FXUS65 KMSO 112141
AFDMSO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
241 PM MST FRI DEC 11 2009

...HEAVY SNOW EXPECTED OVER THE CLEARWATER MOUNTAINS AND THE WEST
GLACIER REGION SATURDAY INTO MONDAY...WITH WHITE-OUT CONDITIONS
POSSIBLE ALONG AND NEAR THE DIVIDE SATURDAY NIGHT AND SUNDAY...

.DISCUSSION...
TONIGHT AND SATURDAY...A CYCLONIC NORTHWEST FLOW ALOFT WILL
CONTINUE TO CIRCULATE MID/HIGH LEVEL MOISTURE INTO THE NORTHERN
ROCKIES TONIGHT. ON SATURDAY...A CANADIAN WEATHER SYSTEM WILL BE
APPROACHING NORTHWEST MONTANA...WHILE A SECOND PACIFIC LOW SYSTEM
WILL SPREAD MOISTURE INTO THE AREA FROM THE WEST. THE COMBINATION
OF THESE TWO WEATHER SYSTEMS WILL BRING HEAVY SNOWFALL TO THE
CLEARWATER MOUNTAINS OF NORTH-CENTRAL IDAHO BY AFTERNOON. FARTHER
NORTH OVER THE WEST GLACIER REGION...ARCTIC AIR PLUNGING SOUTH OUT
OF CENTRAL CANADA WILL BEGIN SPILL ACROSS THE PASSES. CONDITIONS
IN THE WEST GLACIER REGION WILL QUICKLY DETERIORATE MARKING A
PERIOD OF PROLONGED COLD...SNOWY...WINDY WEATHER.

SATURDAY NIGHT THROUGH SUNDAY NIGHT...WEATHER CONDITIONS WILL
CONTINUE TO DETERIORATE THROUGHOUT MUCH OF WESTERN MONTANA AND THE
IDAHO PANHANDLE AS WARMER MOIST AIR ORIGINATING FROM A PACIFIC
SYSTEM TO THE SOUTH INTERACTS WITH THE COLD CANADIAN WEATHER
SYSTEM. WHILE AN INTRUSION OF ARCTIC AIR IS EXPECTED INTO WESTERN
MONTANA...SOME UNCERTAINTY STILL EXISTS AS TO THE EXACT POSITION
AND MOVEMENT OF THE ARCTIC BOUNDARY ACROSS WEST-CENTRAL MONTANA.
THE LOCATION OF THIS BOUNDARY HAS HUGE IMPLICATIONS AS THE AMOUNT
OF SNOW...WIND AND COLD AIR IMPACTS WESTERN MONTANA.
WATCHES/WARNINGS HAVE BEEN ISSUED HIGHLIGHTING AREAS OF CONCERNS.
WARNED AREAS ARE EXHIBITING A GREATER SENSE OF FORECAST
CONFIDENCE IN HEAVY SNOW AND WHITE-OUT CONDITIONS. IN ADDITION...THE
WEST GLACIER REGION WILL HAVE TO DEAL WITH DANGEROUSLY COLD WIND
CHILLS ONCE THE NORTHEAST WINDS KICK IN SATURDAY NIGHT.

SOUTHWEST FLOW DEVELOPS OVER THE PACIFIC NORTHWEST BEHIND THE
EXITING TROUGH. THIS WILL BRING IN WARMER AIR ALOFT BUT GETTING THIS
WARMER AIR INTO THE TIGHTER VALLEYS WILL BE TOUGH WITH THE FRESH
SNOW COVER. THIS AIR WILL BE FAIRLY MOIST KEEPING THE POTENTIAL
FOR MORE SNOW AND POSSIBLY SOME RAIN IN CENTRAL IDAHO AND ALONG
THE MONTANA IDAHO BORDER. TEMPERATURES WILL SLOWLY MODERATE WITH
HIGHS NEARING AVERAGE BY LATE NEXT WEEK.

&&

.AVIATION...LIGHT SURFACE WINDS AND HIGH LEVEL CLOUDS CAN BE
EXPECTED THROUGH FRIDAY EVENING FOR THE NORTHERN ROCKIES. LIGHT
SNOW AND LOWERED CEILINGS WILL START TO SPREAD ACROSS NORTHWEST
MONTANA...BUT THE SIGNIFICANT ACCUMULATING SNOW...IMPACTING VIS
AND CEILINGS...IS NOT EXPECTED UNTIL AFTER THE 12/12Z-15Z WINDOW.
EXPECT SNOW TO CONTINUE AND ALSO SPREAD ACROSS WESTERN MONTANA
AND NORTH-CENTRAL IDAHO THROUGH THE WEEKEND...AS A MOIST PACIFIC
SYSTEM AND ANOTHER ARCTIC PUSH INTERACT OVER THE REGION. THE
PRESSURE GRADIENT WILL TIGHTEN WITH THE PUSH OF THE ARCTIC AIR
ALONG THE CONTINENTAL DIVIDE INCREASING SURFACE WINDS...MAINLY
IMPACTING KGPI AND SMALLER NORTHERN AIRFIELDS SATURDAY EVENING. STRONG
WINDS COUPLED WITH ANY NEW SNOW ACCUMULATION WILL INTRODUCE THE
POSSIBILITY FOR LOCAL WHITEOUT CONDITIONS ACROSS NORTHWEST MONTANA
LATER SATURDAY AFTERNOON.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER STORM WATCH FROM SATURDAY EVENING THROUGH MONDAY
     AFTERNOON FOR LOWER CLARK FORK REGION.

     WINTER STORM WARNING FROM 5 PM SATURDAY AFTERNOON THROUGH
     5 PM MONDAY AFTERNOON FOR LOLO PASS.

     WINTER STORM WATCH FROM SATURDAY AFTERNOON THROUGH LATE SUNDAY 
     NIGHT FOR FLATHEAD/MISSION VALLEYS.

     WINTER STORM WARNING FROM 11 AM SATURDAY TO 5 AM MST MONDAY FOR 
     WEST GLACIER REGION.

     WINTER STORM WATCH FROM SATURDAY EVENING THROUGH MONDAY 
     AFTERNOON FOR POTOMAC/SEELEY LAKE REGION.

     WINTER STORM WATCH FROM SATURDAY EVENING THROUGH MONDAY 
     AFTERNOON FOR BUTTE/BLACKFOOT REGION.

ID...WINTER STORM WARNING FROM 10 AM SATURDAY TO 4 PM PST MONDAY FOR 
     NORTHERN CLEARWATER MOUNTAINS-SOUTHERN CLEARWATER MOUNTAINS.

&&

$$

SHORT TERM...MSJ/FELSCH
LONG TERM....LOEFFELBEIN
AVIATION...NOLTE