127
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
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