Quantitative Precipitation Forecast
Issued by NWS

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477
FOUS11 KWBC 192005
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
405 PM EDT SUN MAR 19 2017

VALID 00Z MON MAR 20 2017 - 00Z THU MAR 23 2017


...PACIFIC NORTHWEST/CA/NORTHERN GREAT BASIN TO NORTHERN ROCKIES...

...DAY 1...
A FRONTAL BOUNDARY WILL LINGER ACROSS THE NORTHERN
ROCKIES...PROVIDING A FOCUS FOR SNOW SHOWERS...WITH ACCUMULATIONS
FOCUSED WHERE FRONTAL ASCENT IS AUGMENTED BY OROGRAPHIC LIFT IN
WINDWARD TERRAIN IN THE NORTHWESTERN WY/SOUTHWESTERN MT RANGES.
AREAS OF 3-5 INCHES ARE EXPECTED IN FAVORED MOUNTAINS IN THE
REGION...SUPPORTED BY THE NAM/SREF MEAN/GFS/GEFS MEAN AND ECMWF
SNOWFALL OUTPUT.

...DAYS 2-3...THE THREAT FOR HEAVY SNOW IS EXPECTED TO RETURN TO
THE CA SIERRA NEVADA RANGE AS A DEEP UPPER TROUGH MOVES INTO
CALIFORNIA ON TUE. ON MONDAY NIGHT THE MODELS INDICATE AN
INCREASING UPPER JET WITH CORRESPONDING INCREASE IN MOISTURE
FLUXES ALOFT PLUS UPPER DIVERGENCE MAXIMA ON TUE THAT WILL COMBINE
WITH TOPOGRAPHIC LIFT IN PRODUCING VALLEY RAIN AND MOUNTAIN SNOWS.
 SNOW LEVELS WHICH ARE EXPECTED TO BE INITIALLY HIGH ARE FORECAST
TO DROP TUE AFTERNOON INTO THE EVENING AS THE UPPER TROUGH MOVES
ONSHORE. THE MODEL CONSENSUS SHOWS POTENTIAL FOR 12-24 INCHES OF
SNOW AT HIGHER ELEVATIONS FOR THE 2 DAY PERIOD.  SNOWS ARE
EXPECTED TO DECLINE IN COVERAGE/INTENSITY WHEN THE UPPER TROUGH
COMES ONSHORE AND MOVES INLAND.

WITH THE UPPER TROUGH OFF THE PACIFIC NORTHWEST COAST...A SEPARATE
NORTHERN STREAM JET MAXIMA COMES INTO THE WA CASCADES TO PRODUCE
SNOW SHOWERS ACROSS THE REGION TUE AFTERNOON AND NIGHT, WHEN THE
UPPER DIVERGENCE EXTENDS INLAND INTO THE NORTHERN ROCKIES, AS A
COUPLED JET DEVELOPS IN THE REGION.  WITH THE WAVE DEPARTURE WED
LEADING TO A DECREASE IN UPPER DIVERGENCE AND A BREAK IN ACTIVITY.

...HIGH PLAINS OF NORTHEAST WY/ADJACENT SOUTHEAST MT ACROSS SD TO
THE NE BORDER...
ON DAY 2 (MON NIGHT-TUESDAY) A LOW LEVEL FRONTAL BOUNDARY ACROSS
WY PERSISTS WITH LOW LEVEL CONVERGENCE IN THE COLD SECTOR NORTH OF
THE FRONT OCCURRING IN BETWEEN THE FRONT AND ARCTIC HIGH BUILDING
ACROSS SOUTHERN CANADA. A STRIPE OF LIGHT TO MODERATE SNOWS IS
FORECAST TO OCCUR IN THE BAND ACROSS NORTHEAST WY AND ADJACENT SD
AND MT.  MODEL QPF VALUES ARE ON THE ORDER OF A QUARTER TO HALF AN
INCH...SO POTENTIAL IS MODEST AND A HEAVY SNOW EVENT IS NOT
EXPECTED.  ALSO...ON THE PLAINS TEMPERATURES MAY NOT BE COLD
ENOUGH TO PRODUCE AL SNOW..WITH COOLING NEEDED TO ENSURE SNOW
FALLS IN MANY CASES.  THE MODELS HAVE TYPICAL DIFFERENCES BUT NO
PARTICULAR OUTLIER SOLUTION...SO A CONSENSUS APPROACH IS
RECOMMENDED.

THE PROBABILITY OF ICE ACCUMULATIONS OF 0.25 INCH OR MORE IS LESS
THAN 10 PERCENT.

PETERSEN



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