Quantitative Precipitation Forecast
Issued by NWS

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000
FOUS11 KWBC 191914
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
313 PM EDT TUE SEP 19 2017

VALID 00Z WED SEP 20 2017 - 00Z SAT SEP 23 2017


DAYS 1-3...

...NORTHERN ROCKIES AND THE WASHINGTON/OREGON CASCADES...

THE SHORTWAVE CURRENTLY MOVING THROUGH THE BASE OF THE LONGWAVE
TROUGH THAT IS CENTERED OVER THE WESTERN U.S. IS FORECAST TO LIFT
OUT TO THE NORTHEAST INTO THE NORTHERN PLAINS TUE...LEAVING
PROGRESSIVE ZONAL FLOW IN ITS WAKE ACROSS THE PACIFIC NORTHWEST
AND NORTHERN ROCKIES.  POSITIVE LOW TO MID LEVEL THETA-E ADVECTION
AHEAD OF A STRING OF EMBEDDED SHORTWAVES WILL HELP TO SUPPORT
AREAS OF LIGHT TO MODERATE PRECIPITATION ACROSS THE NORTHERN
ROCKIES...WITH THE WPC DAY 1 PROBABILITIES (ENDING 00 UTC THU)
INDICATING AT LEAST A SLIGHT RISK FOR SNOW ACCUMULATIONS OF
4-INCHES OR MORE FROM THE HIGHER PEAKS OF THE WASHINGTON AND
OREGON CASCADES TO THE NORTHERN ROCKIES...PRIMARILY ALONG THE
NORTHERN-CENTRAL ID TO THE WESTERN MONTANA AND NORTHWESTERN
WYOMING RANGES.

BY LATE WED...FLOW IN THE WEST IS EXPECTED TO AMPLIFY ONCE AGAIN
AS A WELL-DEFINED SHORTWAVE DROPPING INTO THE BASE OF THE TROUGH
BEGINS TO DIG SOUTH FROM THE PACIFIC NORTHWEST COAST...CARVING OUT
A DEEP TROUGH OVER THE WESTERN U.S. BY LATE THU.  FAVORABLE UPPER
JET DYNAMICS ALONG WITH LOW-MID LEVEL FRONTOGENESIS ARE EXPECTED
TO SUPPORT INCREASING PROBABILITIES FOR HEAVY SNOW ACCUMULATIONS
ACROSS PORTIONS OF THE NORTHERN ROCKIES...WITH WPC DAY 2
PROBABILITIES (ENDING 00 UTC FRI) INDICATING A SLIGHT RISK FOR
8-INCHES OR MORE ACROSS NORTHWESTERN WYOMING...MAINLY ABOVE 8000
FT ALONG THE TETON...ABSAROKA AND WIND RIVER RANGES.

THE TROUGH IS FORECAST TO CONTINUE AMPLIFYING LATE THU INTO EARLY
FRI...WITH THE GFS SHOWING 500 MB HEIGHT ANOMALIES DROPPING TO 1-2
STANDARD DEVIATIONS BELOW NORMAL AS A CLOSED CENTER DEVELOPS OVER
IDAHO.  LOW TO MID LEVEL FRONTOGENESIS ALONG WITH CONTINUED UPPER
JET SUPPORT WILL ENCOURAGE ADDITIONAL HEAVY SNOWS ACROSS
NORTHWESTERN WYOMING...WITH WPC DAY 3 PROBABILITIES (ENDING 00 UTC
SAT) SHOWING AT LEAST SLIGHT RISK FOR A FOOT OR MORE...MAINLY
ABOVE 8000 FT ALONG THE ABSAROKA AND WIND RIVER RANGES.  THE HEAVY
SNOW THREAT IS EXPECTED TO EXTEND EAST LATE IN THE PERIOD...WITH A
SLIGHT RISK FOR A FOOT OR MORE ALONG THE BIG HORNS DURING THE DAY
3 PERIOD.

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

PEREIRA



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