000 FXUS65 KBYZ 101015 CCA AFDBIL SOUTH CENTRAL AND SOUTHEAST MONTANA FORECAST DISCUSSION...CORRECTED CCF NATIONAL WEATHER SERVICE BILLINGS MT 415 AM MDT SAT MAY 10 2003 .SHORT TERM...VALID FOR TDY AND SUN... WILL LET HIGHLIGHTS REMAIN IN PLACE UNTIL NOON TODAY. BROAD AREA OF STEADY LIGHT TO MODERATE RAIN/SNOW FALLING ACROSS MOST OF THE CENTRAL ZONES THIS MORNING. UPPER LEVEL DIFFLUENCE AND LOW LEVEL UPSLOPE/ISENTROPIC FORCING ARE RESPONSIBLE FOR THIS. BOTH MODELS HANDLING THIS SCENARIO REMARKABLY WELL...BUT PREFER THE GFS AS IT LINES UP THE BEST WITH CURRENT PRECIPITATION PATTERN. MODELS INDICATE A SLOW SHIFTING OF UPPER LEVEL DIVERGENCE TO THE SOUTH AND EAST THIS AFTERNOON AS UPPER LOW OVER SOUTHERN UTAH TRANSITIONS INTO THE SOUTHERN PLAINS PULLING DYNAMICS THAT DIRECTION. IN ADDITION WEAK RIDGING MOVES INTO THE NORTHERN AND WESTERN PORTION OF THE STATE AHEAD OF SHORTWAVE TROF OVER PACIFIC NORTHWEST PUSHING DRIER LOW LEVEL AIR IN FROM THE NORTH. THE RESULT OF THESE SHIFTS IS THAT PRECIPITATION WILL TAPPER OFF FROM NORTH TO SOUTH TODAY. FOR THIS MORNING WILL GO WITH CATEGORICAL POPS CENTRAL...LIKELY ACROSS THE WEST AND CHANCE IN THE EAST...WILL SEE AROUND 4 OR 5 INCHES OF SNOW ABOVE 6000 FEET THIS MORNING. BY LATE THIS AFTERNOON WILL CONTINUE WITH LIKELY POPS IN SHERIDAN COUNTY AND REMOVE POPS ACROSS THE NORTHERN ZONES. WILL NOT STRAY FAR FROM YESTERDAYS HIGHS AS CLOUD COVER WILL BE CONSIDERABLE UNTIL LATE AFTERNOON...AND WITH THE WET GROUND WILL NOT SEE MUCH WARMING EVEN IF A BIT OF SUNSHINE IS REALIZED THIS AFTERNOON DUE TO LOW LEVEL EVAPORATION. WILL END ALL POPS BY LATE THIS EVENING. SKIES SHOULD BECOME PARTLY CLOUDY ACROSS THE AREA TONIGHT ALLOWING TEMPERATURES TO FALL BELOW FREEZING ACROSS THE WESTERN HALF OF THE CWA AND ALONG EASTERN RIVER VALLEYS. FOR SUNDAY...PARTLY CLOUDY CONDITIONS IN THE MORNING WILL GIVE WAY TO SHOWERS AND AFTERNOON THUNDERSTORMS IN THE WESTERN TWO THIRDS OF THE CWA. AREAS WHERE CONSIDERABLE PRECIPITATION HAS FALLEN OVER THE PAST FEW DAYS WILL TAKE LITTLE IN THE WAY OF SUNSHINE TO PRODUCE SHOWER ACTIVITY. BY LATE AFTERNOON A NEGATIVELY TILTED 700MB TROF MOVES INTO THE SOUTHWEST ZONES TO PROVIDE ENOUGH EXTRA LIFT FOR ISOLATED THUNDERSTORMS TO DEVELOP. TEMPERATURES SHOULD REBOUND INTO THE UPPER 50S DESPITE INCREASING AFTERNOON CLOUD COVER. CHAMBERS .LONG TERM...VALID FOR MON...TUE...WED...THU...FRI... GFS WAS FAIRLY CONSISTENT WITH ITS PREVIOUS RUN AND AGREED WITH CANADIAN MODEL AND UKMET IN BRINGING UPPER SHORTWAVE TROUGH INTO REGION SUN NIGHT THROUGH MON NIGHT. ETA WAS SIMILAR TO ECMWF WITH A ZONAL/RIDGING PATTERN EARLY IN EXTENDED PERIOD. GFS/CANADIAN REFLECTED GOING FORECAST FAIRLY WELL SO FOLLOWED THEM FOR THIS PACKAGE. ADDED CHANCE SHOWERS/THUNDERSTORMS TO W AREAS SUN EVE DUE TO APPROACHING TROUGH. SOME UNCERTAINTY ABOUT TIMING OF SYSTEM AS LATEST GFS POSITION OF SURFACE LOW DIFFERED FROM PREVIOUS RUN SO LEFT E DRY SUN NIGHT AND MON. KEPT TUE THROUGH THU DRY AS SYSTEM MOVES E OF REGION AND UPPER RIDGING MOVES OVER AREA. RAISED TEMPS THU/THU NIGHT WITH THERMAL RIDGE OVER AREA. GFS BROUGHT SHORTWAVE NE TOWARD AREA ON FRI AND HAD ASSOCIATED SURFACE SYSTEM OVER WY. WENT WITH 30 POPS ENTIRE AREA. THERMAL RIDGE AND SW FLOW WILL CONTINUE OVER REGION ON FRI SO RAISED TEMPS SEVERAL DEGREES. ARTHUR TDY SUN MON TUE WED THU FRI ------------------------------------------------------- BIL 049 032/056 035/061 042/065 042/067 040/068 044/070 +/R 03/T 22/T 20/E 00/B 00/B 03/T LVM 048 028/055 031/062 036/060 036/065 033/066 038/065 6/R 03/T 33/T 30/E 00/B 03/T 33/T HDN 048 032/056 034/060 041/066 041/068 042/069 043/071 +/R 02/T 00/E 00/E 00/B 00/B 03/T MLS 049 034/057 036/062 041/067 041/069 044/070 045/072 6/R 02/T 00/E 00/B 00/B 00/B 03/T 4BQ 047 034/057 035/058 040/062 040/064 044/065 045/067 +/R 32/T 00/E 00/E 00/B 00/B 03/T BHK 048 033/058 033/059 040/063 040/065 045/066 045/068 3/R 00/B 00/E 00/B 00/B 00/B 03/T SHR 046 031/056 033/057 041/062 041/064 041/065 042/067 +/R 42/T 00/E 00/B 00/B 00/B 03/T .BYZ... MT...HEAVY SNOW WARNING FOR THIS MORNING FOR ZONES MTZ034...MTZ038... .MTZ041...MTZ056. .WINTER WEATHER ADVISORY FOR SNOW THIS MORNING FOR ZONES MTZ034... .MTZ038...MTZ056. WY...HEAVY SNOW WARNING FOR THIS MORNING FOR ZONE WYZ098. $$