Winter Storm Watch 2, NWS Missoula, MT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
242 PM MST MON JAN 5 2009

.DISCUSSION...SNOWFALL IS EXPECTED TO CONTINUE ACROSS WESTERN
MONTANA AND CENTRAL IDAHO FOR THE REMAINDER OF TODAY BUT WILL
GRADUALLY LESSEN AS THE NIGHT PROGRESSES. IMMEDIATE CONCERNS WITH
SNOWFALL AMOUNTS DURING THE OVERNIGHT PERIOD WILL BE THE POTENTIAL
FOR BANDING TO OCCUR LATER TONIGHT...AFFECTING LOCATIONS OF WEST-
CENTRAL MONTANA THROUGH EARLY TUESDAY MORNING. THERE IS ENOUGH
STABILITY PRESENT ALONG WITH STRONG WINDS NECESSARY TO GENERATE
SNOW BANDS...BUT UNCERTAINTY LIES IN WHETHER THE POSSIBLE BANDS
WILL REMAIN STATIONARY OR MOVE AS THE WINDS SHIFT OVERNIGHT. THIS
WILL AFFECT SNOW AMOUNTS DEPENDING ON THE LOCATION OF THESE SNOW
BANDS SHOULD THEY SET UP. THAT BEING SAID...THE NEXT CONCERN IS
FOR DURING THE DAY TUESDAY WITH THE ONSET OF THE NEXT
WEATHER SYSTEM. WARM AIR WILL BEGIN TO CREEP INTO SOUTH CENTRAL
IDAHO...WARMING THE VALLEYS IN THE CAMAS PRAIRIE AND SOUTHERN
CLEARWATER REGION OF IDAHO BY TUESDAY EVENING. AT THIS
TIME...MOISTURE WILL BEGIN TO ENTER FROM THE WEST AND FALL IN THE
FORM OF SNOW. AS UPPER LEVELS BEGIN TO WARM THIS SNOW MAY HAVE A
BRIEF CHANGEOVER TO FREEZING RAIN LATE IN THE EVENING TUESDAY.

TUESDAY NIGHT THROUGH THURSDAY...A LOW AMPLITUDE WILL NOT PROVIDE
MUCH PROTECTION AS AN ONSLAUGHT OF PACIFIC MOISTURE BARRELS INTO
THE NORTHERN ROCKIES. THE JET STREAM ARCS NORTHWARD INTO CANADA
ALLOWING FOR AN INFLUX OF WARMER LOW-LEVEL AIR FROM THE SOUTH. A
MESSY FORECAST ON TAP AS PRECIPITATION STARTS OUT AS SNOW OR A MIX
OF RAIN AND SNOW...POSSIBLY WITH A BRIEF PERIOD OF FREEZING
RAIN...BEFORE CHANGING OVER TO ALL RAIN IN THE VALLEYS TUESDAY
NIGHT. MOUNTAINS WILL BE GETTING HAMMERED WITH SNOW TUESDAY NIGHT
AND WEDNESDAY AS SNOW LEVELS GRADUALLY RISING FROM ROUGHLY 3500
FEET TUESDAY EVENING TO BETWEEN 5000 AND 6000 FEET BY WEDNESDAY
AFTERNOON. A WINTER STORM WARNING HAS BEEN ISSUED TO HIGHLIGHT
THIS SNOW POTENTIAL. IN ADDITION TO PRECIPITATION...STRONG WINDS
ALOFT COMBINED WITH A TIGHTENING SURFACE PRESSURE GRADIENT WILL
PROMOTE STRONG SOUTHERLY WINDS IN THE LOWER HELLS CANYON AND CAMAS
PRAIRIE AREAS OF NORTH-CENTRAL IDAHO. THIS SOUTHERLY LOW LEVEL
FLOW WILL ALSO WARM SURFACE TEMPERATURES INTO THE UPPER 30S TO
MID 40S IN MANY VALLEY LOCATION RESULTING IN CONSIDERABLE SNOW
MELT AND POTENTIAL SMALL STREAM AND URBAN FLOODING PROBLEMS.

FRIDAY THROUGH MONDAY...A COOL DOWN IS EXPECTED BY THE END OF THE
WORK WEEK INTO THE COMING WEEKEND. A WEAK FRONTAL SYSTEM WILL PASS
THROUGH THE AREA EARLY FRIDAY BRINGING SOME LIGHT SNOW
ACCUMULATIONS TO THE AREA WITH TEMPERATURES RETURNING TO
SEASONABLE NORMS. A NORTHWEST FLOW ALOFT WILL SET UP OVER THE NORTHERN
ROCKIES OVER THE WEEKEND INTO EARLY NEXT WEEK WITH THE HIGH
PRESSURE RIDGE ALOFT REMAINING ALONG THE WEST COAST. PERIODS OF SNOW
IS POSSIBLE...MAINLY FOCUSED OVER THE MOUNTAINS IN THIS TIME FRAME.

&&

.HYDROLOGY...AS SNOW TRANSITIONS TO RAIN IN MOST VALLEYS OF NORTH
CENTRAL IDAHO AND WESTERN MONTANA TUESDAY NIGHT INTO WEDNESDAY...PONDING
OF WATER AND FLOODING OF ROAD WAYS WILL BE A CONCERN. IN ADDITION...
WARMING TEMPERATURES AND GUSTY SOUTHERLY WINDS WILL ALSO AIDE IN
MELTING SNOW AT THE LOWER ELEVATIONS THROUGH THURSDAY. SNOW LOADING ON
BUILDINGS WITH HEFTY SNOW WILL ALSO BE A CONCERN...ESPECIALLY IN
NORTHWEST MONTANA.

&&

.AVIATION...CURRENT STORM SYSTEM WILL CONTINUE TO BRING SNOW
THROUGH THIS EVENING IN NORTH CENTRAL IDAHO AND WESTERN MONTANA.
SNOW WILL KEEP VISIBILITIES REDUCED THROUGH THE OVERNIGHT IN MOST
OF CENTRAL IDAHO. KMSO WILL SEE LOW CEILINGS AND VISIBILITIES
CONTINUING BUT WITH GRADUAL IMPROVEMENT LATER THIS
EVENING...THOUGH INTERMITTENT SNOW SHOWERS WILL REDUCE FLIGHT
CONDITIONS FROM TIME TO TIME OVERNIGHT. MVFR WILL BE THE DOMINANT
FLIGHT CATEGORY FOR ALL TERMINALS THROUGH THE OVERNIGHT HOURS.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER STORM WATCH FROM TUESDAY AFTERNOON THROUGH WEDNESDAY
     MORNING FOR BITTERROOT/SAPPHIRE MOUNTAINS.

     WINTER STORM WARNING UNTIL 11 PM MST THIS EVENING FOR
     BITTERROOT/SAPPHIRE MOUNTAINS.

     WINTER STORM WARNING UNTIL 11 PM MST THIS EVENING FOR
     POTOMAC/SEELEY LAKE REGION-WEST GLACIER REGION.

     WINTER STORM WATCH FROM TUESDAY AFTERNOON THROUGH WEDNESDAY
     AFTERNOON FOR WEST GLACIER REGION.

     WINTER STORM WATCH FROM TUESDAY AFTERNOON THROUGH WEDNESDAY
     MORNING FOR KOOTENAI/CABINET REGION-LOWER CLARK FORK REGION-
     POTOMAC/SEELEY LAKE REGION.

     WINTER STORM WARNING UNTIL 11 PM MST THIS EVENING FOR LOWER
     CLARK FORK REGION.

ID...WINTER WEATHER ADVISORY UNTIL 4 PM PST THIS AFTERNOON FOR LOWER
     HELLS CANYON/SALMON RIVER REGION.

     WIND ADVISORY FROM 1 PM TUESDAY TO 4 PM PST THURSDAY FOR LOWER
     HELLS CANYON/SALMON RIVER REGION-OROFINO/GRANGEVILLE REGION.

     WINTER STORM WATCH FROM TUESDAY AFTERNOON THROUGH WEDNESDAY
     MORNING FOR NORTHERN CLEARWATER MOUNTAINS-SOUTHERN
     CLEARWATER MOUNTAINS.

     WINTER STORM WARNING UNTIL 10 PM PST THIS EVENING FOR SOUTHERN
     CLEARWATER MOUNTAINS.

     WINTER STORM WARNING UNTIL 4 PM PST THIS AFTERNOON FOR
     OROFINO/GRANGEVILLE REGION.

     WINTER STORM WARNING UNTIL 10 PM PST THIS EVENING FOR NORTHERN
     CLEARWATER MOUNTAINS.

&&

SHORT TERM...ALLEGRETTO
LONG TERM....MSJ
AVIATION...ALLEGRETTO

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