Heavy Snow Warning 12, NWS Missoula, MT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE MISSOULA MT
1054 AM MST MON MAR 3 2008

.DISCUSSION...MOIST UPPER LEVEL TROUGH CURRENTLY MOVING INTO THE
NORTHERN ROCKIES THIS MORNING. AHEAD OF THIS FEATURE A LARGE PLUME
OF MOISTURE HAS INCREASED SHOWER ACTIVITY ACROSS NORTHWEST
MONTANA. ADDITIONALLY...STRONGER WINDS THAN PREVIOUSLY EXPECTED
HAVE DEVELOPED AREA WIDE...MAINLY IMPACTING THE HIGHER TERRAIN AND
MOUNTAIN PASSES. DUE TO SHOWERS BEGINNING EARLIER THAN EXPECTED
...HAVE INCREASED START TIME TO 17Z (10AM MST) FOR THE HAZARDS IN
THE KOOTENAI/CABINET REGION...THE LOWER CLARK FORK REGION AND THE
NORTHERN CLEARWATER MOUNTAINS IN IDAHO. ADDITIONALLY...DUE TO THE
WINDS...HAVE UPGRADED ALL HEAVY SNOW WARNINGS TO WINTER STORM
WARNINGS TO ACCOUNT FOR AREAS OF BLOWING AND DRIFTING SNOW. SHOWER
ACTIVITY WILL GRADUALLY SHIFT SOUTHEASTWARD THROUGH THE DAY WITH
HEAVIER PRECIPITATION STILL EXPECTED TO DEVELOP THIS EVENING.
FORECAST WILL CONTINUE TO BE MONITORED FOR FURTHER UPDATES.

&&

.AVIATION...INCREASING CLOUDS ASSOCIATED WITH AN APPROACHING
UPPER LEVEL TROUGH WILL LEAD RAIN AND SNOW SHOWERS FIRST INTO
NORTHWEST MONTANA AND NORTH CENTRAL IDAHO...AND THEN SOUTHEASTWARD
INTO SOUTHWEST MONTANA BY AFTERNOON. CEILING AND VISIBILITY IS
EXPECTED TO DETERIORATE AREA WIDE THROUGH THE DAY WITH MOUNTAIN
OBSCURATIONS BECOMING WIDESPREAD. EXPECT FLIGHT CONDITIONS TO BE
IMPACTED BY SNOW SHOWERS THROUGH MID DAY ON TUESDAY BEFORE
NOTICEABLE IMPROVEMENT OCCURS.

&&

.PREV DISCUSSION... /ISSUED 418 AM MST MON MAR 3 2008/

..A COLD UPPER LEVEL TROUGH WILL BRING HEAVY SNOW TO THE
MOUNTAINS AND PASSES THIS EVENING THROUGH TOMORROW...

DISCUSSION...A COLD UPPER LEVEL TROUGH IS VISIBLE THIS MORNING
OVER CANADA ON INFRARED SATELLITE IMAGERY. CLOUD COVER ASSOCIATED
WITH THIS SYSTEM HAS ALREADY MOVED INTO NORTHWEST MONTANA WITH
SHOWERS NOT TOO FAR BEHIND VERIFIED BY PRECIPITATION REPORTS
BEGINNING TO SHOW UP IN SOUTHERN BRITISH COLUMBIA THIS MORNING. THE
GFS...NAM AND ECMWF FORECAST MODELS CONTINUE TO BE IN VERY GOOD
AGREEMENT WITH THE PATH OF THE TROUGH AND STORM DYNAMICS. THUS
HAVE ENOUGH FORECAST CONFIDENCE TO ISSUE A VARIETY OF WINTER
WEATHER HIGHLIGHTS. FOR INFORMATION SPECIFIC TO THESE
PRODUCTS...PLEASE VISIT OUR WEBSITE AT WWW.WEATHER.GOV/MISSOULA.

A MIX OF RAIN AND SNOW SHOWERS WILL GRADUALLY SPREAD ACROSS
WESTERN MONTANA MAINLY DURING THE AFTERNOON HOURS. THEN AS NIGHT
FALLS...SHOWERS WILL TURN TO ALL SNOW WITH SNOW TO LIQUID RATIOS
INCREASING THROUGHOUT THE NIGHT. ALTHOUGH THE TROUGH PASSAGE
APPEARS TO BE FAIRLY QUICK...THE COLDEST AIR WILL MOVE IN DURING
THE EARLY MORNING HOURS ON TUESDAY WHICH SHOULD ALLOW MAXIMAL
POTENTIAL COOLING CONSIDERING THE COMPLETE LACK OF INSULATION. ALL
MODELS HAVE BEEN CONSISTENTLY POINTING TOWARDS THE BEST
ACCUMULATING SNOWFALL OVER THE CLEARWATER MOUNTAINS (INCLUDING
LOLO PASS)...THE BITTERROOT MOUNTAINS (INCLUDING THE MONTANA SIDE
OF LOST TRAIL PASS) AND THEN ALONG US-2 FROM ESSEX TO MARIAS PASS
AND THE BOB MARSHALL WILDERNESS AND GLACIER NATIONAL PARK. THESE
LOCATIONS FALL UNDER AREAS OF PERSISTENT LIFT ALONG WITH THE BEST
MOISTURE FEED AS INDICATED BY THE MODELS. AS THE TROUGH SWINGS
THROUGH THE REGION OVERNIGHT TONIGHT THE SURFACE PRESSURE GRADIENT
WILL TIGHTEN AND PROMOTE GUSTY WINDS...ESPECIALLY OVER THE HIGHER
TERRAIN OF THE BITTERROOT MOUNTAINS AND THEN ALONG US-12/MT 141
LEADING UP TO MACDONALD PASS...MT-1 SOUTH TO GEORGETOWN AND ALSO
I-90 FROM GARRISON JCT. SNOW AND BLOWING SNOW MAY ALSO BECOME AN
ISSUE OVER HOMESTAKE PASS EAST OF BUTTE. NUMEROUS SHOWERS WILL
REMAIN ACROSS THE AREA ON TUESDAY AS THE ATMOSPHERE STAYS FAIRLY
UNSTABLE BEHIND THE EXITING TROUGH. HOWEVER SHOWERS WILL GRADUALLY
DISSIPATE...DECREASING IN COVERAGE FROM NORTH TO SOUTH AFTER
SUNDOWN.

TUESDAY NIGHT THROUGH WEDNESDAY NIGHT...THE NORTHERN ROCKIES WILL
REMAIN UNDER NORTHWESTERLY TO NORTHERLY FLOW THROUGH THIS PERIOD. A
FEW DISTURBANCES WILL MAKE THEIR WAY THROUGH THE REGION CAUSING A
FEW HIGH TERRAIN SHOWERS. THEN A RIDGE OF HIGH PRESSURE WILL
BUILD OVER THE AREA ON THURSDAY INTO FRIDAY BRINGING DRYING AND
CLEARING ACROSS THE REGION. MODELS ARE INDICATING THAT THE NORTHERN
ROCKIES WILL BE UNDER AN ACTIVE WEATHER PATTERN DURING THE WEEKEND
INTO NEXT WEEK. BOTH THE GFS AND ECMWF ARE SHOWING SIGNIFICANT
PRECIPITATION BY THE BEGINNING OF NEXT WEEK...SO HAVE INCREASED THE
CHANCE OF PRECIPITATION FOR THIS TIME FRAME. TEMPERATURES FOR THE
EXTENDED PERIOD APPEAR TO BE NEAR SEASON NORMALS.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER STORM WARNING FROM 5 PM THIS AFTERNOON TO 11 PM MST
     TUESDAY FOR BITTERROOT/SAPPHIRE MOUNTAINS-WEST GLACIER
     REGION.

     WINTER STORM WARNING UNTIL 5 PM MST TUESDAY FOR LOWER CLARK FORK
     REGION.

     SNOW ADVISORY UNTIL 5 PM MST TUESDAY FOR KOOTENAI/CABINET REGION.

     SNOW AND BLOWING SNOW ADVISORY FROM 5 PM THIS AFTERNOON TO 11 PM
     MST TUESDAY FOR POTOMAC/SEELEY LAKE REGION.

     SNOW AND BLOWING SNOW ADVISORY FROM 11 PM THIS EVENING TO 11 PM
     MST TUESDAY FOR BUTTE/BLACKFOOT REGION.

ID...WINTER STORM WARNING FROM 4 PM THIS AFTERNOON TO 4 PM PST
     TUESDAY FOR NORTHERN CLEARWATER MOUNTAINS.

     WINTER STORM WARNING FROM 4 PM THIS AFTERNOON TO 10 PM PST
     TUESDAY FOR SOUTHERN CLEARWATER MOUNTAINS.

&&

SHORT TERM...VIA/FOSTER
LONG TERM....DICKERSON/STEGEMILLER
AVIATION...VIA