FXUS65 KBOI 151107
AFDBOI

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

.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.

&&

.AVIATION...MOUNTAINS OBSCURED. WIDESPREAD IFR IN SNOW AND FOG WITH 
LOCAL LIFR DUE TO SNOW AFTER 18Z. GUSTY SURFACE WINDS WITH STRONG 
SOUTHWEST TO WEST WINDS ALOFT.

&&

.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

SHORT TERM...SP
LONG TERM....CD
AVIATION.....CD