Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 210809
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
408 AM EDT MON APR 21 2014

VALID 12Z MON APR 21 2014 - 12Z THU APR 24 2014

...PACIFIC NORTHWEST/NORTHERN ROCKIES...

MODELS REMAIN IN GOOD AGREEMENT REGARDING A 700 MB TROUGH AND
FRONT MOVING EAST ACROSS THE PACIFIC NORTHWEST.  SNOW SHOWERS ARE
EXPECTED AT HIGHER ELEVATIONS OF THE OR/WA CASCADES WITH THE
FRONTAL PASSAGE.  AS COLD ADVECTION OCCURS BEHIND THE FRONT...
SNOW LEVELS DECREASE AND LAPSE RATES STEEPEN TO 7-8 DEG C/KM.
MEAN LAYER RH STILL LOOKS TO BE IN THE 80 TO 90 PCT RANGE AT THE
SAME TIME AN UPPER LEVEL DIVERGENCE MAXIMA IN THE LEFT EXIT REGION
SUPPORT POST FRONTAL SHOWERS UNTIL THE UPPER TROUGH PASSES DURING
DAY 2.  THE 00Z ECMWF REDUCED THE HIGHER QPF AMOUNTS ON DAY 2 IN
THE OR CASCADES TO CLUSTER BETTER WITH THE OTHER MODEL/ENSEMBLE
SOLUTIONS.

AS THE UPPER TROUGH APPROACHES THE NORTHERN ROCKIES TUE
NIGHT...SNOW SHOWERS DEVELOP UNDER THE UPPER DIVERGENCE MAXIMA/MID
LEVEL DEFORMATION IN THE RANGES OF ID AND SOUTHWESTERN MT AND
ADJACENT NORTHWEST WY. THIS CIRCULATION THEN PIVOTS NORTH WITH
SNOW ON WED IN THE WRAPAROUND BAND WEST OF THE CIRCULATION IN
GLACIER NATIONAL PARK...AND ALONG THE SECONDARY UPPER JET MAXIMA
EXTENDING ACROSS THE RANGES OF OR TO SOUTHERN ID/NORTHWEST WY. THE
00Z ECMWF DRIES OUT THESE AREAS AREAS FASTER THAN IT DID ON THE
12Z RUN...CUTTING BACK ON QPF AND RESULTANT SNOW AMOUNTS.

GIVEN GOOD MODEL AGREEMENT...USED A MULTI-MODEL CONSENSUS OF THE
12Z-00Z ECMWF/18Z GFS QPF/TEMPERATURE PROFILES ON THE SPEED OF THE
TROF AND PLACEMENT OF PCPN IN TERRAIN.

THE PROBABILITY FOR SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN


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