Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 050803
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
302 AM EST THU MAR 05 2015

VALID 12Z THU MAR 05 2015 - 12Z SUN MAR 08 2015


DAYS 1 THROUGH 3...


...GREAT LAKES/NORTHERN NEW ENGLAND...

A STRONG SHORT WAVE TRACKING FROM CENTRAL CANADA ACROSS THE GREAT
LAKES INTO NORTHERN NEW ENGLAND PRODUCES AN AREA OF SNOW ON DAY 3.
THERE IS A FAIR AMOUNT SPREAD CONCERNING THE TIMING OF THE SHORT
WAVE (WHICH IS NOT UNCOMMON IN A FAST FLOW)...SO THE THERMAL
FIELDS WERE BASED ON MULTI MODEL BLEND (INCLUDING THE 21Z SREF
MEAN AND 12Z ECMWF ENSEMBLE MEAN). THE QPF PORTION OF THE FORECAST
WAS BASED ON THE MOST RECENT WPC QPF.

A SHORT WAVE DROPPING SOUTHEAST OUT OF THE CENTRAL CANADA CROSSES
THE GREAT LAKES AND NORTHERN NEW ENGLAND BETWEEN 07/18Z AND 18/06Z
AND ENCOUNTERS A FAIRLY DRY AIRMASS IN PLACE (AS PRECIPITABLE
WATER VALUES REMAIN AT OR BELOW 0.25 INCHES). SYNOPTIC SCALE LIFT
IS AUGMENTED BY DIVERGENCE IN THE LEFT FRONT QUADRANT OF A 110
KNOT JET STREAK...AND THE LIFT MAKES THE MOST OF THE AVAILABLE
MOISTURE TO PRODUCE AN AREA OF 2 TO 4 INCHES OF SNOWFALL EXTENDING
FROM WESTERN NY STATES INTO THE HIGHER TERRAIN OF NORTHERN NEW
ENGLAND. MOST OF THE GREAT LAKES HAVE FROZEN TO THE POINT THAT
THEY ARE NOT SIGNIFICANT SOURCES OF MOISTURE...SO NO APPRECIABLE
LAKE EFFECT IS EXPECTED WITH THIS SYSTEM.


...OH VALLEY/MID ATLANTIC/SOUTHERN NEW ENGLAND...

STRONG FRONTOGENETIC FORCING NORTH OF A SURFACE FRONT DROPPING
SOUTH ACROSS THE OH VALLEY AND MID ATLANTIC STATES ON DAY 1
COMBINES WITH DEEP MOISTURE FROM THE PACIFIC AND WESTERN GULF OF
MEXICO FUNNELED ALONG THE FRONT TO PRODUCE AN AXIS OF HEAVY SNOW.
WHILE THERE IS GOOD MODEL AGREEMENT WITH RESPECT TO MASS
FIELDS...THERE ARE STILL SOME SUBTLE BUT IMPORTANT THERMAL
DIFFERENCES...WHICH HAVE AN IMPACT ON SNOWFALL
AMOUNTS...ESPECIALLY ACROSS CENTRAL AND SOUTHERN VA AND THE
SOUTHERN DELMARVA. FOR THE MOST PART...THE THERMAL PORTION OF THE
FORECAST WAS BASED ON THE 00Z NAM...WHICH BETTER FIT NEAR TERM
TRENDS WITH RESPECT TO PRECIPITATION TYPE CHANGES ACROSS THE
WESTERN OH VALLEY. THE QPF PORTION OF THE FORECAST WAS BASED ON
THE MOST RECENT WPC QPF.

DAY 1...
STRONG MID LEVEL FRONTOGENETIC FORCING STRETCHING FROM THE OH
VALLEY INTO THE MID ATLANTIC EARLY ON DAY 1 MAXIMIZES BETWEEN
05/15Z AND 05/21Z ACROSS
VA/MD/SOUTHEAST PA/CENTRAL AND SOUTHERN NJ INTO LONG ISLAND NY.
THE PRECIPITATION PHASE OCCURS FROM NORTH TO SOUTH...REACHING A
LINE FROM
KDCA/KDOV/KACY BETWEEN 05/12Z AND 05/18Z. THE 00Z NAM APPEARED TO
HAVE A BETTER HANDLE ON THE TIMING OF THE CHANGE FROM SLEET TO
SNOW FROM NORTH TO SOUTH...AND THIS HAS AN IMPACT ON SNOWFALL
AMOUNTS...ESPECIALLY ACROSS SOUTHERN MD/DC/CENTRAL VA. THE LATER
TIMING OF THE CHANGEOVER MEANS LESS IN THE WAY OF SNOW (OUTSIDE OF
BANDING...WHERE THE STRONG VERTICAL MOTION WIPES OUT THE REMAINING
WARM LAYER BETWEEN 700 MB AND 650 MB). THIS REQUIRED MOVING THE 6
TO 8 INCH CONTOUR CLOSER TO A DC METRO INTO SOUTHERN DE LINE
(WHICH IS FURTHER NORTH THAN EARLIER FORECASTS).

FURTHER SOUTH...ACROSS SOUTHEAST VA AND NORTHEAST NC...THE
TRANSITION TAKES PLACES LATER (GENERALLY BETWEEN 05/18Z AND
06/00Z. THE MID LEVEL WARM AIR REMAINS IN PLACE THE LONGEST...SO
SLEET MAY END UP BEING THE DOMINANT PRECIPITATION TYPE.
HOWEVER...THE LOW LEVEL COLD AIR ARRIVES FIRST...SO A LIGHT
COATING OF ICE PRECEDES 1 TO 3 INCHES OF SNOW AND SLEET BEFORE THE
BEST MID LEVEL EXITS AFTER 06/00Z.

ACROSS SOUTHEAST PA/CENTRAL AND SOUTHERN NJ AND LONG
ISLAND...WHERE THE PRECIPITATION IS EXPECTED TO BE SNOW AT THE
BEGINNING OF DAY 1...MOISTURE AND LIFT IN THE DENDRITIC GROWTH
ZONE BETWEEN 05/15Z AND 05/21Z SUGGESTS A PERIOD OF HEAVY SNOW.
QPF AMOUNTS SUPPORTS 6 TO 8 INCHES OF SNOWFALL...WITH LOCAL 10
INCH AMOUNTS POSSIBLE ACROSS SOUTH CENTRAL NJ. THESE AMOUNTS ARE
WELL SUPPORTED BY THE LATEST ECMWF ENSEMBLE MEMBERS...AS WELL AS
21Z SREF MEMBERS (WHICH WERE A TAD FURTHER NORTH WITH THE HIGHER
SNOWFALL PROBABILITIES).

DAY 2...

COLD AIR WEDGED ALONG THE NC COAST ON DAY 2 REMAINS IN PLACE AS
MOISTURE FROM LOW PRESSURE PASSING OFF THE MID ATLANTIC COAST IS
DIRECTED BACK TOWARD THE COAST. THE MID LEVELS REMAINS WARM...AND
THE SHALLOW COLD AIR SUPPORTS LIGHT ICE ACCUMULATIONS. THE 00Z NAM
IS THE MOST ROBUST WITH ICE ACCUMULATION HERE...BUT AMOUNTS ARE
EXPECTED TO REMAINS BELOW 0.10 INCHES.


...GULF COAST/SOUTHEAST...

COLD AIR DIVING SOUTH BEHIND A FRONT STRETCHING FROM THE GULF
COAST THROUGH THE SOUTHEAST STATES UNDERCUTS WARMER MID LEVEL
TEMPERATURES TO PRODUCE THE THREAT FOR FREEZING RAIN EXTENDING
FROM NORTHERN LA THROUGH MS...NORTHERN AL AND THE MOUNTAINS OF
NORTH GA AND WESTERN NC. THERE WAS GENERALLY GOOD MODEL AGREEMENT
WITH RESPECT TO THE STRENGTH OF THE LOW LEVEL COLD AIR
ADVECTION...BUT THERE ARE SOME TIMING DIFFERENCES. THE 00Z NAM
APPEARED TO BE HANDLING THE TIMING OF THE COLD AIR ADVECTION
BETTER...SO THE THERMAL PORTION OF THE FORECAST WAS BASED ON THIS
MODEL. THE QPF PORTION OF THE FORECAST WAS BASED ON THE MOST
RECENT WPC QPF.

MOISTURE DRAWN NORTHEAST FROM THE WESTERN GULF OF MEXICO OVERRUNS
A COLD FRONT DROPPING SOUTH ACROSS THE GULF COAST AND THE
SOUTHEAST STATES ON DAY 1. THE COLD AIR ADVECTION BEHIND THE FRONT
WILL DRIVE BOUNDARY LAYER TEMPERATURES NEAR FREEZING TO THE GULF
COAST...AS WELL AS MUCH OF THE SOUTHEAST. THE COLD AIR UNDERCUTS
WARM AND MOIST AIR ABOVE IT (AS SHOWN BY 00Z NAM MODEL SOUNDINGS
EXTENDING FROM NORTH CENTRAL LA INTO THE MOUNTAINS OF NORTH GA).
WARMER GROUND TEMPERATURES ACROSS THIS REGION WILL INITIALLY
INHIBIT ICE ACCUMULATION...BUT AS THE BOUNDARY LAYER LAYER
COOL...A LARGE AREA OF 0.01 TO 0.10 INCHES OF ICE IS EXPECTED FROM
NORTH CENTRAL LA ACROSS CENTRAL MS...NORTHWEST AL INTO THE
MOUNTAINS OF NORTH GA AND WESTERNMOST NC.

ACROSS THE HIGHER TERRAIN...LOCAL UPSLOPE FLOW COULD RESULT IN
HIGHER QPF AMOUNTS WHILE BOUNDARY TEMPERATURES ARE BELOW FREEZING.
THIS COULD RESULT IN LOCAL 0.10 TO 0.25 INCHES OF QPF ACROSS THE
MOUNTAINS OF NORTH GA AND WESTERNMOST NC. THESE AMOUNTS ARE
SUPPORTED STRONGLY BY THE 00Z NAM/21Z SREF MEAN.


HAYES







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