Expires:No;;223179
FXUS65 KMSO 250420
AFDMSO
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE MISSOULA MT
920 PM MST TUE NOV 24 2009
.DISCUSSION...SEVERAL PARAMETERS ARE PLAYING A ROLE IN THE
FORECAST UPDATE THIS EVENING. FIRST...FOG IS FORMING QUICKLY IN
THE FLATHEAD VALLEY AS A RESULT OF AVAILABLE LOW LEVEL MOISTURE
FROM SNOW TODAY COMBINED WITH CONDITIONS REACHING SATURATION DUE
TO CALM WINDS. OTHER VALLEY LOCATIONS ACROSS NORTHWEST MONTANA
ARE NEARING SATURATION ALREADY THIS EVENING...THEREFORE HAVE
PUSHED UP THE ONSET OF FOG TO PRIMARILY AFTER MIDNIGHT RATHER THAN
BY DAYBREAK. THE SECOND UPDATE RELATES TO MID AND HIGH LEVEL
CLOUDS ROTATING AROUND THE HIGH PRESSURE OVER THE PACIFIC SHORE
AND PREVENTING TEMPERATURES FROM DROPPING SUBSTANTIALLY THIS
EVENING. NORTHWEST FLOW ALOFT SHOULD PREVAIL THROUGH THE OVERNIGHT
HOURS AND KEEP THE CLOUDS STREAMING OVER THE NORTHERN ROCKIES.
WITH DEWPOINT TEMPERATURES HOLDING STEADY FROM 25 TO 30 DEGREES ON
AVERAGE ACROSS THE REGION AND MODEL SOUNDINGS INDICATING WARM AIR
ADVECTING INTO THE AREA TONIGHT THROUGH WEDNESDAY...EXPECT VALLEY
TEMPERATURES TO DROP TOWARD CURRENT DEWPOINT VALUES.
&&
.AVIATION...A WEAKENING SHORTWAVE LOW PRESSURE TROUGH WILL KEEP
LIGHT MOUNTAIN SNOW SHOWERS INTO THE OVERNIGHT TONIGHT. HIGH
PRESSURE WILL BEGIN TO BUILD INTO THE REGION FROM THE WEST TONIGHT
AND THROUGH WEDNESDAY...ALTHOUGH MODEL SOUNDINGS AND AREA CROSS
SECTIONS SHOW LINGERING LOW AND MID-LEVEL MOISTURE. THIS MOISTURE
WILL BE ENOUGH FOR FOG DEVELOPMENT ACROSS NORTH CENTRAL IDAHO AND
WESTERN MONTANA. IT SHOULD ALSO KEEP LOWERED CLOUD DECKS ACROSS
PRIMARILY WESTERN MONTANA THROUGH WEDNESDAY MORNING. A DRIER AND
WARMER AIRMASS IS EXPECTED TO MOVE INTO THE NORTHERN ROCKIES BY
WEDNESDAY EVENING.
&&
.PREV DISCUSSION... /ISSUED 233 PM MST TUE NOV 24 2009/ 
DISCUSSION...
TONIGHT THROUGH WEDNESDAY...A SHORTWAVE WEATHER DISTURBANCE WILL
CONTINUE ITS PROGRESSION EASTWARD THIS EVENING AND OVERNIGHT AS
HIGH PRESSURE MOVES IN FROM THE WEST. NORTHWESTERLY FLOW ALOFT WILL
BE THE DOMINANT FLOW OVER THE NORTHERN ROCKIES OVERNIGHT AND
THROUGH WEDNESDAY WITH MODEL CROSS SECTIONS SUPPORTING ENOUGH LOW
LEVEL MOISTURE AND OROGRAPHIC LIFT FOR LIGHT MOUNTAIN SNOW SHOWERS
MAINLY NORTH. LOOKOUT PASS HAS BEEN THE MOST INFLUENCED BY THESE
SHOWERS WITH OFF AND ON LOWERED VISIBILITIES. SOME DRIER AIR WILL
BEGIN TO MOVE INTO THE AREA OVERNIGHT ALLOWING FOR THESE SHOWERS
TO DECREASE BY WEDNESDAY MORNING. LOW LEVEL INVERSIONS WILL ALLOW
FOR SOME FOG DEVELOPMENT WEDNESDAY MORNING WITH THE MAIN FOCUS
BEING IN NORTH CENTRAL IDAHO AND NORTHWEST MONTANA.
THURSDAY...HIGH PRESSURE AND A STABLE ATMOSPHERE SHOULD PREVAIL.
HOWEVER THERE SHOULD BE ENOUGH HIGH CLOUDS TO KEEP FOG FROM
BECOMING WIDESPREAD IN THE VALLEYS. FRIDAY AND SATURDAY...A TROUGH
OF LOW PRESSURE SHOULD IMPACT THE REGION. THERE SHOULD BE
SUFFICIENT MOISTURE AND ENERGY TO PRODUCE SOME SNOW...ESPECIALLY
OVER THE HIGHER TERRAIN.
IN THE EXTENDED PERIODS FROM SUNDAY THROUGH EARLY NEXT WEEK THERE
IS AGAIN A LOT OF UNCERTAINTY. FOR INSTANCE...NEARLY ALL THE
OPERATIONAL FORECAST GUIDANCE A DAY AGO SHOWED A RIDGE OF HIGH
PRESSURE AND DRY CONDITIONS FOR EARLY NEXT WEEK. GUIDANCE TODAY
SHOWS SOMETHING QUITE DIFFERENT WITH MOISTURE MOVING THROUGH A
FLAT RIDGE OF HIGH PRESSURE SUNDAY FOLLOWED BY ANOTHER WEATHER
SYSTEM LATE MONDAY INTO TUESDAY. WHAT TO DO? GIVEN THE
UNCERTAINTY...HAVE ELECTED TO GO WITH A MORE CLIMATOLOGICAL
FORECAST WITH SOME MENTION OF PRECIPITATION AND NEAR NORMAL
TEMPERATURES.
AIR STAGNATION...A RIDGE OF HIGH PRESSURE WILL BUILD OVER NORTH
CENTRAL IDAHO AND WESTERN MONTANA TONIGHT AND THEN PERSIST THROUGH
THURSDAY. THIS WILL SUPPORT A STABLE ATMOSPHERE...ESPECIALLY IN
THE VALLEYS. A WEATHER SYSTEM AND ASSOCIATED COLD FRONT SHOULD
MOVE ACROSS THE AREA FRIDAY AND SIGNIFICANTLY INCREASE THE LOWER
ATMOSPHERE MIXING.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...NONE.
ID...NONE.
&&
$$
UPDATE...DEROSA
PREVIOUS DISCUSSION...KITSMILLER/BAUCK
AVIATION...DEROSA