807
FXUS65 KCYS 211000 AAA
AFDCYS
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHEYENNE WY
300 AM MST SAT NOV 21 2009
SHORT TERM
TODAY THROUGH SUNDAY NIGHT
AN ACTIVE PERIOD WILL BE IN STORE FOR THE CWA THROUGH 12Z MONDAY.
A SHORTWAVE TROUGH CURRENTLY OVER THE PACIFIC NORTHWEST WILL MOVE
EAST AND ACROSS THE CWA THROUGH 18Z SUNDAY. MODELS HAVE BEEN
TRENDING A BIT SLOWER WITH THE FEATURE THE PAST FEW RUNS BUT ARE
NOW FAIRLY CLOSE WITH THE AXIS OVER FAR EASTERN WYOMING AT 12Z
SUNDAY. THE ASSOCIATED PACIFIC COLD FRONT WAS LOCATED FROM
WESTERN MONTANA TO IDAHO EARLY THIS MORNING WITH 2 TO 3 MB THREE
HOURLY SURFACE PRESSURE RISES BEHIND THE FRONT. THE FRONT WILL
MOVE ACROSS THIS AFTERNOON AND EARLY EVENING. CONSIDERABLE MID
AND HIGH CLOUDS WILL BE OVER THE CWA TODAY AND TONIGHT PRIOR TO
THE PASSAGE OF THE SHORTWAVE TROUGH. IT WILL ALSO BE WINDY OVER
AND NEAR THE MOUNTAINS TODAY WITH THE SURFACE LOW PRESSURE TROUGH
OVER FAR EASTERN WYOMING. WIND GUSTS SINCE 06Z HAVE BEEN AS HIGH
AS 52 MPH AT KCPR...KARL AND KRWL. THE WIND SPEEDS MAY BE THE
HIGHEST THROUGH MID MORNING WITH THE TEMPERATURE INVERSION PRESENT
AND BASED AROUND 750 MB. TEMPERATURES ARE CURRENTLY FAIRLY MILD
IN MANY LOCATIONS AND WILL HELP TO PRODUCE ANOTHER DAY WITH ABOVE
NORMAL TEMPERATURES OVER THE EASTERN PART OF THE CWA AHEAD OF THE
FRONT. THERE IS CURRENTLY BANDS OF PRECIPITATION ON THE KMTX AND
KSFX RADARS ASSOCIATED WITH THE BAROCLINIC CLOUD BAND NOW OVER
EASTERN IDAHO AND NORTHERN UTAH. THE MAIN THREAT OF LIGHT SNOW
WILL BE OVER AND NEAR THE SNOWY RANGE TONIGHT WITH THE PASSAGE OF
THE SHORTWAVE TROUGH. OTHERWISE EXPECT MOST OF THE CWA TO REMAIN
DRY. MODELS DIFFER ON THE TIMING AND AMPLITUDE OF THE NEXT
SHORTWAVE TROUGH. THE FEATURE WILL DEEPEN OVER THE ROCKIES SUNDAY
NIGHT WITH THE ASSOCIATED SURFACE COLD FRONT MOVING ACROSS THE CWA
SUNDAY NIGHT. THE 00Z ECMWF CONTINUES TO BE SLOWER THAN MOST OF
THE OTHER MODELS WITH THE SHORTWAVE TROUGH BUT HAS TRENDED FASTER
AND WITH LESS AMPLITUDE THAN ITS 12Z RUN. THE 00Z ECMWF IS FAVORED
BY HPC AND SO WILL USE THAT MODEL AGAIN FOR THIS FORECAST...
ESPECIALLY SUNDAY NIGHT. THE LEFT EXIT REGION OF THE 300 MB JET
WILL AFFECT MAINLY THE SOUTH PART OF THE CWA SUNDAY NIGHT AND
MOISTURE WILL INCREASE. FOR NOW WILL CONTINUE THE TREND OF
INCREASING POPS AND SNOW AMOUNTS OVER THE CWA FOR SUNDAY NIGHT.
TWO TO FIVE INCHES OF NEW SNOW ARE POSSIBLE IN THE SNOWY RANGE
SUNDAY NIGHT...WITH AN INCH OR LESS OVER MOST OF THE REST OF THE
SOUTH PART OF THE CWA.
LONG TERM
MONDAY THROUGH FRIDAY
LEANED TOWARDS THE 00Z ECMWF AS IT SEEMS TO BE HANDLING THE SPLIT
FLOW BETTER THAN THE GFS...GEMS OR UKMET. THE UPPER TROUGH WILL
MOVE ACROSS THE CWA ON MON...WITH THE AXIS MOVING EAST OF THE
FORECAST AREA BY EVENING. AN UPPER LOW WILL FORM OVER THE CENTRAL
PLAINS MON NIGHT. COLD TEMPERATURES CAN BE EXPECTED ACROSS THE AREA
MON WITH BRISK WINDS IN THE MORNING. SOME LIGHT SNOW WILL FALL
ACROSS THE AREA...WITH THE BEST CHANCES IN THE MOUNTAINS...BUT
LOWERED POPS DUE TO BEST DYNAMICS SHIFTING EAST AS THE TROUGH AXIS
MOVES FAIRLY QUICK ACROSS THE CWA DURING THE DAY. AN INCH OR TWO
ADDITIONAL SNOW WILL FALL IN THE MOUNTAINS...WITH LESS THAN AN INCH
AT THE LOWER ELEVATIONS. ON TUES...NW FLOW WILL SET UP ALOFT AS AN
UPPER RIDGE BUILD OVER THE WESTERN U.S. DRY CONDITIONS WILL PERSIST
FROM TUES THROUGH FRIDAY. TEMPERATURES WILL MODERATE A FEW DEGREES
ON TUESDAY UNDER PARTLY CLOUDY SKIES. FOR WED...MILD CONDITIONS
WILL OCCUR AND THEN VERY MILD FOR THUR AND FRI WITH DOWNSLOPE WINDS
AND SUNNY SKIES. THE NW UPPER FLOW WILL FLATTEN OUT FRI AS AN UPPER
TROUGH MOVES INLAND FROM THE WEST COAST. A COLD FRONT WILL MOVE
ACROSS THE CWA LATE FRI NIGHT/EARLY SAT MORNING. HIGH TEMPERATURES
WILL BE FROM 5 TO 8 DEGREES BELOW NORMAL MON...AROUND NORMAL TUES
AND WED...AND 5 TO 8 DEGREES ABOVE NORMAL FOR THANKSGIVING DAY AND
THE FRIDAY HOLIDAY SHOPPING DAY.
AVIATION
VFR CONDITIONS WILL CONTINUE OVER THE CWA TODAY. CLOUD BASES WILL
LOWER TONIGHT BUT SHOULD STILL REMAIN VFR. THE SNOWY RANGE WILL BE
OBSCURED TONIGHT. WEST WINDS WILL GUST AS HIGH AS 50 MPH THROUGH
MID MORNING OVER AND NEAR THE MOUNTAINS. THOSE WINDS WILL PRODUCE
TURBULENT CONDITIONS AT AIRPORTS SUCH AS KRWL...KLAR...KDGW...KEAN
AND KCYS. CLOUDS WILL ONCE AGAIN LOWER SUNDAY NIGHT OVER THE CWA
WITH AT LEAST PERIODS OF MVFR CEILINGS AND VISIBILITIES IN SNOW
OVER THE SOUTH PART OF THE CWA. THE SNOWY RANGE WILL BE OBSCURED
AGAIN SUNDAY NIGHT. ICING WILL BE A CONCERN WITH COOLER
TEMPERATURES AND INCREASED MOISTURE.
FIRE WEATHER
WESTERLY SURFACE WINDS WILL GUST AS HIGH AS 50 MPH OVER AND NEAR THE
MOUNTAINS THROUGH MID MORNING. LOWER WIND SPEEDS ARE THEN EXPECTED
THROUGH SUNDAY. MINIMUM RELATIVE HUMIDITIES WILL BE BE BETWEEN 23
AND 30 PERCENT THIS AFTERNOON AND AGAIN SUNDAY AFTERNOON. EVEN
WITH THE GUSTY WINDS...FIRE WEATHER CONCERNS WILL BE MINIMAL FOR
THIS WEEKEND.
CYS WATCHES/WARNINGS/ADVISORIES
WY...NONE.
NE...NONE.
SHORT TERM...WEILAND
LONG TERM...KMD
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