Quantitative Precipitation Forecast
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000
FOUS11 KWBC 201905
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
304 PM EDT WED SEP 20 2017

VALID 00Z THU SEP 21 2017 - 00Z SUN SEP 24 2017


DAYS 1-3...

...NORTHERN ROCKIES...

BY LATE WED...THE UPPER LEVEL PATTERN IN THE WEST IS EXPECTED TO
AMPLIFY ONCE AGAIN AS A WELL-DEFINED SHORTWAVE DROPPING INTO THE
BASE OF A LONGWAVE 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 1
PROBABILITIES (ENDING 00 UTC FRI) INDICATING AT LEAST A SLIGHT
RISK FOR 8-INCHES OR MORE ACROSS NORTHWESTERN WYOMING...MAINLY
ABOVE 8000 FT ALONG THE TETON...ABSAROKA...BEARTOOTH AND WIND
RIVER RANGES.  LIGHTER AMOUNTS ARE EXPECTED FURTHER TO THE
SOUTHWEST...WITH A SLIGHT RISK FOR 4-INCHES OR MORE ACROSS
PORTIONS OF THE SIERRA AS WELL AS AS THE MOUNTAINS OF
CENTRAL...EAST-CENTRAL AND NORTHEASTERN NEVADA.

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 ACROSS THE NORTHERN ROCKIES 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 2 PROBABILITIES (ENDING 00 UTC
SAT) SHOWING AT LEAST SLIGHT RISK FOR A FOOT OR MORE...MAINLY
ABOVE 8000 FT ALONG THE ABSAROKA...BEARTOOTH AND WIND RIVER
RANGES.  THE HEAVY SNOW THREAT IS EXPECTED TO EXTEND SOUTH AND
EAST LATE IN THE PERIOD...WITH A SLIGHT RISK FOR 8-INCHES OR MORE
ALONG THE UINTAS AND BIG HORNS DURING THE DAY 2 PERIOD.

BY LATE FRI...THE UPPER LOW IN THE WEST IS EXPECTED TO FILL...WITH
SHORTWAVE ENERGY LIFTING OUT TO THE NORTHEAST INTO SOUTHERN
CANADA...DIMINISHING THE POTENTIAL FOR HEAVY SNOWS.  HOWEVER...A
SLIGHT RISK FOR 8-INCHES OR MORE REMAINS ACROSS PORTIONS OF THE
REGION...PARTICULARLY ALONG PARTS OF THE BEARTOOTH...WIND RIVER
AND BIG HORN MOUNTAINS DURING THE DAY 3 PERIOD (ENDING 00 UTC
SUN).

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

PEREIRA

$$





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