Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 172037
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
436 PM EDT TUE OCT 17 2017

VALID 00Z WED OCT 18 2017 - 00Z SAT OCT 21 2017


DAYS 1 THROUGH 3...

...PACIFIC NORTHWEST/NORTHERN ROCKIES...

SEVERAL SHORT WAVES IN THE MID LEVEL FLOW WILL CROSS THE PACIFIC
NORTHWEST DURING DAYS 1 THROUGH 3.  SNOW LEVELS WILL BE FAIRLY
HIGH OVER THE AREA...WITH SNOW ACCUMULATIONS GENERALLY RELEGATED
TO THE HIGHER TERRAIN OF THE NORTHERN CASCADES ON DAY 1 WHEN THERE
IS A WARM ADVECTION/ISENTROPIC UPGLIDE PATTERN AHEAD OF THE
APPROACHING WAVE.
SNOW AMOUNTS ARE NOT EXPECTED TO BE TOO GREAT CONSIDERING THE
HEAVIEST PRECIPITATION WILL HAVE MOVED OUT OF THE AREA BEFORE
HEIGHTS/THICKNESS VALUES DROP TOO MUCH.

SNOW LEVELS START TO DROP IN THE WA CASCADES SO SNOW COVERAGE
STARTS TO INCREASE ON THU
ONCE THE COLD ADVECTION SETS IN BEHIND THE LOW LEVEL COLD FRONT.
THE APPROACHING 700 MB TROUGH PROVIDE CONVERGENCE MAXIMA AND
ASCENT AS COLD AIR ARRIVES. THIS TROUGH WILL BE STRONGER...SO COLD
AIR STARTS PENETRATING FURTHER SOUTH INTO OR/ID.

ON DAY 3/THU NIGHT-FRI) DEEP LAYER WEST-SOUTHWEST FLOW WITH LOW
DIRECTIONAL SHEAR IS FORECAST TO HAVE HIGH RELATIVE HUMIDITY
THROUGH A DEEP LAYER AND ASCENT 850-700 MB.
AN INITIAL PERIOD OF SNOW OCCURS WITH THE ASCENT IN RESPONSE TO
UPPER DIVERGENCE MAXIMA AHEAD OF THE TROUGH.  ONCE THE UPPER
TROUGH PASSES...ASCENT BECOMES MORE TARGETED TO WHERE UPSLOPE FLOW
OCCURS...PARTICULARLY IN THE OR CASCADES AND THEN THE SALMON
RIVER/BOISE MNTNS. CONSEQUENTLY...THE WA/OR CASCADES AND SALMON
ROVER MNTNS ARE WHERE THE HIGHER PROBABILITIES FOR HEAVY SNOW ARE
FORECAST.

DUE TO GOOD MODEL AGREEMENT ON THE TIMING/AMPLITUDE OF THE UPPER
TROUGH...THE MODELS/ENSEMBLES WERE WEIGHTED EQUALLY IN THE
FORECAST.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT DAYS
1-3.

PETERSEN


$$





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