Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 212121
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
520 PM EDT TUE MAR 21 2017

VALID 00Z WED MAR 22 2017 - 00Z SAT MAR 25 2017

...WESTERN U.S....

AN ACTIVE PATTERN WILL CONTINUE ACROSS MUCH OF THE WESTERN U.S.
THROUGH THE END OF THE WEEK...WITH HIGHER ELEVATIONS OF THE
CASCADES...SIERRA NEVADA...CENTRAL GREAT BASIN...AND CENTRAL
ROCKIES ALL EXPECTED TO SEE MODERATE TO LOCALLY HEAVY SNOWFALL
ACCUMULATIONS.

THE INITIAL THREAT FOR ACCUMULATING SNOW WILL BE WITH DEEP AND
MOIST SOUTHWESTERLY FLOW AHEAD OF A LONGWAVE TROUGH PROGRESSING
INLAND OVER THE WESTERN U.S. TUESDAY NIGHT INTO WEDNESDAY...WHICH
SHOULD BRING PERIODS OF HEAVY SNOW TO THE FAVORED TERRAIN OF THE
OLYMPICS...CASCADES...SHASTA...AND ESPECIALLY SIERRA NEVADA RANGES.

BY WEDNESDAY NIGHT...THE GREATEST CHANCE FOR HEAVY SNOW SHOULD
BEGIN TO TRANSITION EASTWARD AS SHORTWAVE ENERGY DIGGING INTO THE
SOUTHERN PORTION OF THE LONGWAVE THROUGH AMPLIFIES OVER CALIFORNIA
AND POTENTIALLY CLOSES OFF OVER THE CENTRAL GREAT BASIN EARLY
THURSDAY.  LOW TO MID LEVEL FRONTOGENESIS ALIGNED WITH FAVORABLE
UPPER FORCING SHOULD SUPPORT AN AXIS OF ORGANIZED SNOW EXTENDING
FROM THE SOUTHERN WASATCH TO THE UNITAS AND TETONS.  WHILE THE
CLOSED ENERGY PROGRESSES EASTWARD AND A SURFACE LOW DEEPENS IN THE
LEE OF THE COLORADO ROCKIES THURSDAY NIGHT INTO FRIDAY...THE FOCUS
FOR HEAVY SNOW WILL SHIFT TO THE CENTRAL ROCKIES...WITH A
POTENTIALLY SIGNIFICANT UPSLOPE SNOW EVENT POSSIBLE ALONG THE
FRONT RANGE.

UPSTREAM...JUST BEFORE THE CLOSED ENERGY EJECTS INTO THE
PLAINS...AN UPPER TROUGH AND ASSOCIATED COLD FRONT WILL BEGIN
PRESSING INLAND OVER THE WESTERN U.S....BRINGING ANOTHER CHANCE
FOR MODERATE TO HEAVY SNOWFALL ACCUMULATIONS TO THE HIGHER
ELEVATIONS OF THE OLYMPICS...CASCADES...AND NORTHERN CALIFORNIA
THURSDAY NIGHT INTO FRIDAY.

PLEASE REFER TO WPC`S MODEL DIAGNOSTIC DISCUSSION (PMDHMD) FOR
DETAILS ON THE LARGE AMOUNT OF MODEL SPREAD THAT STILL EXISTS WITH
THE EVOLUTION OF THE UPPER LOW CLOSING OFF OVER THE CENTRAL GREAT
BASIN AND TIMING DIFFERENCES THAT DEVELOP WITH THE UPSTREAM COLD
FRONT APPROACHING THE WEST COAST THURSDAY NIGHT INTO FRIDAY.

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


...NORTHERN NEW ENGLAND...

SNOW WILL BE POSSIBLE ACROSS PORTIONS OF THE NORTHEAST TUESDAY
NIGHT INTO WEDNESDAY AS A BROAD UPPER TROUGH PROGRESSES EASTWARD
OUT OF THE GREAT LAKES REGION.  SHORTWAVE ENERGY DIGGING INTO THE
BASE OF THE TROUGH COULD HELP PRODUCE A SWATH OF LIGHT TO MODERATE
ACCUMULATIONS WHILE THE ENERGY AMPLIFIES OVER NEW ENGLAND AND A
SURFACE LOW QUICKLY DEEPENS IN THE BAY OF FUNDY.  HOWEVER...THERE
IS STILL SOME MODEL SPREAD WITH THE EXACT LOCATION/DEPTH OF THE
SURFACE LOW AND RESULTING AXIS OF MODERATE SNOW.  THE WPC FORECAST
STAYED CLOSE TO A BLEND BETWEEN THE GFS/ECMWF...WHICH MATCHED WELL
WITH CONTINUITY IN KEEPING THE GREATEST CHANCE FOR SNOWFALL
ACCUMULATIONS IN EXCESS OF FOUR INCHES CONFINED TO NORTHERN MAINE.



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


GERHARDT

$$




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