FXUS65 KBOI 151625
AFDBOI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
925 AM MST TUE DEC 15 2009

.DISCUSSION...UPDATED FOR RAIN IN THE TREASURE VALLEY EAST OF 
CALDWELL. CONTINUED SNOW WEST OF CALDWELL WHERE COLD AIR REMAINS 
ENTRENCHED. PAYETTE REPORTED 4 INCHES NEW SNOW AS OF 8 AM AND GET 8 
INCHES BEFORE SNOW MIXES WITH OR CHANGES TO RAIN MID AFTERNOON. 
WE SEE THAT TWIN FALLS WAS IN THE UPPER 30S AT 9 AM MST WITH A 
MODERATE SOUTH WIND WHILE JEROME WAS 25 DEGS WITH A MODERATE NE 
WIND.  AS OF 9 AM PCPN HAD NOT STARTED BUT JEROME WILL LIKELY GET 
SEVERAL INCHES OF SNOW WHILE MIXED RAIN AND SNOW FALL AT TWIN
FALLS.

ON THE LARGER SCALE THE MAIN PACIFIC STORM WAS PUSHING ITS WARM 
FRONT THROUGH OUR CWA THIS MORNING BUT PCPN WILL PROBABLY CONTINUE 
WELL INTO THE COMING NIGHT...EVEN AFTER THE CURRENT WINTER HIGHLIGHTS
HAVE EXPIRED.  WE WILL PROBABLY HAVE TO EXTEND THOSE HIGHLIGHTS
AND INCREASE TOTAL SNOW AMOUNTS ABOVE THE GENERAL 4000 FOOT SNOW
LEVEL.

&&

.AVIATION...WIDESPREAD IFR /AND LOCAL LIFR/ IN SNOW...MIXED 
PRECIPITATION...AND FOG THROUGH EARLY AFTERNOON. MOUNTAINS OBSCURED. 
LOWER VALLEY LOCATIONS RECEIVING MORNING SNOW WILL SWITCH OVER TO 
RAIN BY LATE AFTERNOON...BUT CONDITIONS EXPECTED TO REMAIN IFR/MVFR. 
GUSTY SE-SW SURFACE WINDS WITH STRONG WESTERLY WINDS ALOFT.

&&

.PREV DISCUSSION...
SHORT TERM...TODAY THROUGH WEDNESDAY...STRONG AND WET STORM COMING 
IN AT THIS TIME. MAJOR QUESTION IS WITH SNOW LEVELS AND TIMING OF 
SNOW CHANGING TO RAIN. HAVE MADE SOME SMALL CHANGES TO THE WSW THAT 
WAS ISSUED. HAVE UPGRADED ORZ063 FROM ADVISORY TO WARNING BASED ON 
INCREASED AMOUNTS OF SNOW TODAY. HAVE BROUGHT THE OWYHEE MTNS DOWN 
FROM A WARNING TO AN ADVISORY AND HAVE REMOVED IDZ015 AND IDZ030 
ALTOGETHER BECAUSE NOT ENOUGH PRECIPITATION WILL FALL IN THESE 
AREAS. LATEST BEST GUESS IS THAT SNOW WILL CHANGE TO RAIN IN THE 
TREASURE VALLEY BETWEEN 20Z AND 00Z TONIGHT. MODEL SOUNDINGS 
INDICATE SNOW AT KBOI THROUGHOUT THIS PERIOD...BUT FEEL THAT THESE 
MAY BE A TAD WARM. THE GRADIENT INVOLVED IS VERY STRONG AND JUST A 
DEGREE OR TWO A FEW THOUSAND FEET ABOVE THE GROUND COULD RESULT IN A 
2 TO 3 HOUR DIFFERENCE IN TIME OF CHANGE FROM SNOW TO RAIN. WITH 
THIS THERMAL STRUCTURE...THERE WILL BE FREEZING RAIN IN ISOLATED 
MTN-VALLEYS...BUT WE CANNOT DETERMINE WHICH ONES. THEREFORE THE 
CHANCE WILL BE MENTIONED IN THE APPROPRIATE PORTIONS OF THE WSW BUT 
WILL NOT BE FOUND IN THE GRIDS. SNOWFALL AMOUNTS IN THE TREASURE 
VALLEY WILL VARY FROM NEAR 1 INCH TO AS MUCH AS 5 INCHES IN THE LAST 
AREAS TO CHANGE OVER. COMPARED TO OUR EARLIER FORECAST...THIS ONE 
HOLDS BACK THE ONSET OF PRECIPITATION UNTIL LATE MORNING...AND THERE 
WILL LIKELY BE A PERIOD OF HEAVY SNOW FROM AROUND NOON TO AROUND 3 
PM THROUGHOUT MUCH OF THE POPULATED CORRIDOR FROM ONTARIO THROUGH 
BOISE AND OVER TOWARD MOUNTAIN HOME. WE COULD SEE SNOWFALL AT OR 
ABOVE 1 INCH PER HOUR DURING THIS PERIOD. HOWEVER...THIS WILL 
QUICKLY BE FOLLOWED BY A CHANGE TO RAIN AND A QUICK SHRINKAGE OF THE 
ACCUMULATED SNOW. TEMPS OVERNIGHT TONIGHT ARE EXPECTED TO REMAIN AT 
OR ABOVE FREEZING OVER MUCH OF THIS AREA...SO WITH ANY DIRT OR 
TREATMENT ON THE ROADWAYS THERE WILL BE VERY LITTLE REFREEZING. 
TEMPS WILL WARM UP ENOUGH WEDNESDAY FOR AMPLE MELTING IN THESE AREAS.

IN THE MOUNTAINS...SNOWFALL WILL BE HEAVY BECAUSE THIS IS A VERY
WET SYSTEM. HIGHER ELEVATIONS WILL PICK UP A FOOT OR MORE OF NEW
SNOW THROUGH WEDNESDAY. SOME ISOLATED MTN-VALLEYS WILL SEE
FREEZING RAIN TODAY. TEMPS WILL MODERATE EVEN AS THE HEAVY SNOW
CONTINUES DUE TO WARM AIR ADVECTION ASSOCIATED WITH THE STORM.
HOWEVER...MOST MTN LOCATIONS WILL REMAIN COLD ENOUGH FOR SNOW.

LONG TERM...WEDNESDAY NIGHT THROUGH MONDAY...ANOTHER SHORTWAVE 
TROUGH WILL MOVE THROUGH THE FORECAST AREA WEDNESDAY NIGHT IN MOIST 
WESTERLY FLOW. THE PROGRESSIVE NATURE OF THIS PATTERN WILL PUSH THE 
FEATURE NORTH AND EAST OF THE AREA EARLY THURSDAY - FOR NEAR CLIMO 
POPS IN THE WEST CENTRAL AND BOISE MOUNTAINS BUT DRYING ELSEWHERE. 
MODELS CONTINUE WITH DECENT AGREEMENT TOWARD AN AMPLIFIED UPPER 
RIDGE OVER THE PAC NW FOR THURSDAY THROUGH SATURDAY. FORECAST 
QUESTION IS TO WHAT DEGREE THE SHORTWAVE ENERGY IN SOUTHWEST FLOW 
WILL BE ABLE TO FLATTEN THE RIDGE TO BRING IN PERIODS OF 
PRECIPITATION AND WEAKEN THE INVERTED TEMPERATURES. FORECAST PACKAGE 
WILL FAVOR ABOVE NORMAL POPS FOR SOUTHEAST OREGON ZONES AND WEISER 
BASIN...BUT NEAR TO BELOW CLIMO POPS FOR THE IDAHO VALLEYS. BY 
SUNDAY...GFS AND ECMWF DEPICT ANOTHER TROUGH APPROACHING FROM THE 
PACIFIC. THERE ARE SLIGHT TIMING AND STRENGTH DIFFERENCES...BUT 
QUITE REASONABLE AGREEMENT OVERALL FOR THE EXTENDED. WILL KEEP 
TEMPERATURES FOR SUNDAY AND MONDAY NEAR CLIMATOLOGY...WHILE PRECIP 
WILL BE HELD JUST BELOW CLIMO SUNDAY TO ACCOUNT FOR THE TIMING 
UNCERTAINTY. BY MONDAY...BOTH MODELS SUPPORT AT OR ABOVE CLIMATOLOGY 
PRECIP.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...WINTER STORM WARNING UNTIL 11 PM MST THIS EVENING FOR IDZ011-
      IDZ013-IDZ033.
     WINTER WEATHER ADVISORY FROM 7 AM THIS MORNING TO 5 PM MST THIS 
      AFTERNOON FOR IDZ012-IDZ014-IDZ016-IDZ028.
     WINTER WEATHER ADVISORY FROM 7 AM THIS MORNING TO 5 PM MST THIS 
      AFTERNOON FOR IDZ029.
OR...WINTER STORM WARNING UNTIL 5 PM MST /4 PM PST/ THIS AFTERNOON 
      FOR ORZ063.
     WINTER STORM WARNING UNTIL 5 PM MST THIS AFTERNOON FOR ORZ064.
     WINTER STORM WARNING UNTIL 4 PM PST THIS AFTERNOON FOR ORZ061-
      ORZ062.
&&

$$

WEATHER.GOV/BOISE

DISCUSSION...LC
AVIATION.....MT
PREV SHORT TERM...SP
PREV LONG TERM....CD