Issued by NWS Great Falls, MT
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000 FXUS65 KTFX 210544 AFDTFX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE GREAT FALLS MT 1100 PM MST FRI NOV 20 2009 UPDATED AVIATION SECTION .UPDATE... PROGRESSION OF SHORTWAVE TROUGH SEEMS TO HAVE SLOWED A BIT MORE SO HAVE LOWERED OVERNIGHT POPS ALONG THE CONTINENTAL DIVIDE AND PORTIONS OF CENTRAL MONTANA. WEAK COLD FRONT STILL EXPECTED TO PUSH THROUGH THE AREA OVERNIGHT...WITH TEMPERATURES FALLING CLOSER TO SEASONAL AVERAGES ON SATURDAY. LIGHT SNOW WILL DEVELOP ALONG THE CONTINENTAL DIVIDE AFTER MIDNIGHT WITH LIGHT SNOW ACCUMULATION OF 1 TO 2 INCHES POSSIBLE. GUSTY WINDS AND FALLING SNOW MAY CAUSE TIMES OF REDUCED VISIBILITIES ALONG THE MOUNTAIN PASSES. MLS && .AVIATION... UPDATED 0600Z. AN UPPER LEVEL TROF OF LOW PRESSURE WILL MOVE EASTWARD TOWARDS CENTRAL/SOUTHWEST MONTANA LATE TONIGHT. EXPECT GENERALLY OVERCAST CONDITIONS UNTIL 00Z SUNDAY...WITH THE CHANCE FOR SNOW DEVELOPING AFTER 09Z OVER WESTERN AND SOUTHWESTERN MONTANA. THE BEST CHANCE FOR SNOW ON SATURDAY WILL BE ON SATURDAY FROM 13Z TO 20Z IN THE BOZEMAN AND WEST YELLOWSTONE AREA. THUS IFR/LIFR CONDITIONS WILL BE POSSIBLE AT TIMES OVER THIS REGION. MOUNTAINS WILL BE OBSCURED AT TIMES THROUGHOUT THE REGION THOUGH THE PERIOD. ADDITIONALLY...EXPECT SURFACE WINDS TO BECOME VERY BREEZY OVER NORTH CENTRAL MONTANA AFTER 09Z SATURDAY. EXPECT A GENERALLY CLEARING TREND WITH LIGHTER WINDS EXPECTED FROM 0Z SUNDAY UNTIL 6Z SUNDAY. BRUSDA && .PREV DISCUSSION... /ISSUED 500 PM MST FRI NOV 20 2009/ AN UPPER LEVEL TROUGH OF LOW PRESSURE...CURRENTLY MOVING ONTO THE WEST COAST...WILL TRACK EAST ACROSS THE REGION LATE TONIGHT THROUGH EARLY SATURDAY AFTERNOON. A BRIEF PERIOD OF RIDGING BEHIND THIS INITIAL SYSTEM SATURDAY NIGHT WILL BE QUICKLY FOLLOWED BY ANOTHER TROUGH MOVING INTO THE NORTHERN ROCKIES ON SUNDAY. A COLD FRONT ASSOCIATED WITH THE INITIAL TROUGH WILL MOVE EAST ACROSS THE REGION LATE TONIGHT. MOISTURE WILL INCREASE ALONG AND BEHIND THE FRONT WITH SNOW SHOWERS DEVELOPING AFTER MIDNIGHT OVER THE ROCKY MTN FRONT AND GLACIER PARK AREA...SPREADING EAST OVER SW MT LATE TONIGHT THROUGH SATURDAY MORNING. WESTERLY DOWNSLOPE FLOW WILL LIMIT PRECIPITATION CHANCES OVER THE PLAINS...HOWEVER OCCASIONAL SNOW SHOWERS OVER THE MTNS MAY BRING ACCUMULATIONS OF 1 TO 3 INCHES TO ELEVATIONS ABOVE 6000 FT. SIGNIFICANT COOLING WILL OCCUR BEHIND THE FRONT OVER THE FORECAST AREA ON SATURDAY...HOWEVER THIS WILL ONLY MEAN TEMPERATURES DROPPING BACK TO SEASONAL AVERAGES. MODELS HAVE BEEN TRENDING SLOWER AND DEEPER WITH THE SECOND SYSTEM TO ARRIVE ON SUNDAY AND THIS SYSTEM WILL LIKELY BRING ANOTHER ROUND OF SNOW SHOWERS TO THE MTNS WITH PERHAPS A LITTLE BETTER CHANCE OF SNOW SHOWERS EXTENDING FARTHER EAST ONTO THE PLAINS DUE TO DEEPER MOISTURE AND MARGINAL INSTABILITY. ACCUMULATING SNOW AGAIN LOOKS TO BE CONFINED MAINLY TO THE MOUNTAINOUS AREAS WITH ANOTHER 1 TO 3 INCHES POSSIBLE ABOVE 6000 FEET. GUSTY SOUTHWEST WINDS WILL SHIFT TO THE WEST BEHIND TONIGHTS COLD COLD FRONT WITH WESTERN PORTIONS OF NORTH CENTRAL MT AND MTN PASSES SEEING THE STRONGEST WINDS WITH GUSTY WINDS CONTINUING THROUGH THE WEEKEND. COMBINATION OF GUSTY WINDS AND PERIODS OF LIGHT SNOW MAY HAVE AN IMPACT TO TRAVELERS OVER MTN PASSES THIS WEEKEND AND WILL HIGHLIGHT THIS IN THE HAZARDOUS WEATHER OUTLOOK. HOENISCH SUNDAY NIGHT THROUGH FRIDAY NIGHT...THE MEDIUM RANGE MODELS ARE IN DECENT AGREEMENT WITH MOVING THE UPPER LEVEL TROUGH OUT OF THE AREA SUNDAY NIGHT INTO MONDAY FOR A DECREASING CHANCE OF PRECIPITATION. AS THE COLD FRONT ASSOCIATED WITH THIS SYSTEM ALSO MOVES THROUGH THE AREA SUNDAY NIGHT...WEST WINDS WILL BECOME BREEZY AND GUSTY. THESE WINDS WILL DECREASE ON MONDAY AS THE FRONT EXITS THE AREA TO THE EAST...AS TEMPERATURES CONTINUE TO HOVER AROUND NORMAL. THE UPPER LEVEL HIGH PRESSURE RIDGE OVER THE EASTERN PACIFIC WILL THEN BEGIN TO BUILD BACK INTO THE WESTERN UNITED STATES MONDAY NIGHT THROUGH TUESDAY NIGHT. HOWEVER...A WEAK SHORTWAVE WILL MOVE DOWN THE EAST SIDE OF THE RIDGE AND THROUGH MONTANA DURING THIS TIME. THIS WILL BRING A CHANCE OF PRECIPITATION TO THE AREA ONCE AGAIN...BUT IT WILL MAINLY BE IN THE MOUNTAINS AS WESTERLY DOWNSLOPE WINDS WILL INCREASE WITH THE PASSAGE OF THE SHORTWAVE. THESE WINDS WILL ALSO WARM TEMPERATURES TO ABOUT 5 TO 10 DEGREES ABOVE NORMAL ON TUESDAY. THE UPPER LEVEL RIDGE WILL THEN BUILD INTO MONTANA TUESDAY NIGHT THROUGH THURSDAY (THANKSGIVING DAY)...WHICH WILL KEEP ONLY A SLIGHT CHANCE OF PRECIPITATION IN THE WESTERN MONTANA MOUNTAINS. ALSO...WESTERLY DOWNSLOPE WINDS WILL INCREASE ONCE AGAIN THROUGH THIS PERIOD...WHICH WILL KEEP TEMPERATURES ABOUT 10 DEGREES ABOVE NORMAL. THE MODELS BEGIN TO DIVERGE FOR THE REMAINDER OF THIS FORECAST PERIOD. THE GFS IS MUCH FASTER THAN THE ECMWF WITH BRINGING THE NEXT PACIFIC WEATHER SYSTEM THROUGH THE AREA. THE GFS MOVES IT THROUGH MONTANA ON FRIDAY...WHILE THE ECMWF IS 12 TO 24 HOURS SLOWER IN MOVING IT THROUGH FRIDAY NIGHT INTO SATURDAY. WITH THIS UNCERTAINTY...HAVE GONE WITH A WIDESPREAD CHANCE OF PRECIPITATION FOR FRIDAY AND FRIDAY NIGHT...WITH THE CHANCE BEING HIGHER THAN CLIMATOLOGICAL AVERAGES. HAVE ALSO COOLED TEMPERATURES CLOSER TO NORMAL. THE INCREASES IN WIND SUNDAY NIGHT...MONDAY NIGHT AND TUESDAY...AND THANKSGIVING DAY INTO FRIDAY WILL NEED TO BE MONITORED FOR POSSIBLE HIGHLIGHTS. COULSTON && .PRELIMINARY POINT TEMPS/POPS... GTF 36 42 25 41 / 0 10 0 30 CTB 31 40 24 39 / 10 10 0 20 HLN 29 41 23 41 / 20 20 0 40 BZN 23 37 13 37 / 20 50 0 40 WEY 22 29 8 30 / 50 90 30 90 DLN 27 32 14 34 / 40 50 0 50 HVR 29 49 21 45 / 0 10 10 20 LWT 32 42 20 38 / 0 10 0 20 && .TFX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ WEATHER.GOV/GREATFALLS
