Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 202034
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
434 PM EDT MON OCT 20 2014

VALID 00Z TUE OCT 21 2014 - 00Z FRI OCT 24 2014


...DAY 1...

THE MODELS SHOW A MID-UPPER LEVEL TROUGH CROSSING THE NORTHERN
ROCKIES TONIGHT AND EARLY TUESDAY. COUPLETS OF UPPER DIVERGENCE
AND LOW LEVEL CONVERGENCE PRODUCE ASCENT WITHIN A MOISTENING AIR
MASS.  THIS LEADS TO WIDESPREAD PRECIPITATION.  AS HEIGHTS FALL
RAIN CHANGES TO SNOW AT HIGHER ELEVATIONS OF THE SAWTOOTH AND
BITTERROOT RANGES. THE MODELS DIFFER AS TO HOW LOW THE SNOW
ELEVATIONS GET...WITH A BLEND OF MODEL/SHORT RANGE ENSEMBLE
SOLUTIONS USED TO MITIGATE TEMPERATURE PROFILE DIFFERENCES.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


...DAY 2...

DEEP LAYER SOUTHWEST FLOW CONTINUES ACROSS THE PACIFIC NORTHWEST...
SO COLD ADVECTION WILL BE SLOW TO DEVELOP AS HEIGHTS ARE SLOW TO
LOWER. THIS LEAVES THE PRIMARY THREAT FOR MODERATE TO HEAVY SNOW
IN THE NORTHERN WA CASCADES.
THE NEXT SURGE OF WARM/MOISTURE ADVECTION IN THE LOW-MID LEVELS
WITH THE 700 MB JET CROSSING THE NORTHERN WA CASCADES LEADING TO A
RESURGENCE IN PRECIPITATION.
THE MODELS CLUSTER WELL ON THE TIMING OF THE UPPER TROUGH SO A
BLEND OF THE MODEL/ENSEMBLE QPF AND TEMPERATURE PROFILES WAS USED
IN THE SNOW FORECAST.

IN THE RANGES OF SOUTHWEST MT AND NORTHWEST WY...THE DEPARTURE OF
THE UPPER WAVE LEADS TO A TREND TOWARDS DRYING ALOFT AND SINKING
MOTION DEVELOPING...SO SNOW AMOUNTS SHOULD BE LIGHT GIVEN THE
SHORT DURATION.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

...DAY 3...

THE ONLY RISK OF LIGHT TO MODERATE SNOW SHOULD LINGER IN THE
NORTHERN PORTION OF THE CASCADES IN WA WHERE THE MODELS AND
ENSEMBLES STILL SHOWED LOW-MID LEVEL SOUTHWEST FLOW CONTINUING.
GRADUAL HEIGHT FALLS LEAD TO A GRADUAL LOWERING OF SNOW LEVELS
WITH TIME...SO NONE OF THE GLOBAL MODELS OR SREF MEAN SHOW HEAVY
SNOW POTENTIAL RIGHT NOW.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN




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