Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 120851
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
350 AM EST TUE DEC 12 2017

VALID 12Z TUE DEC 12 2017 - 12Z FRI DEC 15 2017


DAYS 1 THROUGH 3...

THE SYNOPTIC SCALE PATTERN ACROSS THE CONTINENTAL U.S. WILL REMAIN
AMPLIFIED THROUGH THURSDAY NIGHT, WITH A LARGE RIDGE OVER THE WEST
AND A LARGE-SCALE TROUGH OVER THE CENTRAL AND EASTERN PART OF THE
NATION.  MULTIPLE SHORTWAVE IMPULSES AND SURFACE LOWS WILL BE
EMBEDDED WITHIN THE FLOW AROUND THE TROUGH, AND THESE WILL PRODUCE
ROUNDS OF SNOW OVER THE NEXT FEW DAYS, ALONG WITH PERIODS OF LAKE
EFFECT SNOW.  THE ONLY PARTS OF THE U.S. THAT ARE EXPECTED TO HAVE
WINTRY PRECIPITATION THROUGH THURSDAY ARE THE UPPER MIDWEST, GREAT
LAKES REGION, CENTRAL AND NORTHERN APPALACHIANS, AND NEW ENGLAND.
THE WEST IS EXPECTED TO BE VOID OF ANY NOTEWORTHY SNOW OWING TO
THE EXPANSE OF THE AFOREMENTIONED RIDGE.

A STRONG LOW PRESSURE SYSTEM DEVELOPING OVER NEW YORK AND THEN NEW
ENGLAND WILL BE THE MAIN STORY IN TERMS OF SIGNIFICANT SNOWFALL
DURING THIS FORECAST PERIOD.  PHASING BETWEEN THE ALBERTA CLIPPER
FROM THE OHIO VALLEY AND A STRONG ARCTIC SHORTWAVE FROM THE HUDSON
BAY REGION, ALONG WITH LEFT EXIT UPPER LEVEL JET DYNAMICS, WILL
SUPPORT RAPID CYCLOGENESIS AND DEEP LAYERED ASCENT.  LOW LEVEL
WARM AIR ADVECTION WILL RESULT IN SURFACE TO 850MB TEMPERATURES
RISING ABOVE FREEZING FOR MOST OF THE EVENT NEAR THE COAST, SO THE
MAJOR METROPOLITAN AREAS ALONG THE INTERSTATE 95 CORRIDOR SHOULD
BE MAINLY RAIN.  THERE WILL LIKELY BE A PERIOD OF MIXED
PRECIPITATION FROM CENTRAL MASSACHUSETTS TO CENTRAL MAINE AROUND
00Z AS A 925-850MB WARM NOSE APPEARS ON NAM FORECAST SOUNDINGS,
WITH SNOW EARLIER IN THE DAY ON TUESDAY.  FARTHER INLAND,
WIDESPREAD 6 TO 10 INCH AMOUNTS APPEAR LIKELY FROM NORTHERN NEW
YORK TO NORTHERN MAINE, WITH STRONG ASCENT IN THE DENDRITIC GROWTH
ZONE.  THERE WILL ALSO BE SIMILAR AMOUNTS DOWNWIND OF LAKE ERIE
AND LAKE ONTARIO OWING TO LAKE ENHANCEMENT BEHIND THE COLD FRONT.
THERE WAS ENOUGH AGREEMENT ON THE THERMAL PROFILES IN THE MODELS
TO WARRANT A GENERAL BLEND.

BY MIDWEEK, ANOTHER CLIPPER SYSTEM AND SHORTWAVE TRACKING
SOUTHEASTWARD ACROSS THE MIDWEST STATES AND OHIO VALLEY IS
EXPECTED TO PRODUCE A SWATH OF LIGHT TO MODERATE SNOW FROM THE
WESTERN GREAT LAKES TO PENNSYLVANIA.  THE MODELS HAVE TRENDED
HEAVIER WITH THE QPF, AND THUS THE SNOWFALL FORECAST HAS INCREASED
SINCE 24 HOURS AGO.  THERE IS ALSO ENSEMBLE MEMBER SUPPORT FOR 4+
INCHES FROM SOUTHEAST WISCONSIN TO NORTHERN OHIO, AND THE WPC
PROBABILITIES INDICATE A MODERATE CHANCE OF 4+ INCHES FOR THIS
AREA.  LOCALLY ENHANCED SNOWFALL IS ALSO LIKELY ACROSS THE HIGHER
ELEVATIONS OF THE CENTRAL APPALACHIANS.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT
NATIONWIDE.

D. HAMRICK

$$





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