Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 250743
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
343 AM EDT SAT OCT 25 2014

VALID 12Z SAT OCT 25 2014 - 12Z TUE OCT 28 2014


DAYS 1 THROUGH 3...


...PACIFIC NORTHWEST/NORTHERN AND CENTRAL ROCKIES...

A STRONG SHORT WAVE APPROACHING THE NORTHERN CA COAST EARLY THIS
MORNING WILL CROSS THE PACIFIC NORTHWEST AND NORTHERN ROCKIES ON
DAY 1...BEFORE STRETCHING INTO A LONG WAVE TROUGH ACROSS THE
ROCKIES ON DAY 2. ANOTHER STRONG SHORT WAVE APPROACHES THE PACIFIC
NORTHWEST LATE ON DAY 3. FOR THE MOST PART...THE QPF PORTION OF
THE FORECAST WAS DERIVED FROM A BLEND OF THE 00Z GFS/12Z ECMWF (AS
WELL AS THE MOST RECENT WPC QPF)...WHILE THE THERMAL PORTION OF
THE FORECAST WAS GLEANED FROM A BLEND OF THE 00Z NAM/GFS/21Z SREF
MEAN.

A STRONG SHORT WAVE CROSSES THE PACIFIC NORTHWEST DURING THE FIRST
HALF OF DAY 1...REACHING THE NORTHERN ROCKIES BY THE END OF THE
PERIOD. AHEAD OF THE SHORT WAVE AND ITS ATTENDANT SURFACE LOW AND
COLD FRONT...BROAD WARM AIR ADVECTION RESULTS IN A GRADUAL RISE IN
THE SNOW LEVEL...WHICH STARTS CLOSE TO 6500 FEET...AND REACHES
BETWEEN 8000 AND 9000 FEET AROUND 26/00Z. THE BEST MOISTURE AND
LIFT WITH THE SHORT WAVE AFFECTS THE REGION DURING THE FIRST HALF
OF THE PERIOD. THE RISING SNOW LEVELS AND QPF ASSOCIATED WITH THE
SHORT WAVE SHOULD RESULT IN A SWATH OF 2 TO 4 INCHES OF SNOWFALL
ACROSS THE NORTHERN CASCADES OF WA...AND AN INCH OR TWO OF
SNOWFALL ACROSS THE HIGHER TERRAIN OF THE BITTEROOTS OF ID.

THE BEST LIFT WITH THE SHORT WAVE EARLY ON DAY 2 AFFECTS
ID/MT/WY...AS THE SURFACE LOW TRACKS INTO MANITOBA. SNOW LEVELS
START OUT BETWEEN 8000 AND 9000 FEET EARLY DURING THE PERIOD OF
BEST MID LEVEL MOISTURE AND LIFT. THE COMBINATION OF SYNOPTIC
SCALE LIFT AND UPSLOPE FLOW SHOULD PRODUCE THE HIGHEST QPF ACROSS
THE CASCADES OF WA/OR...AS WELL AS THE BITTERROOTS IN ID AND THE
GRAND TETONS IN WY. THE COMBINATION OF EXPECTED QPF AND SNOW
LEVELS SHOULD RESULT IN A NARROW AXIS OF 3 TO 6 INCHES OF SNOW
ACROSS THE NORTHERN CASCADES OF WA...AND AN INCH OR TWO OF
SNOWFALL ACROSS THE ROCKIES OF NORTH CENTRAL CO.

COLD AIR ADVECTION BEHIND THE SHORT WAVE (WHICH BEGINS TO ELONGATE
INTO A LONG WAVE TROUGH DURING DAY 2 ACROSS THE ROCKIES) WILL
ALLOW SNOW LEVELS ACROSS THE BITTERROOTS AND GRAND TETONS TO FALL
DURING THE SECOND HALF OF THE DAY (REACHING NEAR 6000 FEET BY THE
END OF THE PERIOD). LOWERING SNOW LEVELS AND PERSISTENT UPSLOPE
FLOW WILL MAKE THE MOST OF THE AVAILABLE MOISTURE AND PRODUCE
AREAS OF 4 TO 8 INCHES OF SNOW ACROSS THE HIGHER TERRAIN OF
NORTHWEST WY AND CENTRAL ID. SEVERAL MEMBERS OF THE LATEST ECMWF
ENSEMBLE OUTPUT SUPPORT 8+ INCH SNOWFALL AMOUNTS ON DAY 2 ACROSS
THESE AREAS. A DWINDLING MOISTURE SUPPLY AND MOVEMENT OF THE LONG
WAVE TROUGH SHOULD RESULT IN A LOW PROBABILITY OF
8+ INCHES OF SNOW OVER THE HIGHER TERRAIN OF THE BITTERROOTS AND
GRAND TETONS.

THE LONG WAVE TROUGH MOVES INTO THE NORTHERN PLAINS ON DAY 3.
WHILE FREEZING LEVELS REMAINS CLOSE TO 6000 FEET ACROSS THE
NORTHERN AND CENTRAL ROCKIES...MOISTURE REMAINS THE LIMITING
FACTOR FOR SIGNIFICANT SNOWFALL HERE. SNOWFALL AMOUNTS OF 1 TO 3
INCHES ARE POSSIBLE ACROSS THE ROCKY MOUNTAIN FRONT RANGE IN MT
AND THE GRAND TETONS IN WY. A STRONG SHORT WAVE APPROACHING THE
PACIFIC NORTHWEST LATE WILL PRODUCE A LARGE SWATH OF WARM AIR
ADVECTION PRECIPITATION...AS WELL AS RISING SNOW LEVELS. THE BEST
THRUST OF MOISTURE SHOULD INITIALLY OCCUR ACROSS SOUTHERN BRITISH
COLUMBIA AND NORTHWEST WA. HOWEVER...THE HIGHER PEAKS ACROSS THE
NORTHERN CASCADES OF WA COULD SEE 1 TO 3 INCHES OF SNOWFALL BEFORE
THE END OF DAY 3.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES

$$





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