Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 150915
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
415 AM EST FRI DEC 15 2017

VALID 12Z FRI DEC 15 2017 - 12Z MON DEC 18 2017

DAYS 1 THROUGH 3...

...MID-ATLANTIC...

SFC LOW CURRENTLY ANALYZED ACROSS THE CAROLINAS EARLY THIS MORNING
IS EXPECTED TO TRACK QUICKLY NORTHEASTWARD TODAY. THE LOW WILL
BEGIN TO DEEPEN OFF THE MID-ATLANTIC COASTLINE AS SHORTWAVE ENERGY
ROUNDS THE BASE OF A BROAD UPPER TROUGH CENTERED OVER ONTARIO. AS
THE SHORTWAVE BEGINS TO GO NEGATIVELY TILTED ACROSS THE
MID-ATLANTIC AND THE SURFACE LOW DEEPENS OFFSHORE, AN AREA OF
PRECIPITATION IS EXPECTED TO DEVELOP BY MIDDAY ACROSS PORTIONS OF
THE MID-ATLANTIC STATES. TEMPERATURES WILL BE SUFFICIENTLY COLD
FOR MOST AREAS FOR SNOW TO BE THE DOMINANT P-TYPE. LATEST HREF
SHOWS A RAPID EXPANSION OF SNOW PROBABILITIES ACROSS DELMARVA
REGION BY 18Z TODAY THEN QUICKLY MOVING NORTHEASTWARD ALONG MUCH
OF THE I-95 CORRIDOR BY 21Z. GIVEN THE DYNAMIC NATURE OF THE
SYSTEM, FRONTOGENETIC FORCING COULD SUPPORT LOCALLY HEAVY SNOWFALL
RATES, BUT THE QUICK MOVEMENT OF THE SYSTEM WILL SERVE AS THE
LIMITING FACTOR ON ACCUMULATIONS. NONETHELESS, WPC PROBABILITIES
SUGGEST AT LEAST AN INCH OF SNOW IS POSSIBLE FROM THE DC METRO
AREA NORTH TO THE BOSTON VICINITY ON DAY 1 (12Z FRI-12Z SAT).
MODELS SHOW THE BEST FRONTOGENETIC FORCING DEVELOPING BY LATE
AFTERNOON ACROSS NORTHERN PORTIONS OF THE DELMARVA PENINSULA AND
SOUTHERN NEW JERSEY, AND THIS IS LIKELY WHERE THE BEST CHANCE FOR
HEAVIER SNOW RATES COULD OCCUR.


...GREAT LAKES...

A CLIPPER SYSTEM CROSSING THE GREAT LAKES TODAY WILL BRING SNOW TO
MUCH OF THE REGION, WITH LAKE EFFECT SNOWS LINGERING IN ITS WAKE
INTO SATURDAY. HEAVIEST SNOWS ARE EXPECTED DOWNWIND OF LAKE ERIE
AND ALSO ACROSS NORTHERN PORTIONS OF LOWER MICHIGAN. MODERATE TO
HIGH PROBABILITIES OF EXCEEDING 4 INCHES ARE SHOWN ACROSS THESE
TWO AREAS ON DAY 1 WPC PROBABILITIES. TOTALS ACROSS DAYS 1 AND 2
MAY EXCEED 8 INCHES ACROSS PORTIONS OF EXTREME WESTERN NY/NWRN PA
DOWNWIND OF LAKE ERIE.


...PACIFIC NORTHWEST/INTERMOUNTAIN WEST AND NRN/CTRL ROCKIES...

UPPER TROUGH IS EXPECTED TO MOVE INLAND ACROSS THE PAC NW LATE
TODAY AND THEN AMPLIFY AS IT CROSSES THE INTERIOR WESTERN U.S. ON
SAT. A BAND OF PRECIPITATION WILL ACCOMPANY THE FRONT ACROSS THE
NORTHWEST AND NORTHERN ROCKIES TODAY INTO EARLY SAT, WITH 4-8 INCH
SNOWFALL (AND LOCALLY HIGHER) AMOUNTS POSSIBLE ACROSS THE NORTHERN
CASCADES AND THE NORTHERN ROCKIES. AS THE UPPER TROUGH DIGS
SOUTHWARD ON SATURDAY THE COLD FRONT WILL RUSH SOUTHWARD ACROSS
THE ROCKIES SPREADING COLDER AIR ACROSS THE CENTRAL ROCKIES, AND
EVEN THE SOUTHERN ROCKIES BY SUN MORNING. EXPECT HEAVIEST SNOWS ON
DAY 2 (12Z SAT-12Z SUN) TO FOCUS ACROSS MUCH OF WY AS SURFACE
CYCLOGENESIS TAKES PLACE SOUTH OF THE REGION ALLOWING SOME MODEST
UPSLOPE ENHANCEMENT, AND FAVORABLE UPPER DIFFLUENT FLOW PERSISTS
OVERHEAD. MODERATE TO HIGH PROBABILITIES FOR AT LEAST 4 INCHES OF
SNOW ARE SHOWN ON DAY 2 ACROSS MUCH OF WY, EVEN PORTIONS OF THE
HIGH PLAINS IN EASTERN WY, WITH AMOUNTS EXCEEDING 8 INCHES
POSSIBLE AT THE HIGHER ELEVATIONS SUCH AS THE BIG HORN RANGE.


...HIGHER ELEVATIONS OF AZ/NM INCLUDING THE MOGOLLON RIM...

STILL A BIT OF UNCERTAINTY AS TO THE EXACT EVOLUTION OF THE UPPER
TROUGH AS IT DIGS INTO THE SOUTHWEST, WITH THE ECMWF CONTINUING TO
CUT OFF AN UPPER LOW SAT NIGHT/SUN MORNING AND THE GFS/NAM A BIT
WEAKER. NONETHELESS, MODELS ARE IN GENERAL AGREEMENT THAT SOME
RELATIVELY LIGHT SNOWFALL AMOUNTS OF AN INCH OR TWO WILL BE
POSSIBLE ON DAY 3 (12Z SUN-12Z MON) ACROSS PORTIONS OF THE HIGHER
TERRAIN OF NEW MEXICO AND EASTERN AZ, WITH SLIGHT TO MODERATE
PROBABILITIES FOR EXCEEDING 4 INCHES SHOWN ON THE DAY 3 OUTLOOK.


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

RYAN

$$





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