. OROGRAPHIC SNOW
SHOWERS IN THE HIGHER TERRAIN OF SOUTHEAST WYOMING SHOULD BE THE
ONLY PRECIPITATION REMAINING ACROSS THE AREA THURSDAY...AS SNOW
ASSOCIATED WITH THE EXITING STORM SYSTEM WILL BE EAST OF THE
NEBRASKA PANHANDLE.
AFTER NEAR NORMAL LOWS TONIGHT...TEMPERATURES WEDNESDAY WILL BE
AROUND 10 DEGREES BELOW NORMAL. EVEN COLDER AIR WILL BE IN PLACE
WEDNESDAY NIGHT AS LOW TEMPERATURES WILL DROP TO AROUND 5 TO 10
DEGREES BELOW NORMAL. CHRISTMAS EVE WILL HAVE HIGH TEMPERATURES
AROUND 20 DEGREES BELOW NORMAL...WITH MOST LOWER ELEVATION AREAS
EXPECTED TO BE IN THE TEENS ABOVE ZERO.
.LONG TERM...THURSDAY NIGHT THROUGH TUESDAY...
MAIN PROBLEM THIS PERIOD IS THE EARLY PART...SPECIFICALLY HOW
QUICKLY THE WINTER STORM SYSTEM WILL DEPART THE CWA. MODELS WERE IN
PRETTY GOOD AGREEMENT IN PULLING OUT ESSENTIALLY ALL PCPN THURSDAY
EVENING AND DEPICTING MAINLY DRY CONDITIONS AFTER THAT. LATEST
ECMWF NOW HOLDING SOME LIGHT PCPN OVER THE EASTERN CWA THROUGH
CHRISTMAS DAY AS IT IS A LITTLE MORE WIDESPREAD WITH ITS WRAP-AROUND
PCPN SHIELD. WILL KEEP SOME LOW POPS OVER MAINLY THE PANHANDLE
FRIDAY IN LIGHT OF THIS. WILL ALSO NOT OPT TO EXTEND THE WINTER
STORM WARNING INTO THURSDAY NIGHT OVER THE PANHANDLE AT THIS TIME AS
MOST OF THE SNOW SHOULD END AND WINDS SHOULD DROP OFF A BIT...THOUGH
AN ADVY FOR BLOWING SNOW MAY BECOME NECESSARY. QUITE COLD AND
RATHER WINDY CONDITITIONS WILL CONTINUE THROUGH CHRISTMAS DAY AS THE
NEARLY STACKED SFC/UPPER LOW ONLY DRIFT SLOWLY EAST OVER THE
MIDWEST. A SLOW WARMUP SHOULD OCCUR OVER THE WEEKEND INTO EARLY NEXT
WEEK AS THE LAR UPPER LOW CONTINUES ITS SLOW EASTWARD DRIFT...
PICKING UP SPEED SUNDAY WHILE SHORT UPPER RIDGE MOVES OVER THE AREA.
NEXT UPPER SYSTEM IN THE SOUTHERN STREAM SHOULD TRACK ACROSS THE
DESERT SW INTO THE SRN PLAINS EARLY NEXT WEEK WITH MODELS CURRENTLY
KEEPING EFFECTS OF THIS SYSTEM WELL SOUTH OF THE CWA.
&&
.AVIATION...
AREA OF IFR LOW CIGS CURRENTLY OVER PARTS OF THE NEB PANHANDLE
EXPECTED TO EXPAND IN COVERAGE TO MOST AREAS OVERNIGHT AS SFC
UPSLOPE FLOW INCREASES. SOME LIGHT SNOWS SHOULD ALSO DEVELOP
MAINLY OVER SE WY OVERNIGHT WITH WIDESPREAD SNOW EXPECTED BY WEDS
NIGHT.
&&
.FIRE WEATHER...
WINTRY WEATHER WILL IMPACT THE AREA THROUGH CHRISTMAS EVE WITH
ACCUMULATING SNOWFALL EXPECTED ALONG WITH STRONG NORTHWEST WINDS
BY WEDNESDAY NIGHT.
&&
.CYS WATCHES/WARNINGS/ADVISORIES...
WY...WINTER WEATHER ADVISORY FROM 6 PM TUESDAY TO 6 AM THURSDAY FOR
WYZ103-WYZ106-WYZ107-WYZ116-WYZ117-WYZ118-WYZ119.
WINTER WEATHER ADVISORY FROM 6 PM TUESDAY TO MIDNIGHT THURSDAY
FOR WYZ110-WYZ114-WYZ115.
WINTER WEATHER ADVISORY FROM 11 AM WEDNESDAY TO 11 AM THURSDAY
FOR WYZ101-WYZ102-WYZ108.
NE...WINTER STORM WARNING FROM 3 PM WEDNESDAY TO 6 PM THURSDAY FOR
NEZ002-NEZ003-NEZ019-NEZ020-NEZ021-NEZ054-NEZ055-NEZ095-
NEZ096.
&&
$$
SHORT TERM/FIRE WEATHER...CARPENTER
LONG TERM/AVIATION...RE
