Issued by NWS Cheyenne, WY
000 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
