Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 200743
QPFHSD

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

VALID 12Z WED SEP 20 2017 - 12Z SAT SEP 23 2017


DAYS 1-3...

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

THE AMPLIFYING WESTERN US TROUGH RESULTS IN BACKING FLOW IN WY
WITH LOW TO MID LEVEL THETA-E ADVECTION ON LOW SHEAR DEEP
SOUTHWEST FLOW. THE UPPER JET CROSSING SOUTHERN ID INTO ADJACENT
SOUTHWEST MT SUPPORTS UPPER DIVERGENCE MAXIMA CROSSING FROM ID
INTO NORTHWEST WY.
A PERIOD OF SNOW IS LIKELY WITH SEVERAL INCHES EXPECTED IN THE
NORTHERN ROCKIES...WITH HIGHER AMOUNTS/PROBABILITIES CENTERED ON
THE WY TETONS/YELLOWSTONE NATIONAL PARK. LIGHTER SNOWS EXTEND FROM
THE HIGHER PEAKS OF THE WASHINGTON AND OREGON CASCADES TO THE
NORTHERN ROCKIES...PRIMARILY IN THE RANGES OF SOUTHERN TO CENTRAL
ID TO SOUTHWESTERN MONTANA.

THE TROUGH CONTINUES TO AMPLIFY ON THU (DAY 2)...WITH THE
GFS/ECMWF SHOWING AN UPPER LEVEL CIRCULATION DEVELOPING OVER THE
GREAT BASIN.  A WAVY SFC AND NEAR SFC FRONT PROVIDES A LOW-MID
LEVEL FRONTOGENESIS MAXIMA EXTENDING FROM NORTHERN NV AND SOUTHERN
ID ACROSS MUCH OF NORTHERN UT AN WY.   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 DAY 2 PROBABILITIES
INDICATING A SLIGHT TO MODERATE RISK FOR 8 INCHES OR MORE ACROSS
NORTHWESTERN WYOMING...MAINLY IN THE TETON...ABSAROKA AND WIND
RIVER RANGES.

ON FRI THE LOW LEVEL FRONTOGENESIS MAXIMA IS FORECAST TO BE
FOCUSED ON UT AND WY SO HIGHER SNOW AMOUNTS ARE EXPECTED WITHIN
THE FRONTAL CIRCULATION AND WHERE ADDITIONAL LIFT FROM TOPOGRAPHY
OVER NORTHERN UT AND THE RANGES OF WESTERN WY AND SOUTH CENTRAL
MT.  THE HEAVY SNOW THREAT IS EXPECTED TO EXTEND INTO THE BIG
HORNS FRI AND FRI NIGHT. LIGHTER SNOWS OCCUR IN THE DIFLUENT FLOW
ALOFT NEAR THE UPPER LOW/UPPER TROUGH AXIS ACROSS HIGHER
ELEVATIONS OF SOUTHERN ID TO SOUTHWESTERN MT. CONFIDENCE IS NOT AS
HIGH THROUGH FRI NIGHT DUE TO GROWING SPREAD ON QPF AND RESULTANT
SNOW AMOUNTS.

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

PETERSEN



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