Area Forecast Discussion
Issued by NWS Boise, ID

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FXUS65 KBOI 201030
AFDBOI

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BOISE ID
330 AM MST SAT DEC 20 2014

.SHORT TERM...TODAY THROUGH SUNDAY...LONG FETCH OF MOISTURE
STRETCHING BACK TO THE DATELINE WILL PUSH INTO THE PAC NW TODAY.
THE PRECIPITABLE WATER VALUES ARE WELL ABOVE NORMAL /150-200
PERCENT/ AND WILL BE ACCOMPANIED BY SIGNIFICANT WARM-AIR ADVECTION
LATER TONIGHT. PRECIPITATION WILL FILL IN ACROSS SE OREGON BY LATE
MORNING AND SW IDAHO BY EARLY AFTERNOON. SNOW LEVELS WILL START
OUT BETWEEN 4-5K FEET. THE STEADIER AND HEAVIER PRECIPITATION
ARRIVES WITH A WARM FRONT THIS EVENING. THE WARM AIR ADVECTION ON
WESTERLY WINDS OF 30-60 KTS AT 700 MB WILL PUSH SNOW LEVELS TO
6-7K FEET BY SUNDAY MORNING AND MAXIMIZE PRECIPITATION TOTALS
ALONG WEST FACING SLOPES IN THE MTNS OF EAST-CENTRAL OREGON AND SW
IDAHO. SW IDAHO VALLEYS AND SE OREGON COULD SEE A SHORT BREAK IN
THE PRECIPITATION SUNDAY MORNING AS THE WARM ADVECTION WANES AND
PRECIPITATION PROCESS BECOMES MAINLY OROGRAPHIC. BY SUNDAY
AFTERNOON THE FLOW SHIFTS MORE TOWARD THE NORTHWEST AND A WEAK
SHORTWAVE WILL REINTRODUCE PRECIPITATION TO LOWER VALLEYS. SNOW
LEVELS PEAK MIDDAY SUNDAY AND MAY START TO EDGE DOWN SOME IN THE
WEST CENTRAL MTNS UNDER THE NORTHWEST FLOW. HIGHER FORECAST SNOW
LEVELS AND SURFACE TEMPERATURES TODAY AND SUNDAY HAVE CUT DOWN ON
MTN VALLEY SNOW TOTALS...ESPECIALLY ACROSS THE BOISE MTNS AND
CAMAS PRAIRIE. THUS HAVE TAKEN DOWN THE HIGHLIGHTS FOR THESE
ZONES. ALSO THE WARMING OVERNIGHT WILL LEAD TO LESS OF AN IMPACT
FOR THE WEST CENTRAL MTNS SO HAVE SHORTENED UP THE ADVISORY PERIOD
THERE.

.LONG TERM...SUNDAY NIGHT THROUGH FRIDAY...LINGERING MOISTURE WILL
BEGIN TO TAPER OFF SUNDAY NIGHT INTO MONDAY FROM NORTH TO SOUTH. A
DRIER NORTHWEST FLOW WILL SET IN WITH HIGH PRESSURE SITTING OFF OF
THE CALIFORNIA COAST. THIS RIDGE WILL AMPLIFY ON TUESDAY KEEPING
CONDITIONS DRY AND TEMPERATURES ABOVE NORMAL. HOWEVER...MOISTURE IS
STILL EXPECTED TO SPILL OVER THIS RIDGE...AND MODELS VARY AND HOW
FAR NORTH THE RIDGE AMPLIFIES AND THUS...HOW FAR NORTH THE MOISTURE
STAYS.  THE ECMWF BRINGS BACK PRECIP CHANCES IN THE WEST CENTRAL
MOUNTAINS ON TUESDAY NIGHT AND WEDNESDAY...WHILE THE GFS KEEPS THE
ENTIRE AREA DRY. BOTH THE GFS AND ECMWF ARE IN GOOD AGREEMENT OF A
TROUGH AND COLD FRONT MOVING IN FROM COASTAL BRITISH COLUMBIA AND
FLATTENING THE RIDGE...SOMETIME ON WEDNESDAY EVENING TO THURSDAY
MORNING. THE GFS IS ABOUT 6 TO 12 HOURS SLOWER THAN THE ECMWF...AND
THE GFS IS WARMER. THEREFORE...TIMING OF PRECIP AND WHETHER OR NOT
TEMPERATURES WILL BE COLD ENOUGH FOR SNOW IN THE VALLEYS IS STILL
UNCERTAIN. HOWEVER...ONCE THAT TROUGH PASSES...TEMPERATURES WILL
LIKELY RETURN TO NEAR NORMAL.

.AVIATION...VFR CONDITIONS WITH ISOLATED MVFR/IFR CEILINGS WILL
CONTINUE THIS MORNING AHEAD OF RAIN AND SNOW MOVING IN. AS SATURDAY
WEARS ON...EXPECT VFR CONDITIONS TO LOWER TO MVFR WITH AREAS OF IFR
AND MOUNTAIN OBSCURATION. SNOW LEVELS ARE AROUND 4500 NOW AND WILL
RISE TO 9000FT IN SE OREGON AND 5500 FT IN CENTRAL IDAHO AROUND
21/06Z. SURFACE WINDS...SW 10-20 KTS IN SE OREGON...AND SE AROUND 10
KTS IN IDAHO. WINDS ALOFT NEAR 10K FT MSL...W 25-35 KTS...SHIFTING
TO NW 45-55 KTS AFTER 21/06Z.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...WINTER WEATHER ADVISORY FROM 11 AM THIS MORNING TO 11 PM MST
     THIS EVENING IDZ011.
OR...NONE.

&&

$$

WEATHER.GOV/BOISE

SHORT TERM...DG
LONG TERM....EP
AVIATION.....EP



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