AREA FORECAST DISCUSSION

 
 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.
 
 
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 .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.
 
 
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 .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.
 
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 .MSO WATCHES/WARNINGS/ADVISORIES...
 MT...NONE.
 ID...NONE.
 &&
 
 $$
 
 UPDATE...DEROSA
 PREVIOUS DISCUSSION...KITSMILLER/BAUCK
 AVIATION...DEROSA