Winter Weather Advisory 6, NWS Missoula, MT

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE MISSOULA MT
435 AM MST MON JAN 16 2012

..MAJOR WINTER WEATHER IMPACTS STARTING TONIGHT THROUGH FRIDAY...

..SEVERAL FEET OF MOUNTAIN SNOW WILL IMPACT ALL PASSES ALL
WEEK WITH SIGNIFICANT VALLEY SNOW LIKELY...

.DISCUSSION...COLD TEMPERATURES WILL BE THE MAIN FEATURE TODAY
AND WILL BE IN PLACE AS PRECIPITATION INCREASES TODAY AND TONIGHT
ACROSS THE NORTHERN ROCKIES. SNOW HAS BEGUN FALLING IN CENTRAL
IDAHO...IMPACTING WHITEBIRD GRADE AS WELL AS THE CAMAIS PRAIRIE
REGION. SNOW IS ALSO IMPACTING THE MOUNTAINS ALONG THE MONTANA AND
IDAHO BORDER. ACCUMULATIONS WILL BE LIGHT TO MODERATE TODAY AND
TONIGHT...BUT THESE INITIAL CONDITIONS SIGNAL THE BEGINNING OF
WHAT WILL PROVE TO BE A DIFFICULT AND SERIOUS WEATHER WEEK.

ON TUESDAY THE FIRST IN A SERIES OF STRONG WINTER STORMS WILL MOVE
ACROSS THE REGION. SNOW WILL FIRST BEGIN ACROSS NORTHWEST MONTANA
AND GRADUALLY SPREAD SOUTH AS AN EXTREMELY STRONG UPPER LEVEL JET
WILL USHER IN INCREASING AMOUNTS OF MOISTURE. AND WITH SNOW
LEVELS ALREADY TO VALLEY BOTTOMS...EXPECT THE FIRST ROUND OF LIGHT
TO MODERATE SNOW TO PRODUCE THE FOLLOWING SNOWFALL TOTALS:

NORTHWEST MONTANA VALLEYS...3 TO 6 INCHES
IDAHO/MONTANA MOUNTAINS...8 TO 16 INCHES
WEST CENTRAL MONTANA VALLEYS...3 TO 5 INCHES

A BREEZY WIND MAY ACCOMPANY THE SNOWFALL IN THE NORTHERN FLATHEAD
VALLEY AND RESULT IN SOME BLOWING SNOW THROUGH BADROCK CANYON.

THE SECOND WINTER STORM...EXPECTED INTO MONTANA WEDNESDAY AND
PERSISTING WELL INTO THURSDAY CONTINUES TO BE ADVERTISED IN THE
MODELS AS SIGNIFICANTLY STRONGER AND FAR MORE POTENT GIVEN ITS
HIGHLY ANOMALOUS MOISTURE TAP OVER THE PACIFIC. FORECAST MODELS
SUGGEST THAT ON WEDNESDAY A VERY STRONG ARCTIC AIR MASS EAST OF
THE DIVIDE AND NORTH OF THE CANADIAN BORDER WILL BEGIN TO SEEP
INTO NORTHWEST MONTANA. THE ARCTIC AIR MASS WILL MAKE IT INTO THE
FLATHEAD AND MISSION VALLEYS AND MODELS ARE SHOWING THAT IT SHOULD
REACH MISSOULA LATE THURSDAY AFTERNOON. MEANWHILE THE APPROACHING
WARM...MOIST AIR FROM THE PACIFIC WILL PUSH OVER THE ARCTIC AIR
AND PRODUCE AN AREA WHICH WILL BE VERY FAVORABLE FOR EXTREMELY
HEAVY...PROLONGED SNOWFALL. THIS TREND HAS BEEN SHOWN OVER THE
PAST SEVERAL DAYS AND BY NUMEROUS MODELS...THUS FORECAST
CONFIDENCE IS HIGH IN THIS SCENARIO. HOWEVER WHERE THE HIGHEST
AMOUNTS ARE EXPECTED IS STILL IN QUESTION...WITH THE BEST REGION
CENTERED SOMEWHERE BETWEEN THE NORTHERN BITTERROOT AND KALISPELL.
NONETHELESS...THE DETAILS AT THIS POINT ARE PURE SEMANTICS AS ITS
HIGHLY LIKELY THAT THIS AREA WILL SEE SNOWFALL TOTALS OF 6 TO 8
INCHES WITH A NARROW BAND OF UP TO 16 INCHES POSSIBLE WHERE THE
BEST CONVERGENCE SETS UP. THIS HAS ALL THE MAKINGS OF A MAJOR
WINTER STORM AND AS SUCH A WINTER STORM WATCH HAS BEEN ISSUED FOR
ALMOST ALL OF WESTERN MONTANA AND CENTRAL IDAHO...INCLUDING THE
MISSOULA AND KALISPELL AREAS.

WARMER AIR WILL CONTINUE TO RIDE OVER COLDER AIR PREVIOUSLY
TRAPPED IN VALLEYS THURSDAY MORNING. WILL WILL GRADUALLY CHANGE
SNOW OVER TO RAIN...BUT PERIODS OF FREEZING RAIN ARE POSSIBLE AS
VALLEYS OF WESTERN MONTANA UNDERGO THIS TRANSITION. THIS MAY
CAUSE SOME MAJOR IMPACTS DURING THIS TRANSITION...THUS HEAVILY
IMPACTING THE THURSDAY MORNING COMMUTE. THE EXACT MOMENT OF THIS
TRANSITION IS ALWAYS VERY DIFFICULT TO TIME...PARTICULARLY WITH A
SIGNIFICANT WEATHER SYSTEM SUCH AS THIS. BUT FOR NOW IT APPEARS
THAT THE MISSOULA VALLEY...THE I-90 CORRIDOR...AND POINTS
NORTH WILL TRANSITION FROM SNOW TO RAIN DURING THE LATE MORNING
HOURS OF THURSDAY. KALISPELL MAY REMAIN SNOW FOR MUCH OF THE
MORNING...BUT SHOULD THEN TURN TO A RAIN/SNOW MIX BY THE
AFTERNOON. POCKETS OF FREEZING RAIN SHOULD BE ANTICIPATED IN THE
MORNING HOURS UNTIL THE CHANGEOVER IS COMPLETE.

BEYOND THIS THE FORECAST BECOMES EVEN MORE DIFFICULT: WITH WARM
AIR CONSISTENTLY BEING USHERED IN FROM THE SOUTHWEST...AND WITH
COLDER AIR TRYING TO ENTER FROM EAST OF THE DIVIDE...THERE MAY BE
A PERIOD OF PROLONGED PRECIPITATION CHANGES OVERNIGHT THURSDAY
INTO FRIDAY MORNING FOR NORTHWEST MONTANA. IT COULD BE THAT FRIDAY
MORNING THE FLATHEAD VALLEY WILL SEE ADDITIONAL PERIODS OF RAIN AND
SNOW MIXING ON AND OFF...MAKING FOR YET ANOTHER DIFFICULT MORNING
COMMUTE. FURTHER SOUTH INTO WEST CENTRAL MONTANA AND CENTRAL IDAHO
PRECIPITATION WILL REMAIN AS RAIN DURING THE AFTERNOON HOURS.

COLD AIR RETURNS FRIDAY NIGHT AND SATURDAY ACROSS THE NORTHERN
ROCKIES...CHANGING PRECIPITATION BACK TO ALL SNOW FOR THE WEEKEND.
WITH MUCH COLDER AIR EXPECTED AFTER A PROLONGED TIME PERIOD OF WET
SNOW MIXED WITH RAIN...ROADWAYS ARE LIKELY TO FREEZE WITH AREAS OF
BLACK ICE COVERED BY FALLING SNOW SATURDAY MORNING. WITH A MESS
LIKE THIS...THOSE WHO MUST TRAVEL DURING THIS PERIOD SHOULD
EXERCISE EXTREME CAUTION.

ANOTHER IMPACT EXPECTED ARE STRONG SOUTH TO SOUTHWEST WINDS ACROSS
THE OROFINO/GRANGEVILLE AND WHITEBIRD HILL REGION OF CENTRAL
IDAHO...ALONG WITH THE SALMON AND LEADORE REGION OF LEMHI COUNTY.
WINDS COULD BECOME QUITE STRONG SATURDAY MORNING ACROSS THIS
REGION.

&&

.AVIATION...SCATTERED SNOW SHOWERS WILL REMAIN AN ISSUE THROUGH
THIS MORNING WITH BRIEF REDUCTIONS TO CEILINGS AND VISIBILITIES
THROUGHOUT THE REGION DUE TO THESE SNOW SHOWERS. CONFIDENCE IS
MODERATE TO HIGH THAT PROLONGED PERIOD OF SNOW AND OBSCURED
TERRAIN WILL CONTINUE FOR THE DURATION OF THIS WEEK. MAJOR
AVIATION IMPACTS INCLUDING HEAVY SNOW FOR ALL TERMINALS SHOULD BE
ANTICIPATED TO BEGIN AS EARLY AS TUESDAY AND LINGER THROUGH THE
WEEKEND. STAY TUNED.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER STORM WATCH FROM LATE TONIGHT THROUGH THURSDAY MORNING
     BITTERROOT/SAPPHIRE MOUNTAINS.

     WINTER STORM WATCH FROM LATE TUESDAY NIGHT THROUGH THURSDAY
     MORNING MISSOULA/BITTERROOT VALLEYS.

     WINTER STORM WATCH FROM THIS EVENING THROUGH THURSDAY MORNING
     LOWER CLARK FORK REGION.

     WINTER STORM WATCH FROM LATE TUESDAY NIGHT THROUGH THURSDAY
     MORNING FLATHEAD/MISSION VALLEYS.

     WINTER STORM WATCH FROM LATE TONIGHT THROUGH THURSDAY MORNING
     WEST GLACIER REGION.

     WINTER STORM WATCH FROM THIS EVENING THROUGH THURSDAY MORNING
     KOOTENAI/CABINET REGION.

     WINTER STORM WATCH FROM LATE TONIGHT THROUGH THURSDAY MORNING
     POTOMAC/SEELEY LAKE REGION.

ID...WINTER WEATHER ADVISORY UNTIL 1 PM PST THIS AFTERNOON
     OROFINO/GRANGEVILLE REGION.

     WINTER STORM WATCH FROM LATE TONIGHT THROUGH THURSDAY MORNING
     NORTHERN CLEARWATER MOUNTAINS...SOUTHERN CLEARWATER
     MOUNTAINS.

     WINTER STORM WATCH FROM LATE TONIGHT THROUGH THURSDAY MORNING
     FOR LOST TRAIL PASS.

&&

DISCUSSION...ALLEGRETTO/DICKERSON
AVIATION...ALLEGRETTO