Issued by NWS Billings, MT
000 FXUS65 KBYZ 210946 AFDBYZ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE BILLINGS MT 246 AM MST SAT NOV 21 2009 .SHORT TERM...VALID FOR TDY AND SUN... PROGRESSIVE WEATHER PATTERN TO BE OVER THE FORECAST AREA THIS WEEKEND AND INTO THE DAY MONDAY. LARGE TROUGH CURRENTLY MOVING ONTO AND ACROSS THE PACIFIC NORTHWEST AND INTO THE NORTHERN ROCKIES REGION. SATELLITE SHOWING A DEEP SOUTHWEST FLOW ALOFT ACROSS THE STATE WITH PLENTY OF MID AND HIGH LEVEL CLOUDS MOVING ACROSS OUR AREA. RADARS ACROSS IDAHO SHOWING CONVECTIVE TYPE PRECIP NOW MOVING UP INTO THE SOUTHWEST PORTIONS OF THE STATE AND APPROACHING OUR SOUTHWEST MOUNTAINS. A FEW LIGHTING STRIKES HAVE ALSO BEEN OBSERVED ACROSS CENTRAL IDAHO INDICATING FAIRLY STRONG DYNAMICS AND INSTABILITY FOR THIS TIME OF YEAR. WILL CONTINUE WITH LIKELY POPS FOR THE SOUTHWEST MOUNTAINS WITH CHANCE POPS FOR THE CENTRAL AND WESTERN ZONES THIS MORNING WITH BEST CHANCES WEST. SNOWFALL COULD BE HEAVY FOR A SHORT WHILE OVER THE BEARTOOTH/ABSAROKA MOUNTAINS RESULTING IN SEVERAL INCHES OF SNOW THIS MORNING. A COLD FRONT IS EXPECTED TO MOVE ACROSS THE CENTRAL AND WESTERN PORTIONS OF THE FORECAST AREA BETWEEN 12-15Z AND THE EASTERN ZONES BY EARLY AFTERNOON. DYNAMICS AND MOISTURE DECREASE BEHIND THE FRONT RESULTING IN ONLY CHANCE POPS OVER THE MOUNTAINS...WITH SLIGHT CHANCES SPREADING EAST DURING THE AFTERNOON. EXPECT WINDY CONDITIONS TO DEVELOP BEHIND THE FRONT ACROSS THE PLAINS. THIS DOWNSLOPE FLOW WILL HELP TO INHIBIT PRECIP ACROSS THE PLAINS. DYNAMICS ALSO LOOK TO WEAKEN AS THE TROUGH MOVES ACROSS THE FORECAST AREA SO FOR NOW WILL CONTINUE WITH ONLY A SLIGHT CHANCE FOR SHOWERS ACROSS THE PLAINS TODAY. RIDGING QUICKLY MOVES ACROSS THE CENTRAL PORTIONS OF THE STATE TONIGHT BRINGING AN END TO ANY SHOWER ACTIVITY. HOWEVER...LEE- SIDE TROUGHING DEVELOPS ACROSS THE FORECAST AREA LATE TONIGHT IN ADVANCE OF THE NEXT FAST MOVING TROUGH ACROSS THE PACIFIC NORTHWEST. PRESSURE GRADIENTS TIGHTEN ONCE AGAIN ACROSS THE UPPER YELLOWSTONE AND STILLWATER VALLEYS LATE TONIGHT THROUGH SUNDAY MORNING. AS A RESULT...HAVE PUT SUB-ADVISORY WINDS ACROSS THESE AREAS FOR THIS TIME. ANOTHER COLD FRONT MOVES THROUGH LATE SUNDAY MORNING WHICH WILL SHUT THE STRONGER WINDS OFF IN THE LIVINGSTON AREA. THE GFS CONTINUES TO SHIFT THE BULK OF PRECIP SOUTH ACROSS WESTERN WYOMING SO WILL CONTINUE WITH JUST A SLIGHT CHANCE TO CHANCE POPS FOR OUR PLAINS WITH LIKELY POPS RETURNING FOR THE MOUNTAINS. TEMPERATURES TODAY WILL BE COOLER BUT STILL REMAIN ABOVE NORMAL BEFORE MORE NORMAL TEMPERATURES ARRIVE FOR SUNDAY WITH READINGS IN THE UPPER 30S TO LOWER 40S. HOOLEY .LONG TERM...VALID FOR MON...TUE...WED...THU...FRI... A PROGRESSIVE PATTERN DEVELOPS OVER THE FORECAST AREA FOR THE EXTENDED. 3 SYSTEMS WILL IMPACT THE AREA NEXT WEEK...BUT RIDGING ON THANKSGIVING BRINGS WARM...DRY AND BREEZY CONDITIONS TO THE AREA. THE FIRST SYSTEM IS OVER THE AREA MONDAY...SHIFTING SOUTHEAST MONDAY NIGHT. INCREASED POPS OVER EASTERN AREAS MONDAY AS WELL AS KEEPING SOME POPS FOR WESTERN MOUNTAIN AND FOOTHILLS LOCATIONS. TEMPERATURES DON`T LOOK TERRIBLY COLD WITH THIS SYSTEM IN THE LOWER 40S...AND COULD BE A TAD COOLER IN SOME LOCATIONS WHERE CLOUDS AND PRECIPITATION OCCUR. AN ALBERTA CLIPPER SYSTEM QUICKLY MOVES THROUGH EASTERN AREAS ON TUESDAY AFTERNOON...THOUGH NOT BEFORE TEMPERATURES CLIMB INTO THE LOW TO MID 40S. IF TIMING OF THE CLIPPER IS FASTER WILL NEED TO COOL EASTERN AREAS 5 TO 10 DEGREES. RIDGING BUILDS INTO THE AREA WEDNESDAY AND THURSDAY WITH DOWNSLOPE WIND GRADIENT IN PLACE PUSHING TEMPERATURES BACK INTO THE 50S. THE FINAL SYSTEM ARRIVES FRIDAY OR SATURDAY DEPENDING ON MODEL PREFERENCE. THE ECMWF DIGS THE LATE WEEK SYSTEM FURTHER TO THE SOUTH AND BRINGS COOLER AIR INTO THE AREA FRIDAY AFTERNOON...WHILE THE GFS IS LESS DEEP WITH THE SYSTEM KEEPING THE ENERGY OVER MONTANA AND WYOMING AND BRINGS COLDER AIR IN FOR SATURDAY. 00Z ENSEMBLE DATA SUPPORTIVE OF THE OPERATIONAL GFS KEEPING TROF FURTHER NORTH...AND TRENDED FORECAST TOWARD THE GFS FOR THE LATER PERIODS...INCLUDING INCREASING POPS AND LOWERING TEMPERATURES JUST A TAD. CHAMBERS && .AVIATION... A COLD FRONT WILL MOVE THROUGH THE AREA THIS MORNING. MVFR WITH OCCASIONAL IFR CONDITIONS WILL DEVELOP EARLY THIS MORNING OVER WESTERN AREAS...INCLUDING KLVM...LASTING INTO EARLY AFTERNOON. AS THE FRONT MOVES EASTWARD CONDITIONS SHOULD REMAIN PRIMARILY VFR WITH LOCAL MVFR CONDITIONS ASSOCIATED WITH ISOLATED SHOWER ACTIVITY. MOUNTAIN OBSCURATION WILL LAST INTO LATE AFTERNOON FOR WESTERN MOUNTAINS...AND INTO THIS EVENING FOR THE BIG HORN MOUNTAINS...AS CONDITIONS IMPROVE FROM WEST TO EAST. GUSTY NORTHWEST WINDS ARE EXPECTED ACROSS THE AREA BEHIND THE FRONT THIS AFTERNOON. CHAMBERS && .PRELIMINARY POINT TEMP/POPS... TDY SUN MON TUE WED THU FRI ------------------------------------------------------- BIL 048 023/040 028/042 024/046 030/051 036/055 035/051 2/W 02/W 21/B 11/B 00/B 00/U 02/W LVM 041 020/036 020/035 018/043 025/050 029/054 031/046 4/J 04/J 32/J 12/W 10/B 00/B 03/W HDN 051 024/043 024/044 019/047 025/052 031/057 030/053 2/W 02/W 22/W 12/W 00/B 00/U 02/W MLS 053 022/045 024/043 020/047 023/046 030/056 031/051 1/N 02/W 22/W 22/W 10/B 00/U 01/U 4BQ 051 023/042 021/042 019/045 022/046 028/056 029/051 1/N 22/W 22/J 12/W 00/B 00/U 01/U BHK 053 021/045 021/040 019/043 020/040 024/054 027/051 1/N 11/B 22/J 22/W 10/B 00/U 01/U SHR 047 020/043 022/039 014/046 024/050 028/056 028/051 3/W 22/W 22/J 11/B 00/B 00/U 01/B && .BYZ WATCHES/WARNINGS/ADVISORIES... MT...NONE. WY...NONE. && $$ WEATHER.GOV/BILLINGS
