AREA FORECAST DISCUSSION

 
 FXUS65 KBOI 241619 CCA
 AFDBOI
 
 AREA FORECAST DISCUSSION
 NATIONAL WEATHER SERVICE BOISE ID
 919 AM MST TUE NOV 24 2009
 
 .DISCUSSION...MID CLOUDS PASSING THROUGH OUR CWA THIS MORNING AS A 
 WEAK SHORT WAVE TROUGH PASSES BY TO OUR NORTH.  NO PCPN.  12Z KBOI 
 RAOB SHOWED SIGNIFICANT LOW-LEVEL WARMING SINCE 12Z MONDAY...EXCEPT 
 RIGHT AT THE SURFACE...WHILE DEW POINTS HAVE NOT CHANGED MUCH BELOW 
 CLOUD LEVEL.  THEREFORE WE HAVE STARTED A DRY LOW-LEVEL INVERSION.  
 MOS INDICATES LITTLE CHANGE IN SURFACE DEW POINTS THROUGH WEDNESDAY 
 SO WE EXPECT THE INVERSION TO REMAIN DRY THROUGH THEN. NAM RH CHARTS 
 HINT AT FOG/LOW CLOUDS WEDNESDAY MORNING DEVELOPING FROM EAST TO 
 WEST ACROSS THE WRN MAGIC VALLEY THROUGH THE TREASURE AND LOWER 
 MALHEUR VALLEYS BUT THOSE RH CHARTS ARE NOT SUPPORTED BY MET MOS 
 STATS WHICH SHOW NO MOISTURE INCREASE.  FOR NOW...NO SHORT TERM 
 FORECAST CHANGES.
 
 THE INVERSION EVENT SHOULD PEAK THURSDAY BEFORE A PACIFIC TROUGH 
 COMES INLAND.  THE TROUGH WILL BRING COOLING ALOFT AND PCPN FRIDAY 
 NIGHT AND SATURDAY...ALTHOUGH DETAILS ARE UNCLEAR...AND FOLLOWED
 BY A NEW CLEANER AIR MASS.  MORE ON THAT IN AFTERNOON PACKAGE.
 
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 .AVIATION...VFR UNDER MID-HIGH LEVEL CEILINGS TODAY. CLEARING SKIES 
 WILL ALLOW FOR MVFR-IFR STRATUS AND FOG TO DEVELOP IN THE VALLEYS OF 
 SW IDAHO AND SE OREGON. IF KBOI SEES ANY NON-VFR CONDITIONS... 
 GROWING CONFIDENCE THAT IT WILL BE FROM LOW STRATUS ADVECTING FROM 
 THE WESTERN MAGIC VALLEY. IF THIS IS THE CASE CEILINGS LIKELY WON/T 
 SET UP BELOW 400 FT AGL. ANY STRATUS/FOG THAT FORMS BREAKS OUT 
 WEDNESDAY AFTERNOON...BEFORE REDEVELOPING WEDNESDAY NIGHT IN THE 
 SAME LOCATIONS.     
 
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 .PREV DISCUSSION...
 SHORT TERM...TODAY THROUGH WEDNESDAY...LIGHT NORTHWEST FLOW ALOFT IS 
 OVER OUR AREA TODAY AHEAD OF A BUILDING UPPER LEVEL RIDGE. WE ARE 
 SEEING THE START OF A TEMPERATURE INVERSION WHICH WILL INTENSIFY 
 TONIGHT AND WEDNESDAY. EARLY THIS MORNING MID LEVEL AND HIGH CLOUDS 
 ASSOCIATED WITH A WEAK SHORTWAVE ARE APPROACHING FROM WESTERN AND 
 CENTRAL OREGON. THESE CLOUDS WILL PARTIALLY DISSIPATE CROSSING THE 
 DEVELOPING RIDGE AXIS AND NO PRECIPITATION IS EXPECTED...BUT THERE 
 WILL BE ENOUGH SHADING...COMBINED WITH THE INVERSION AND LIGHT 
 SURFACE WINDS...TO HOLD TEMPERATURES BACK. SO HIGHS SHOULD BE A FEW 
 DEGREES BELOW YESTERDAY/S. THE RIDGE...ALONG WITH THE INVERSION... 
 WILL REACH  MAXIMUM DEVELOPMENT WEDNESDAY AFTERNOON. THE COMBINATION 
 OF MORE SUN AND A BETTER DOWN VALLEY PRESSURE GRADIENT SHOULD 
 BALANCE THE DEEPER INVERSION...SO WE EXPECT HIGH TEMPERATURES 
 WEDNESDAY WILL BE CLOSE TO TODAY/S.  WE ARE KEEPING PATCHY VALLEY 
 FOG AND STRATUS FOR LATE TONIGHT AND WEDNESDAY MORNING...BUT WITH 
 THE DRY AIRMASS CONFIDENCE IS RATHER LOW. 
 
 LONG TERM...WEDNESDAY NIGHT THROUGH TUESDAY...UPPER RIDGE BUILDS 
 OVER THE AREA WEDNESDAY NIGHT AND EARLY THURSDAY IN ADVANCE OF NEXT 
 PACIFIC SHORT WAVE. DURING THANKSGIVING DAY...RIDGE AXIS MOVES EAST 
 TO WYOMING AND MONTANA ALLOWING A SOUTHWEST FLOW OF WARMER AND DRY 
 AIR INTO THE REGION. EXTENDED MODELS IN GOOD AGREEMENT IN THIS TIME 
 RANGE BUT WILD CARD AT THIS TIME IS STRENGTH OF ASSOCIATED INVERSION 
 AND THE POSSIBILITY OF LOW CLOUDS AND FOG. FEELING IS THAT SINCE WE 
 HAVE BEEN QUITE DRY THIS MONTH AND ESPECIALLY RECENTLY..LOW CLOUD 
 INVERSION CONDITIONS MAY NOT SET UP YET. THIS MAY LEAD TO A QUITE 
 MILD...BREEZY AND DRY DAY FOR THANKSGIVING AND THIS IS WHAT WE HAVE 
 FORECAST. A PACIFIC SHORT WAVE MOVES ACROSS THE AREA FRIDAY AND 
 FRIDAY NIGHT FOR LIGHT AMOUNTS OF PRECIPITATION WITH SYSTEM ONCE 
 AGAIN EMBEDDED WITHIN A FAST PROGRESSIVE FLOW. BY SUNDAY...MODELS 
 AGREE WITH HIGH AMPLITUDE RIDGE REBUILDING OVER THE PACIFIC 
 NORTHWEST WHICH REMAINS THROUGH THE LAST DAY IN NOVEMBER. 
 TEMPERATURES AT SEASONAL VALUES THROUGH THE PERIOD BUT PRECIP LOOKS 
 TO BE BELOW NORMAL.          
 
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 .BOI WATCHES/WARNINGS/ADVISORIES...
 ID...NONE.
 OR...NONE.
 
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 WEATHER.GOV/BOISE
 
 DISCUSSION...LC
 AVIATION.....DG
 PREV SHORT TERM...JT
 PREV LONG TERM....GS