FXUS65 KBOI 232132
AFDBOI

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

.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 UNFLOLDING 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.

&&

.AVIATION...MODERATE TO STRONG NORTH TO NORTHWEST WINDS ALOFT. 
MODERATE WESTERLY SURFACE WINDS EAST OF KMUO UNTIL 03Z WITH LIGHT 
SURFACE WINDS ELSEWHERE. VFR CEILINGS AND VISIBILITY TO DETERIORATE 
BETWEEN 03Z AND 06Z TONIGHT WITH FORMATION OF VALLEY FOG AND LOCAL 
LIFR CONDITIONS UNTIL AT LEAST 15Z THURSDAY.

&&   

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...NONE.
OR...AIR STAGNATION ADVISORY THROUGH SATURDAY AFTERNOON FOR ZONES
       ORZ061-ORZ062.

&&

$$

WEATHER.GOV/BOISE

SHORT TERM...LC
LONG TERM....CD/BB
AVIATION.....CD