Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS11 KWBC 200711
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
310 AM EDT SUN APR 20 2014

VALID 12Z SUN APR 20 2014 - 12Z WED APR 23 2014

...PACIFIC NORTHWEST/NORTHERN ROCKIES...

MODELS ARE IN GOOD AGREEMENT REGARDING A 700 MB TROUGH AND FRONT
MOVING EAST ACROSS THE PACIFIC NORTHWEST ON DAY 2.  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.
CONTINUED MOIST CONDITIONS WITH HIGH LAYER RH OF 80-90 PERCENT AND
UPPER DIVERGENCE MAXIMA IN THE LEFT EXIT REGION SUPPORT POST
FRONTAL SHOWERS UNTIL THE UPPER TROUGH PASSES.  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.
BETTER THAN USUAL AGREEMENT OF THE 18Z-00Z GFS AND 12Z ECMWF ON
THE TIMING OF THE UPPER TROUGH AND MID LEVEL FRONT SO MORE
WEIGHTING WAS GIVEN TO THE 00Z GFS/12Z ECMWF QPF/TEMPERATURE
PROFILES FOR THE FORECAST.  CONFIDENCE WAS NOT HIGH IN THE FASTER
500 MB TROUGH PROGRESSION IN THE 00Z ECMWF RUN.

THE PROBABILITY FOR SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN


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