Winter Storm Watch 5, NWS Missoula, MT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
355 AM MST MON FEB 21 2011

...ACCUMULATING SNOWS TONIGHT THROUGH MID WEEK...

...ARCTIC AIR INTRUSION WEDNESDAY NIGHT THROUGH THURSDAY NIGHT...

.DISCUSSION...WINTER WILL MOST DEFINITELY CONTINUE THIS WEEK AS A
COUPLE INTERESTING WEATHER FEATURES WILL IMPACT THE REGION. A
SLOW MOVING PACIFIC TROUGH IS STILL EXPECTED TO MOVE OVER THE
NORTHERN ROCKIES TONIGHT. IT WILL HAVE THE MOST IMPACT ON
NORTHERN IDAHO AND NORTHWEST MONTANA. THE GREATEST SNOW AMOUNTS
ARE TARGETED OVER CLEARWATER COUNTY AND NORTHERN IDAHO
COUNTY...AND THUS A WINTER STORM WATCH HAS BEEN ISSUED FOR THAT
AREA FROM THIS EVENING THROUGH TUESDAY NIGHT. THAT'S NOT TO SAY
THE SNOW WILL END COMPLETELY AT THAT TIME...BUT IT SHOULD TAPER
ENOUGH BY WEDNESDAY MORNING TO MAINLY BE A NUISANCE.

SNOW ACCUMULATIONS IN NORTHWEST MONTANA TONIGHT AND TUESDAY WILL
NOT BE AS GREAT...BUT STILL ENOUGH TO CAUSE PROLONGED PERIODS OF
DIFFICULT OR SLOW TRAVEL CONDITIONS. THEREFORE A WINTER WEATHER
ADVISORY HAS BEEN ISSUED FOR THAT REGION. AGAIN...SNOW IS NOT
EXPECTED TO END AFTER TUESDAY...BUT IMPACTS SHOULD LESSEN BRIEFLY
TUESDAY NIGHT AND WEDNESDAY.

LIGHT SNOW...BUT POTENTIALLY STILL SLICK ROADS...WILL DEVELOP AT
TIMES OVER WEST CENTRAL AND SOUTHWEST MONTANA INCLUDING THE I-90
AND HIGHWAY 93 CORRIDORS EAST AND SOUTH OF MISSOULA.

AN ARCTIC BOUNDARY IS EXPECTED TO TRACK SOUTHWARD AND PULL COLD
CANADIAN AIR THROUGH GAPS IN THE CONTINENTAL DIVIDE WEDNESDAY NIGHT
THROUGH THURSDAY NIGHT. THIS SET UP IS A BIT DIFFERENT THAN PREVIOUS
ARCTICS SO FAR THIS WINTER. FIRST...THERE IS NO SUBTROPICAL MOISTURE
FEED WITH THIS FEATURE...WHICH SHOULD RESULT IN LESSER AMOUNTS OF
LIQUID PRECIPITATION EXPECTED. SECOND...COLD AIR ALREADY IN PLACE
WILL RESULT IN HIGHER SNOW RATIOS AS PRECIPITATION INTENSIFIES
ALONG THE ARCTIC BOUNDARY...RESULTING IN LIGHT AND FLUFFY
SNOWFALL. AS EASTERLY WINDS INTENSIFY WEDNESDAY NIGHT...BLOWING
AND DRIFTING OF FLUFFY SNOW...RAPIDLY REDUCED VISIBILITIES AND
DANGEROUS WIND CHILLS WILL BE THE MAIN IMPACT WITH THIS FEATURE.
FIRST ALONG HIGHWAY 2 FROM MARIAS PASS THROUGH BADROCK CANYON AND
INTO THE FLATHEAD VALLEY. THEN ALONG THE CONTINENTAL DIVIDE IN
SOUTHWEST MONTANA...INCLUDING INTERSTATE 90 FROM GARRISON JUNCTION
THROUGH HELLGATE CANYON TO DOWNTOWN MISSOULA AND HIGHWAY 200 FROM
BONNER TOWARD LINCOLN. A HAZARDOUS WEATHER OUTLOOK HAS BEEN ISSUED
FOR THIS FEATURE IN ORDER TO AVOID CONFUSION WITH CURRENT WINTER
WEATHER HIGHLIGHTS.

THE NORTHERN ROCKIES WILL REMAIN UNDER A LONGWAVE TROUGH FRIDAY
THROUGH THE WEEKEND. AS A RESULT...VALLEY TEMPERATURES WILL BE
SLOW TO WARM EACH DAY AS STABLE CONDITIONS PERSIST THROUGH THIS
TIME.

&&

.AVIATION...A WEAK UPPER WAVE WILL CROSS THE AREA IN THE WESTERLY
FLOW THIS MORNING. THIS COULD CAUSE LIGHT SNOW WITH VISIBILITIES
DOWN TO 3 MILES AT TIMES AT KMSO/KBTM/KSMN. THE POTENTIAL FOR THIS
WILL DIMINISH BY EARLY AFTERNOON. A MUCH MORE ORGANIZED SYSTEM
WILL MOVE OVER THE AREA FROM THE NORTHWEST TONIGHT. THIS WILL
BEGIN AN ACTIVE SNOW PATTERN PERSISTING BEYOND TUESDAY. KGPI AND
KMSO WILL BE MOST IMPACTED.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER WEATHER ADVISORY FROM 11 PM THIS EVENING TO 5 PM MST
     TUESDAY FLATHEAD/MISSION VALLEYS...KOOTENAI/CABINET
     REGION...LOWER CLARK FORK REGION...WEST GLACIER REGION.

     WINTER WEATHER ADVISORY FROM 5 AM TO 11 PM MST TUESDAY
     BITTERROOT/SAPPHIRE MOUNTAINS...POTOMAC/SEELEY LAKE REGION.

ID...WINTER WEATHER ADVISORY FROM 4 AM TUESDAY TO 4 AM PST WEDNESDAY
     OROFINO/GRANGEVILLE REGION.

     WINTER STORM WATCH FROM THIS EVENING THROUGH LATE TUESDAY NIGHT
     NORTHERN CLEARWATER MOUNTAINS...SOUTHERN CLEARWATER
     MOUNTAINS.

&&

SHORT TERM...WHITMORE
LONG TERM....DEROSA
AVIATION...WHITMORE