FXUS65 KBOI 240435
AFDBOI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
935 PM MST WED DEC 23 2009

.DISCUSSION...MOSTLY CLEAR SKIES ACROSS THE REGION TONIGHT...WITH
JUST A COUPLE OF AREAS OF MID AND HIGH CLOUDS MOVING FROM
NORTHWEST TO SOUTHEAST. THERE IS AN AREA OF LOW CLOUDS JUST TO THE
EAST OF THE TWIN FALLS AND JEROME AREA. THESE CLOUDS WILL LIKELY MOVE
WESTWARD INTO THE WESTERN MAGIC VALLEY OVERNIGHT. ADDITIONAL FOG
AND/OR LOW CLOUDS MAY FORM JUST ABOUT ANYWHERE IN THE TREASURE
VALLEY...BUT RIGHT NOW THE BEST BET IS THAT IT WILL BE LIMITED IN
EXTENT IF IT HAPPENS AT ALL. THIS IS PROPERLY PORTRAYED IN THE
CURRENT FORECAST...AND NO UPDATE IS PLANNED FOR THIS EVENING.

AIR STAGNATION ADVISORY CONTINUES FOR BAKER AND HARNEY COUNTIES IN
SOUTHEAST OREGON. AT THIS TIME...THE AIR QUALITY REMAINS GOOD AT
THE OFFICIAL REPORTING SITES. HOWEVER...CONDITIONS ARE EXPECTED TO
DETERIORATE THROUGH THE NEXT FEW DAYS.

&&

.AVIATION...VFR CONDITIONS WITH BROKEN HIGH CLOUDS.  LOW STRATUS IN 
THE SNAKE RIVER VALLEY EAST OF KJER/KJER WILL SLOWLY MOVE WEST 
DURING THE NIGHT. STRATUS WILL BRING MVFR CEILING TO KJER AROUND
06Z AND KTWF AROUND 10Z. STILL SOME UNCERTAINTY IF VISIBILITY WILL
BE REDUCED BY FOG UNDER THIS STRATUS DECK. OTHERWISE LOCAL VALLEY
FOG WILL DEVELOP AFTER 06Z WITH ISOLATED LIFR CONDITIONS FROM 11Z
TO 16Z. SURFACE WINDS WILL BE LIGHT WITH LIGHT TO MODERATE
NORTHWEST WINDS ALOFT.

&&

.PREV DISCUSSION...
SHORT TERM...TEMPERATURE INVERSION FORMING IN THE VALLEYS.  LATEST 
MAV AND MET MOS SUGGEST A FOG AND STRATUS EVENT FOR THE TREASURE AND 
WRN MAGIC VALLEYS BEGINNING IN THE PRE-DAWN HOURS THURSDAY AND 
LASTING THROUGH NOON...AND REPEATING ON CHRISTMAS DAY.  SIMILAR 
CONDITIONS IN THE LONG VALLEY...AND PATCHY FOG/STRATUS NEAR BAKER 
AND BURNS OREGON.  THERE IS ALREADY A STRATUS SHIELD IN THE MAGIC 
VALLEY AND ANOTHER NORTH OF MYL.  BUT MODELS ARE NOT ENTIRELY 
CONSISTENT ABOUT A FOG EVENT AS LOW-LEVEL RH PROGS ARE NOT AS MOIST 
AS THEY COULD OR SHOULD BE.  A FOG EVENT ALSO HAS IMPLICATIONS ON 
TEMPERATURE FORECASTS.  LATEST MAV AND MET HAVE MIN TEMPS HIGHER AND 
MAX TEMPS LOWER IN THE VALLEYS THAN THEY WOULD BE WITH CLEAR SKIES.  
MET MOS EVEN HAS POPS OF 10-20 PERCENT IN THE VALLEYS SUGGESTING 
FLURRIES FROM THE FOG/STRATUS.  FOR NOW WE WILL INDICATE ONLY PATCHY 
FOG AND STRATUS DURING THE NIGHTS AND MORNINGS AND LET TEMPERATURES 
GO BOTH SLIGHTLY HIGHER AND SLIGHTLY LOWER THAN IN A FOG EVENT.  
WILL KEEP POPS LESS THAN 10 PERCENT THROUGHOUT.  ON THE LARGER 
SCALE... NORTHERLY FLOW ALOFT WILL KEEP TEMPS BELOW NORMAL THROUGH 
CHRISTMAS. CAMAS PRAIRIE WILL BE ESPECIALLY COLD.  OTHER HIGH BASINS 
IN IDAHO WILL ALSO BE QUITE COLD.  AIR MASS WILL STAGNATE IN THE 
COLD VALLEYS AND BASINS IN HARNEY AND BAKER COUNTIES THROUGH 
SATURDAY AND WE HAVE ISSUED AN AIR STAGNATION ADVISORY FOR THOSE 
COUNTIES IN CONNECTION WITH OREGON DEQ.

LONG TERM...FRIDAY NIGHT THROUGH WEDNESDAY...HIGHLY AMPLIFIED 
NORTHERLY FLOW ALOFT ON THE LEADING SIDE OF A HIGH PRESSURE RIDGE 
WILL PROLONG INVERTED CONDITIONS FOR BELOW NORMAL VALLEY 
TEMPERATURES INTO SATURDAY. BUT THEN THE FLOW WILL TAKE ON AN 
EASTERLY PROGRESSION SATURDAY AND SUNDAY AS A HISTORICALLY 
SIGNIFICANT LOW PRESSURE SYSTEM ON THE CENTRAL PLAINS WILL 
RETROGRADE WESTWARD INTO THE GREAT BASIN TO MIX THE ATMOSPHERE 
TOWARD NORMAL TEMPERATURES. BY LATE SUNDAY A PACIFIC LOW WILL CUT 
OFF AS IT APPROACHES THE FORECAST AREA TO SQUEEZE OFF THE FLOW AND 
FORCE THE HIGH PRESSURE NORTHWARD INTO ALBERTA CANADA. BASED ON THIS 
RARE PATTERN WITH EASTERLY FLOW THERE IS RELATIVELY LOW CONFIDENCE 
IN THE POP FORECAST SO NEAR CLIMATOLOGY POPS THROUGH MONDAY IS THE 
CHOSEN ROUTE FOR THIS FORECAST PACKAGE. THE EXTENDED MODELS ARE NOW 
IN MUCH BETTER AGREEMENT FOR THE POTENTIAL OF A WARM FRONTAL PRECIP 
EVENT UNFOLDING FOR TUESDAY AND WEDNESDAY. BEING THAT FAR OUT IN 
TIME HOWEVER MAKES FOR GREATER UNCERTAINTY BUT WILL TREND UPWARD 
WITH SLIGHTLY HIGHER THAN CLIMO POPS. TEMPERATURES SUNDAY THROUGH 
WEDNESDAY WILL BE AT OR BELOW NORMAL IN THE POST-INVERSION TROFFY 
PATTERN.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...NONE.
OR...AIR STAGNATION ADVISORY UNTIL 4 PM PST SATURDAY FOR ORZ061-
     ORZ062.
&&

$$

WEATHER.GOV/BOISE

DISCUSSION...SP
AVIATION.....JA
PREV SHORT TERM...LC
PREV LONG TERM....CD/BB