000
FXUS65 KMSO 070240
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
740 PM MST TUE JAN 6 2009
.UPDATE...
A QUICK UPDATE IS FORTHCOMING TO MOVE UP THE START TIME FOR THE
FREEZING RAIN ADVISORY FOR THE MISSOULA AND BITTERROOT VALLEYS.
VALLEY SNOW HAS RECENTLY TRANSITIONED TO RAIN AND FREEZING RAIN.
&&
.PREV DISCUSSION... /ISSUED 228 PM MST TUE JAN 6 2009/
..MAJOR HAZARD IMPACTS OF APPROACHING WEATHER SYSTEM CAN BE
EXPECTED BEGINNING TONIGHT THROUGH THURSDAY...
DISCUSSION...ANOTHER VERY MOIST PACIFIC WEATHER SYSTEM IS
APPROACHING THE NORTHERN ROCKIES AND WILL BRING WITH IT A HOST
OF HAZARD IMPACTS OVER THE NEXT FEW DAYS.
THE MOST IMPORTANT CONCERN WITH THIS SYSTEM WILL BE PRECIPITATION
CHANGES THAT WILL OCCUR TONIGHT AND THE HAZARDOUS DRIVING
CONDITIONS THAT WILL RESULT. A DEEP AMOUNT OF MOISTURE COMBINED
WITH AN INFLUX OF WARM AIR WILL AID IN PRECIPITATION CHANGES
ACROSS CENTRAL IDAHO AND WESTERN MONTANA BEGINNING TONIGHT.
TEMPERATURES HAVE ALREADY BEGUN TO CLIMB THROUGHOUT THE DAY ARE
NOT EXPECTED TO DECREASE MUCH OVERNIGHT TONIGHT. AS PRECIPITATION
BEGINS TO FALL THIS EVENING THROUGH A WARMING COLUMN OF AIR ABOVE
A SHALLOW COLDER LAYER OF AIR NEAR THE SURFACE...FREEZING
PRECIPITATION WILL OCCUR. VALLEY LOCATIONS OF CENTRAL IDAHO HAVE
ALREADY BEGUN TO EXPERIENCE THE CHANGE- OVER...BEGINNING AS A
SLUSHY-SNOW MIXED WITH INTERMITTENT PERIODS OF RAIN. THIS
CHANGEOVER WILL CONTINUE INTO WEST CENTRAL MONTANA BY
MIDNIGHT...FOLLOWED FINALLY BY PORTIONS OF NORTHWEST MONTANA IN
THE EARLY HOURS OF WEDNESDAY MORNING.
AS A RESULT...HAZARDOUS ICY ROAD CONDITIONS CAN BE EXPECTED
OVERNIGHT AND FOR THE MORNING COMMUTE ON WEDNESDAY. AS
TEMPERATURES CONTINUE RISING...PRECIPITATION WILL CHANGE TO ALL
RAIN BY MID- MORNING WEDNESDAY. EVEN THOUGH TEMPERATURES WILL BE
WARM ENOUGH FOR RAIN - ANY RAIN THAT FALLS ON ICE AND SNOW COVERED
ROADS WILL FREEZE AND CREATE VERY HAZARDOUS DRIVING CONDITIONS.
MOTORISTS ARE URGED TO USE CAUTION AT THIS TIME AS MANY LOCALIZED
ROADWAYS MAY BE ESPECIALLY PRONE TO FREEZING. THIS WILL VARY
DEPENDING ON THEIR CURRENT ICE/SNOW COVER OR IF THEY ARE LOCATED
IN AREAS THAT ARE SHADED FROM THE SUN.
THE SECOND CONCERN WILL BE SNOW LOADING FROM RAINFALL EXPECTED
THROUGHOUT THE DAY WEDNESDAY AND CONTINUING THROUGH THURSDAY.
SNOW THAT IS CURRENTLY LOCATED ON THE ROOFS OF STRUCTURES WILL BE
SUSCEPTIBLE TO SNOW LOADING FROM RAIN ABSORPTION AND POSE THE RISK
FOR STRUCTURAL DAMAGE. HOMES AND BUSINESSES WITH SNOW-COVERED
ROOFS SHOULD TAKE PREPARATION FOR THIS CONTINUOUS RAINFALL.
WEDNESDAY NIGHT AND THURSDAY...THE RELATIVELY WARM AND VERY WET
AIR MASS WILL CONTINUE TO MOVE THROUGH A LOW AMPLITUDE RIDGE.
SNOW LEVELS WILL REMAIN QUITE HIGH WITH ACCUMULATING SNOW CONFINED
TO THE MOUNTAINS...WITH SLUSHY WET SNOW OVER THE PASSES.
FINALLY...A PACIFIC WEATHER SYSTEM WILL ADVANCE INLAND COOLING
TEMPERATURES ALOFT AND GRADUALLY LOWERING SNOW LEVELS THURSDAY. IN
ADDITION... WINDS IN THE LOWER HELLS CANYON AND CAMAS PRAIRIE
AREAS OF NORTH- CENTRAL IDAHO REMAIN WINDY. IN FACT...THE APPROACH
AND PASSAGE OF OF THIS NEXT SYSTEM COULD VERY WELL CAUSE WINDS
GUSTS TO PEAK AS HIGH AS 55 MPH.
THURSDAY NIGHT THROUGH TUESDAY...THE UPPER LEVEL PATTERN
TRANSITIONS TO A MORE NORTHWESTERLY FLOW ALOFT. A COOLER AND
CONSIDERABLY DRIER AIR MASS IS ANTICIPATED DURING THIS PERIOD.
CHANCES OF PRECIPITATION WILL STILL IN THE FORECAST MIX TARGETING
THE NORTHERN IDAHO PANHANDLE AND NORTHWEST MONTANA.
AVIATION...MAIN THREAT FOR TERMINALS TONIGHT WILL BE POTENTIAL
FOR FREEZING PRECIPITATION BEGINNING THIS EVENING AND CONTINUING
OVERNIGHT INTO MID-MORNING WEDNESDAY. CURRENT MODEL SOUNDINGS
SHOW INCREASING MOISTURE WITH A SATURATED PROFILE WHICH SHOULD
ALLOW FOR SNOW AND LOWERED VISIBILITIES/CEILINGS AT KMSO/KGPI/KSMN
LATER THIS EVENING. WITH WARMING MID LEVEL TEMPERATURES AND
COOLER AIR AT THE SURFACE...EXPECT SNOW THAT FALLS TONIGHT TO
CHANGE OVER TO RAIN THROUGH THE OVERNIGHT HOURS. KMSO WILL BE THE
FIRST TO SEE THIS CHANGEOVER...BEGINNING SOMETIME AROUND
5Z...WITH KGPI CHANGING TO RAIN BY 9Z. KSMN AND KBTM WILL SEE SNOW
LATER TONIGHT AND WILL LIKELY REMAIN SNOW AT LEAST THROUGH THE
OVERNIGHT HOURS BEFORE ALSO CHANGING TO RAIN BY 12Z. AS TEMPS
CONTINUE TO RISE THROUGH WEDNESDAY MORNING...RAIN FALLING ON
COLD/SNOW COVERED RUNWAYS WILL ALSO BE A CONCERN. IFR CONDITIONS
SHOULD PREVAIL AS LOWERED CEILINGS AND VISIBILITIES ARE EXPECTED
TO PERSIST THROUGH THE DAY WEDNESDAY.
HYDROLOGY...AS SNOW TRANSITIONS TO RAIN IN MOST VALLEYS OF NORTH
CENTRAL IDAHO AND WESTERN MONTANA TONIGHT INTO WEDNESDAY...PONDING
OF WATER AND FLOODING OF ROAD WAYS WILL BE A CONCERN. IN ADDITION...
WARMING TEMPERATURES AND GUSTY SOUTHERLY WINDS WILL ALSO AID IN
MELTING SNOW AT THE LOWER ELEVATIONS THROUGH THURSDAY.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER STORM WARNING UNTIL 11 AM MST WEDNESDAY ABOVE 4500 FEET
FOR BITTERROOT/SAPPHIRE MOUNTAINS.
FREEZING RAIN ADVISORY UNTIL 10 AM MST WEDNESDAY FOR
MISSOULA/BITTERROOT VALLEYS.
WINTER STORM WARNING UNTIL 11 AM MST WEDNESDAY FOR
FLATHEAD/MISSION VALLEYS.
WINTER STORM WARNING UNTIL 5 PM MST WEDNESDAY ABOVE 3500 FEET
FOR WEST GLACIER REGION.
WINTER STORM WARNING UNTIL 11 AM MST WEDNESDAY FOR
KOOTENAI/CABINET REGION.
WINTER STORM WARNING UNTIL 11 AM MST WEDNESDAY FOR
POTOMAC/SEELEY LAKE REGION.
WINTER STORM WARNING UNTIL 11 AM MST WEDNESDAY ABOVE 3500 FEET
FOR LOWER CLARK FORK REGION.
ID...WIND ADVISORY UNTIL 4 PM PST THURSDAY FOR LOWER HELLS
CANYON/SALMON RIVER REGION-OROFINO/GRANGEVILLE REGION.
WINTER STORM WARNING UNTIL 10 AM PST WEDNESDAY ABOVE 4500 FEET
FOR NORTHERN CLEARWATER MOUNTAINS-SOUTHERN CLEARWATER
MOUNTAINS.
&&
$$
UPDATE...ZUMPFE
SHORT TERM...ALLEGRETTO
LONG TERM....MSJ
AVIATION...ZUMPFE
