Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 192018
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
318 PM EST FRI DEC 19 2014

VALID 00Z SAT DEC 20 2014 - 00Z TUE DEC 23 2014


DAYS 1-3...

...PACIFIC NORTHWEST TO THE NORTHERN AND CENTRAL ROCKIES...

THE MODELS AGREEMENT IN BRINGING A MID-UPPER SHORTWAVE AND ITS
FRONTAL SYSTEM ACROSS THE NORTHERN ROCKIES AND FURTHER EAST ONTO
THE PLAINS SAT.  THE BREAK AFTER THIS SYSTEM WILL BE SHORT LIVED
AS A POWERFUL WEST TO EAST UPPER JET MOVES ONSHORE FROM THE
PACIFIC...ACCOMPANIED BY STRONG LOW-MID LEVEL WARM ADVECTION.
WHILE PRECIPITATION INCREASES...THE WARM ADVECTION RAISES SNOW
LEVELS.  THIS WARMING LIMITS AREAL COVERAGE OF HEAVY SNOW
POTENTIAL ACROSS THE OR-WA CASCADES.

ON DAY 2...THE 12Z ECMWF FORECASTS AN IMPRESSIVE 150 KT 300 MB JET
MAX CROSSING WA STATE WITH 130-150 KT INTO WESTERN MT AND
NORTHWEST WY.  STRONG LOW-MID LEVEL MOISTURE FLUXES AND SUSTAINED
LOW-MID LEVEL ASCENT FROM UPPER DIVERGENCE/LOWER CONVERGENCE
MAXIMA AND UPSLOPE FLOW LEAD TO WIDESPREAD MODERATE TO HEAVY SNOW
ACROSS THE RANGES OF NORTHWEST WA...SOUTHERN TO CENTRAL ID...AND
FINALLY INTO NORTHWEST WY.

ON DAY 3...WITH THE BUILDING MID-UPPER RIDGE OFF THE WEST
COAST...THE 300 MB JET BECOMES MORE NORTHWEST TO
SOUTHEAST...CONTINUING ACROSS WESTERN MT TO WESTERN WY AND CO.
WITH DRYING DOWNSTREAM FROM THE RIDGE IN THE SNOW GROWTH LAYER
ACROSS WA AND THEN ID AND WESTERN MT...HIGHER AMOUNTS ARE FCST
WHERE MOISTURE IN MORE PERSISTENT IN UT AND CO.  THE STRONGER AND
LONGER LASTING 700 MB CONVERGENCE AND ASCENT SUGGESTS THE RANGES
OF NORTHERN CO WILL HAVE THE HIGHEST SNOW AMOUNTS...WITH SECONDARY
MAXIMA IN THE UT WASATCH/UINTAS AND RANGES OF SOUTHWEST WY...WHERE
A FEW OF MODELS SUGGEST ASCENT TAPERS THE LATTER HALF OF DAY 3
(MON).

QPF AND THERMAL PROFILES FROM THE NAM...GFS...SREF MEAN AND ECMWF
SERVED AS A BASIS FOR THE WPC SNOW ACCUMULATION FORECASTS.
MULTI-DAY TOTALS SHOW POTENTIAL OF 2-3 FEET OF SNOW IN THE
NORTHERN WA CASCADES TO RANGES OF SOUTHERN ID AND NORTHWEST WY.

...ARROWHEAD OF MN...
THE MODELS INDICATE A DEVELOPING MID-UPPER LEVEL CYCLONE MOVES
SLOWLY SOUTHEAST FROM THE DAKOTAS TOWARDS THE UPPER MS VALLEY.
THE NAM AND GFS PARALLEL SHOW MODEST UPPER DIVERGENCE/LOWER
CONVERGENCE MAXIMA DEVELOPING IN MN WITH A LAKE ENHANCEMENT AS
CROSS LK SUPERIOR SFC WINDS ARE FCST TO ENHANCE LOW LEVEL MOISTURE
AND LEE SHORE LIFT ON THE LAKE SHORE WHERE TOPOGRAPHY AIDS RISING
MOTION.  CONSEQUENTLY THE THREAT IS CONFINED TO AREAS NEAR THE
LAKES.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN

$$





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