000
FXUS65 KMSO 032143
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
243 PM MST WED DEC 3 2008
.DISCUSSION...DRIER AIR ADVANCING IN FROM THE NORTH HAS ERODED
AWAY MOST OF THE CLOUD COVER OVER NORTHERN IDAHO AND NORTHWEST
MONTANA. THE EXCEPTIONS ARE A FEW LOCATIONS ALONG THE CONTINENTAL
DIVIDE IN MONTANA AND NORTHEAST OREGON THROUGH THE SOUTHERN
CLEARWATER MOUNTAINS OF IDAHO. THESE LOCATIONS MAY PICK UP SOME LIGHT
PRECIPITATION THROUGH THE EVENING. COLD AIR EAST OF THE ROCKY
MOUNTAINS IS CONTINUING TO BUILD AS HIGH PRESSURE SLIDES SOUTH OUT
OF CANADA. SOME HIGH VALLEYS SUCH AS ESSEX HAVE ALREADY STARTED TO
SEE WINDS PICK UP AS THIS COLD AIR MAKES ITS WAY WEST OF THE
DIVIDE. A SHORTWAVE MOVING THROUGH TONIGHT WILL PLACE ITS MOST
SIGNIFICANT FORCING OVER CENTRAL IDAHO BUT IT SHOULD HELP PULL THE
FRONT INTO WESTERN MONTANA. VALLEYS ORIENTED EAST/WEST WILL SEE
THE STRONGEST WINDS AS THE COLDER AIR RUSHES THROUGH
THEM...SPECIFICALLY THE COLUMBIA FALLS...OVANDO...BUTTE...AND THE
I-90 CORRIDOR EAST OF MISSOULA. THE BEST POTENTIAL FOR
PRECIPITATION WILL BE ALONG THE FRONT AND OVER CENTRAL IDAHO.
UPSLOPE FLOW OVER THE GRANGEVILLE AND WHITE BIRD GRADE WILL
ENHANCE THE POTENTIAL FOR SNOWFALL IN THESE AREAS THROUGH THE
FIRST HALF OF TONIGHT...BUT LIMITED MOISTURE SHOULD KEEP AMOUNTS
FROM REACHING OVER AN INCH. DRIER AIR MOVING IN ON THURSDAY WILL
ALLOW SUNSHINE DURING THE DAY AND GOOD RADIATIONAL COOLING
THURSDAY NIGHT...ESPECIALLY IN THE BUTTE AREA.
FRIDAY THROUGH SATURDAY WILL BE AN INTERESTING WEATHER PERIOD AS
STABLE LOWER LEVELS WILL ALLOW THE COLD AIR MASS TO REMAIN TRAPPED
IN MANY VALLEYS OF WESTERN MONTANA. THE UPPER RIDGE OF HIGH
PRESSURE WILL FLATTEN FRIDAY NIGHT AND ALLOW WARM MOIST PACIFIC
AIR TO STREAM INTO THE INTERMOUNTAIN REGION. WITH THE COLD AIR
TRAPPED IN VALLEYS AND WARM MOIST AIR RIDING OVER THIS AIR...THE
STAGE IS BEING SET FOR A POSSIBLE LIGHT FREEZING RAIN AND SNOW
EVENT SATURDAY THROUGH SATURDAY EVENING. HAVE ISSUED A HAZARDOUS
WEATHER OUTLOOK TO HIGHLIGHT THE POTENTIAL.
SUNDAY THROUGH MONDAY WILL BE UNDER A PROGRESSIVE NORTHWEST FLOW
WITH A TROUGH DIGGING OVER WESTERN MONTANA MONDAY. THE MODELS ARE
DIFFERING WITH THE STRENGTH AND TIMING OF THE TROUGH...BUT ARE
PERSISTENT WITH PLENTY OF MOISTURE STREAMING OVER THE INTERMOUNTAIN
REGION THIS TIME FRAME. THE SNOW LEVELS ON SUNDAY ARE EXPECTED TO
FALL TO THE VALLEY FLOORS AND THE TROUGH DIGS OVER THE AREA...BUT
BEST ACCUMULATING SNOW IS EXPECTED TO BE CONFINED TO THE MOUNTAINS
AND PASSES. TRAVELERS SHOULD STAY ALERT TO LATER FORECAST FOR
POSSIBLE IMPACTS TO MANY MOUNTAIN PASSES.
BEYOND MONDAY NIGHT...MODELS VARY ON UPPER LEVEL PATTERN...BUT A
PROGRESSIVE FLOW IS ANTICIPATED. TEMPERATURES WILL BE FLUCTUATING
THROUGH THE EXTENDED AS WEAK INTRUSIONS OF MODIFIED ARCTIC AIR IS
QUICKLY REPLACED WITH WARM PACIFIC AIR FROM PASSING UPPER LEVEL
WAVES.
&&
.AVIATION...DRIER AIR MOVING IN HAS CLEARED MOST LOW LEVEL CLOUDS
OUT OF NORTHWEST MONTANA WITH THE EXCEPTION ALONG THE CONTINENTAL
DIVIDE AND CENTRAL IDAHO. WINDS WILL PICK UP TONIGHT IN EAST/WEST
VALLEYS AS COLD AIR SURGES IN FROM THE EAST. SOME PRECIPITATION IS
POSSIBLE TONIGHT AS THIS COLD AIR MOVES IN AND A SYSTEM PASSES
OVER CENTRAL IDAHO. THIS WILL ALSO INCREASE MID LEVEL CLOUDS.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...NONE.
ID...NONE.
&&
$$
SHORT TERM...LOEFFELBEIN
LONG TERM....BAUCK/MEAD
AVIATION...LOEFFELBEIN
