Heavy Snow Warning 22, NWS Missoula, MT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
419 AM MST WED DEC 19 2007

...MODERATE TO HEAVY SNOW EXPECTED OVER LOOKOUT...LOLO AND LOST
TRAIL PASSES THIS EVENING THROUGH THURSDAY. SEE HTTP://WWW.WEATHER.GOV/MISSOULA
OR GTFWSWMSO FOR DETAILS...

.DISCUSSION...TODAY...INSTABILITY BEHIND THE SHORT WAVE THAT MOVED
THROUGH TUESDAY WILL CONTINUE TO PRODUCE MOUNTAIN SNOW SHOWERS IN
NORTHWEST AND WEST CENTRAL MONTANA MUCH OF THE DAY. ALL
ADVISORIES AND WARNINGS THROUGH THIS MORNING HAVE BEEN CANCELLED
AS WIDESPREAD SNOW ACCUMULATIONS ARE NO LONGER EXPECTED. LOCALIZED
LIGHT SNOWFALL AMOUNTS ARE POSSIBLE. SOUTHERLY WINDS OVERNIGHT
HAVE KEPT VALLEY TEMPERATURES STEADY IN THE MID TO UPPER 30S.
CONTINUED SOUTHERLY WINDS AND RELATIVELY WARMER AIR WILL KEEP VALLEY
TEMPERATURES ABOVE FREEZING THROUGH THE DAY PRODUCING A MIX OF
RAIN AND SNOW.

THIS EVENING THROUGH THURSDAY WILL BRING ANOTHER ROUND OF
WIDESPREAD MOUNTAIN SNOWS. THIS SYSTEM WILL BE THE STRONGEST OF
THE WEEK AS A STRONG UPPER LEVEL JET WILL HELP ENHANCE OROGRAPHIC
EFFECTS TO PRODUCE MODERATE TO HEAVY MOUNTAIN SNOWS...ESPECIALLY
ABOVE 4000 FEET. LOOKOUT...LOLO AND LOST TRAIL PASSES ARE ALL
EXPECTED TO RECEIVE SIGNIFICANT SNOW MAKING TRAVEL DIFFICULT.
VALLEYS IN MONTANA WILL ONCE AGAIN BE UNDER THE INFLUENCE OF SOME
DOWNSLOPING WITH THE SOUTHWEST TO WEST FLOW...THUS LIGHT
PRECIPITATION IS EXPECTED. PRECIPITATION IN THE FLATHEAD...
MISSION...MISSOULA AND BITTERROOT VALLEYS WILL CONTINUE TO BE A
MIX OF RAIN AND SNOW THROUGH THE EVENING HOURS AS COLDER AIR IS
NOT EXPECTED TO MOVE IN UNTIL AFTER MIDNIGHT THURSDAY.

THE COLD FRONT ASSOCIATED WITH THIS STORM WILL MOVE THROUGH BY LATE
AFTERNOON THURSDAY. POST FRONTAL INSTABILITY AND COOLER
TEMPERATURES WILL PRODUCE SCATTERED SHOWERS ACROSS THE NORTHERN
ROCKIES.

FRIDAY AND FRIDAY NIGHT...THE NORTHWEST FLOW ALOFT CONTINUES WITH
A RIDGE OF HIGH PRESSURE APPROACHING FROM THE WEST. THE AIR MASS
WILL GRADUALLY DRY...BUT REMAIN QUITE COLD WITH SURFACE-BASED
INVERSIONS DEVELOPING.

SATURDAY...THE UPPER RIDGE FINALLY MOVES OVER THE INTERMOUNTAIN
REGION WITH COLD STABLE CONDITIONS EXPECTED. SOME MOISTURE WILL
BEGIN TO SPREAD INTO NORTHWEST MONTANA DURING THE DAY...OFFERING
SOME CLOUDS AND LOW CHANCE POPS.

SATURDAY NIGHT THROUGH TUESDAY....THE LARGE SCALE PATTERN REMAINS
QUITE PROGRESSIVE AND UNSETTLED. TIMING OF INDIVIDUAL WEATHER
FEATURES BECOMES MORE DIFFICULT. HOWEVER...THE OVERALL PATTERN DOES
SUGGEST PERIODIC SURGES OF PACIFIC MOISTURE TO IMPACT THE REGION.

&&

.AVIATION...SCATTERED RAIN AND SNOW SHOWERS ARE EXPECTED TODAY
AND WILL BECOME WIDESPREAD LATE THIS AFTERNOON AND TONIGHT IN THE
MOUNTAINS. PERIODS OF REDUCED VISIBILITY AND OCCASIONALLY LOWERED
CEILINGS ARE POSSIBLE WITH THESE SHOWERS AT KMSO AND KGPI. EXPECT
MOUNTAIN OBSCURATIONS DUE TO SNOW SHOWERS THROUGH THURSDAY
MORNING.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...HEAVY SNOW WARNING FROM 5 PM THIS AFTERNOON TO 3 PM MST THURSDAY
     ABOVE 4000 FEET FOR BITTERROOT/SAPPHIRE MOUNTAINS-LOWER
     CLARK FORK REGION.

ID...HEAVY SNOW WARNING FROM 4 PM THIS AFTERNOON TO 2 PM PST THURSDAY
     ABOVE 4000 FEET FOR NORTHERN CLEARWATER MOUNTAINS-SOUTHERN
     CLEARWATER MOUNTAINS.

     WINTER STORM WATCH FROM THIS AFTERNOON THROUGH THURSDAY
     AFTERNOON ABOVE 4000 FEET FOR EASTERN LEMHI COUNTY.

&&

SHORT TERM...RPN
LONG TERM....RPN
AVIATION...RPN

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