Quantitative Precipitation Forecast
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FOUS11 KWBC 240914
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
414 AM EST MON NOV 24 2014

VALID 12Z MON NOV 24 2014 - 12Z THU NOV 27 2014

DAYS 1 TO 3...

...PACIFIC NORTHWEST/GREAT BASIN/NORTHERN AND CENTRAL
ROCKIES/NORTHERN PLAINS...

THE FIRST 2 DAYS OF THE FORECAST WILL BE PARTICULARLY ACTIVE
ACROSS THE WEST AS A COUPLE OF DISTINCT SHORTWAVE SYSTEMS RIDE
OVER THE OVER RIDGE OVER THE EASTERN PACIFIC AND DIVE
SOUTHEASTWARD ACROSS THE WESTERN STATES.  ON DAY 1...THE SHORTWAVE
THAT LATER AMPLIFIES INTO THE POTENTIAL HEAVY SNOWSTORM ACROSS THE
EASTERN US IS CONSOLIDATING WITHIN THE UPPER LEVEL NORTHWESTERLY
FLOW. THIS SYSTEM IS RESPONSIBLE FOR SIGNIFICANT SNOWFALL OVER THE
COLORADO ROCKIES WHERE 4 TO 8 INCHES WILL BE COMMON IN THE HIGHER
ELEVATIONS.

THE NEXT SYSTEM WILL ALSO DIVE SOUTHEASTWARD FROM SOUTHWESTERN
CANADA WITH A SIGNIFICANT PLUME OF PACIFIC MOISTURE RIDING EAST
SOUTHEASTWARD FROM THE PACIFIC NORTHWEST ACROSS THE NORTHERN
ROCKIES WITHIN THE ENTRANCE REGION OF A STRONG UPPER LEVEL JET.
HEAVY SNOW IS EXPECTED FROM THE CASCADES IN WASHINGTON ACROSS THE
BITTERROOTS AND MUCH OF WESTERN MONTANA INTO THE MOUNTAINS OF
WYOMING. THERE ARE LOW TO MODERATE PROBABILITIES FOR A FOOT OF
SNOW OR GREATER ACROSS THE NORTHERN CASCADES AND A GOOD PORTION OF
WESTERN MONTANA INTO NORTHWESTERN WYOMING. LOW PRESSURE IN THE
EXIT REGION OF THE UPPER JET WILL DIVE SOUTHEASTWARD ACROSS NORTH
DAKOTA TO IOWA BUT MOISTURE IS EXPECTED TO BE LIMITED AND MOST
SNOWFALL TOTALS WILL BE LESS THAN 4 INCHES ACROSS THE NORTHERN
PLAINS.

DAY 1 TO DAY 2...

...MIDWEST/GREAT LAKES...

A COMPLEX LOW WILL MOVE NORTHEASTWARD FROM THE MIDWEST INTO
SOUTHERN CANADA ON MONDAY INTO EARLY TUESDAY. WHILE THE PRIMARY
CYCLONE CENTER WILL MOVE AWAY FROM THE MIDWEST...A RESIDUAL SHORT
WAVE TROUGH OVER THE UPPER MISSISSIPPI VALLEY WILL LIFT
NORTHEASTWARD ACROSS THE GREAT LAKES PRODUCING MORE SNOW AS COLDER
AIR MOVES BACK OVER THE REGION.  SIGNIFICANT SNOW IS EXPECTED TO
FALL ACROSS EASTERN WISCONSIN INTO THE NORTHWEST HALF OF LOWER
MICHIGAN AND MUCH OF UPPER MICHIGAN...WHERE THERE IS A HIGH
PROBABILITY OF GREATER THAN 4 INCHES BUT MOST AMOUNTS SHOULD BE
LESS THAN 8 INCHES.

DAYS 2 TO 3...

...SOUTHERN TO CENTRAL APPALACHIANS/MID ATLANTIC/NEW ENGLAND....

GIVEN THE POTENTIAL FOR MAJOR TRAVEL DISRUPTION DURING THE PEAK
PRE THANKSGIVING TRAVEL PERIOD ON TUESDAY AND WEDNESDAY...THE
FORECAST FOR THIS STORM IS QUITE CRUCIAL. A SIGNIFICANT CYCLONE IS
EXPECTED TO DEVELOP IN THE EASTERN GULF OF MEXICO ON TUESDAY
EVENING AND THEN RIDE UP ALONG THE EAST COAST ON WEDNESDAY. A BAND
OF SIGNIFICANT QPF IS EXPECTED TO DEVELOP WITHIN THE ENTRANCE
REGION OF A 200 KT 250 MB JET OVER THE NORTHEAST. NOT
UNEXPECTEDLY...THERE ARE QUITE A NUMBER OF DIFFERENT SOLUTIONS
WITH VERY DIFFERENT IMPACTS. RIGHT NOW...THE MOST SIGNIFICANT
DIFFERENCES INVOLVE  WHETHER THE STORM BECOMES RATHER INTENSE ON A
SLIGHTLY MORE WESTERLY TRACK VERSUS A SLIGHTLY WEAKER MORE
EASTERLY TRACK...THOUGH THERE MAY ME A TREND TOWARD SOME MIDDLE
GROUND BETWEEN THESE FORECASTS.  IN EITHER EVENT...SIGNIFICANT
SNOW IS POSSIBLE ALONG THE I95 CORRIDOR..WHETHER OR NOT IT IS
ALONG THE CORRIDOR OR SLIGHTLY WESTWARD. THE DETERMINISTIC
FORECAST MADE SHOWED THE HEAVIEST SNOW JUST INLAND WITH SOME
ACCUMULATIONS IN THE BIG CITIES REFLECTING A COMBINATION OF RAIN
CHANGING TO SNOW.

SNOW IS EXPECTED TO INITIALLY DEVELOP ALONG THE APPALACHIANS ON
TUESDAY EVENING/NIGHT...POSSIBLY AS FAR SOUTH AS THE HIGHER
ELEVATIONS OF NORTHEAST GEORGIA ACROSS NW SOUTH CAROLINA/WESTERN
NORTH CAROLINA AND WESTERN VIRGINIA AND EASTERN WEST VIRGINIA.
SNOW AND COASTAL RAIN SHOULD CONTINUE TO SPREAD NORTHEASTWARD
ACROSS THE REST OF THE EASTERN SEABOARD ON WEDNESDAY/DAY 3...THE
PEAK OF THE PRE HOLIDAY TRAVEL AND THEN END ACROSS EASTERN MAINE
ON THANKSGIVING MORNING.

HOWEVER...THIS SCENARIO IS MERELY ONE OF MANY THAT TYPIFY
POTENTIAL SIGNIFICANT SNOW EVENTS ALONG THE EAST COAST.  TO
GENERATE THE FORECAST...THERE SEEMED TO BE MORE SIMILARITIES
BETWEEN THE 00Z ECMWF ENSEMBLE MEAN/THE 00Z GFS/THE 21Z SREF THAN
WITH THE 12Z/00Z ECMWF AND UKMET.  THAT SAID...THE ECMWF/UKMET
CANNOT BE IGNORED AND PRESENTS SOME INTERESTING ALTERNATIVES.
THESE RUNS HAVE A CYCLONE SLIGHTLY MORE INTENSE AND CLOSER TO THE
COAST WITH A 1000/500 THICKNESS PATTERN THAT FIRST APPEARS TOO
HIGH FOR SIGNIFICANT SNOWFALL IN THE BIG CITIES. HOWEVER...THE
MODELS ARE INDICATING THAT LOW LEVEL TEMPERATURES WILL BE QUITE
LOW FOR THOSE THICKNESSES WITH COLD AIR ADVECTION IN THE LOW
LEVELS AS THE STORM MOVES NORTH.  THIS IS NOT A CLASSIC SET UP IN
THAT RELATIVELY COLD HIGH PRESSURE WILL BUILD NORTHEASTWARD FROM
THE MIDDLE OF THE COUNTRY TO THE NORTH OF NEW YORK WITHIN A
PRONOUNCED AREA OF UPPER LEVEL CONFLUENCE.  THIS SUGGESTS A FLOW
OF NORTHERLY TO NORTHEASTERLY AIRFLOW RATHER THAN A MORE EASTERLY
SURFACE COMPONENT.  HOWEVER...THE KEY TO QPF TYPE WILL NOT LIE AT
THE SURFACE BUT MORE LIKELY ALOFT AND THAT SCENARIO REFLECTS THE
INTERACTION OF A SHORT WAVE DROPPING SOUTHEASTWARD ACROSS THE
MIDWEST WITH THE LIFTING TROUGH ALONG THE ATLANTIC COAST. SUBTLE
CHANGES IN THAT INTERACTION COULD SPELL THE DIFFERENCES ALREADY
DISCUSSED.

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

KOCIN




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