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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2054Z Nov 24, 2014)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


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

VALID 00Z TUE NOV 25 2014 - 00Z FRI NOV 28 2014


DAYS 1 THROUGH 3...


...MID ATLANTIC/NORTHEAST...

LOW PRESSURE FORMING OFF THE MID ATLANTIC COAST WILL BRING THE
THREAT FOR HEAVY SNOW NORTHWEST OF THE I 95 CORRIDOR ON DAY
2...AND ACROSS MUCH OF NEW ENGLAND ON DAY 3. THERE IS STILL A FAIR
AMOUNT OF SPREAD CONCERNING THE TRACK OF THE SURFACE LOW...AND THE
PLACEMENT OF THE COLDER AIR NEAR THE SURFACE...ESPECIALLY ACROSS
NEW ENGLAND. FOR THE MOST PART...THE QPF PORTION OF THE FORECAST
WAS BASED ON A BLEND OF THE LATEST WPC QPF/09Z SREF MEAN/00Z ECMWF
ENSEMBLE MEAN...AND THE THERMAL PORTION OF THE FORECAST WAS BASED
ON A MULTI MODEL BLEND.

LOW PRESSURE IS EXPECTED TO FORM ON A LOW LEVEL FRONTAL BOUNDARY
OFF THE MID ATLANTIC COAST NEAR 26/12Z...AS A SOUTHERN STREAM
SHORT WAVE STARTS TAKING ON A NEGATIVE TILT AS ITS RIDES NORTHEAST
IN THE DEEP MID LEVEL SOUTHWEST FLOW. THE SHORT WAVE CROSSES THE
MID ATLANTIC NEAR 26/18Z...THEN INTO NEW ENGLAND AFTER 27/00Z. THE
BEST MID LEVEL FRONTOGENETIC LIFT ASSOCIATED WITH THE SHORT
WAVE...AND THE TRACK OF THE BEST MID LEVEL LIFT IS IMPORTANT SINCE
THE THERMAL PROFILE WILL BE CRITICAL TO THE PRECIPITATION TYPE
ALONG THE I 95 CORRIDOR...ESPECIALLY ACROSS VA/MD/DE/EASTERN
PA/NJ/SOUTHEAST NY STATE...WHERE PRECIPITATION WILL FALL DURING
THE DAY.

MODEL SOUNDINGS ALONG THE I 95 CORRIDOR FROM VA INTO SOUTHEAST NY
STATE SUGGEST THAT THE WET BULB EFFECT AS THE COLUMN MOISTENS WILL
BRING THE COLUMN TO NEAR ISOTHERMAL. THIS LOOKS REASONABLE...AS
LONG AS THE BEST VERTICAL VELOCITY PACKET RISES ALONG THIS I 95
CORRIDOR. THE THERMAL FIELDS WERE BASED ON BLEND OF THE AVAILABLE
GUIDANCE...AS THE 12Z NAM HAD THE COLDEST SOLUTION ACROSS THE MID
ATLANTIC...AND THE 12Z ECMWF HAD THE WARMEST. ANY MIXED
PRECIPITATION ALONG THE CORRIDOR SHOULD CHANGE TO WET SNOW AS THE
VERTICAL MOTION INCREASES...BUT THE SOUTHEAST EDGE OF THE HEAVIEST
SNOW WILL BE TIED TO THE BEST VERTICAL MOTION.

FOR NOW...THE 4+ INCH AMOUNTS WERE DRAGGED BACK NORTH AND WEST OF
DCA/PHL TO ACCOUNT FOR THE UNCERTAINTY...AS WELL AS FACT THAT SOME
OF THE SNOWFALL WILL OCCUR WITH TEMPERATURE NEAR OR JUST ABOVE
FREEZING. ACTUAL ACCUMULATIONS MAY BE PROBLEMATIC BECAUSE OF
BOUNDARY LAYER ISSUES.
THE HIGHEST SNOWFALL AMOUNTS (IN THE 6 TO 10 INCH RANGE) WERE
PLACED ON THE WESTERN EDGE OF THE BEST VERTICAL MOTION ACROSS
PORTIONS OF WESTERN VA INTO CENTRAL AND NORTHEAST PA AND INTERIOR
SOUTHEAST NY STATE.

IT IS IMPORTANT TO NOTE THAT THE THERMAL PORTION OF THE FORECAST
IS STILL THE MOST UNCERTAIN PART...AND IS SUBJECT TO CHANGE
PENDING NEWER INFORMATION. TRAVEL IMPACTS BECOME INCREASINGLY
LIKELY FORTH AND WEST OF THE I 95 CORRIDOR AS DAY 2 WEARS ON.

THE TRACK OF THE BEST VERTICAL MOTION CONTINUES INTO CENTRAL AND
SOUTHEAST NEW ENGLAND DURING DAY 3. AT THE SAME TIME...THE SURFACE
LOW IS EXPECTED TO TRACK OVER THE BENCHMARK ON ITS WAY TO SOUTHERN
NOVA SCOTIA DURING THE SECOND HALF OF THE PERIOD. THE TRACK OF THE
SURFACE AND 850 MB LOW WOULD SUGGEST THE WARMING COULD RESULT IN
BOUNDARY LAYER WARMING ACROSS EASTERN LONG ISLAND...AS WELL AS
SOUTHEAST MA. SNOWFALL IN THESE AREAS MAY BE MINIMAL ON THE FRONT
SIDE OF THE LOW...BUT AS COLD AIR RETURNS IN THE LOWER TO MID
LEVELS...A TIGHT GRADIENT OF SNOWFALL RANGING FROM LITTLE TO 3 TO
6 INCHES SHOULD EXIST OVER SOUTHEAST MA AND SOUTHERN RI.

FURTHER INLAND...THE BEST FRONTOGENETIC FORCING SHOULD EXTEND FROM
WESTERN MA ACROSS CENTRAL AND SOUTHERN NH INTO MUCH OF ME. MODEL
SOUNDINGS HERE SHOULD THAT...AWAY FROM THE IMMEDIATE COAST AND
BOUNDARY LAYER ISSUES EARLY...THAT THE BULK OF THE PRECIPITATION
FALLS AS SNOW. THE AXIS MENTIONED ABOVE SHOULD BE IN THE BEST
DENDRITIC GROWTH ZONE...AND THIS ALONG WITH THE BEST MID LEVEL
LIFT SHOULD SUPPORT AN AXIS OF 8 TO 12 INCHES OF SNOWFALL FROM
WESTERN MA INTO CENTRAL AND EASTERN ME.


...GREAT LAKES/NORTHERN PLAINS/MIDWEST...

THE LAST OF THE MID LEVEL LIFT EXITS THE GREAT LAKES DURING THE
FIRST HALF OF DAY 1. AFTER THAT...CONDITIONS FOR LAKE EFFECT
BECOME FAVORABLE FOR THE REMAINDER OF THE DAY 1 INTO DAY 2. A
SHORT WAVE DROPPING OUT OF WEST CENTRAL CANADA WILL CROSS THE
NORTHERN PLAINS AND MIDWEST DURING DAY 2...PRODUCING SNOWFALL
ACROSS THE REGION. FOR THE MOST PART...THERE WAS DECENT MODEL
AGREEMENT WITH RESPECT TO THE TRACK OF THE SHORT WAVE AND ITS
ATTENDANT SURFACE LOW. THE QPF PORTION OF THE FORECAST WAS BASED
MAINLY ON A BLEND OF THE LATEST WPC QPF/12Z NAM/12Z UKMET. THE
THERMAL PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE 12Z
NAM/09Z SREF MEAN.

THE BEST MID LEVEL FRONTOGENETIC FORCING ASSOCIATED WITH STRONG
ISENTROPIC LIFT AT THE I295 LEVEL ASSOCIATED WITH THE NEGATIVELY
TILTED SHORT WAVE CROSSING THE GREAT LAKES SHOULD OCCUR THIS
EVENING. THE COMBINATION OF LIFT AND MOISTURE ON THE I295 LEVEL
SUPPORTS A SWATH OF 4 TO 6 INCHES OF SNOWFALL ACROSS THE LOWER UP
OF MI AND THE NORTHERN LP OF MI. AFTER THE LIFT EXITS...LOW LEVEL
CONDITIONS BECOME MORE FAVORABLE FOR LAKE EFFECT
PRECIPITATION...FIRST ACROSS THE UPPER GREAT LAKES...THEN ACROSS
THE LOWER GREAT LAKES.

MODEL SOUNDINGS ACROSS THE UPPER GREAT LAKES SHOWED A DEEPENING
CAPPING INVERSION WITH THE LOW LEVEL FLOW FROM THE WEST NORTHWEST.
THIS SUPPORTS MULTI BANDING ON THE SHORT AXIS OF THE OF LAKE
SUPERIOR. MOISTURE AND DURATION OF THE FLOW SHOULD BE MITIGATING
EFFECTS FOR HEAVIER SNOWFALL AMOUNTS...BUT FAVORED LOCATIONS
ACROSS THE UP OF MI COULD RECEIVE AN ADDITIONAL 3 TO 6 INCHES OF
SNOWFALL. THE FLOW BECOMES LESS FAVORABLE DURING DAY 2...AHEAD OF
A SYNOPTIC SCALE SYSTEM.

OVER THE LOWER LAKES...THE COLDER AIR COMES IN DURING THE SECOND
HALF OF DAY 1...AS THE LOW LEVEL FLOW BECOMES MORE OR LESS ALIGNED
WITH THE LONG AXIS OF LAKES ERIE AND ONTARIO. AS WAS THE CASE
ACROSS THE UPPER GREAT LAKES...LOWERING INVERSION HEIGHTS AND LACK
OF DURATION IN THE FLOW SHOULD CAP SNOWFALL AMOUNTS AT 4 INCHES OR
LESS.

A SHORT WAVE TRACKING FROM WESTERN CENTRAL CANADA INTO THE MIDWEST
DURING DAY 2 WILL FOCUS LOW LEVEL MOISTURE FOR SNOWFALL ACROSS THE
NORTHERN PLAINS AND MIDWEST. MOISTURE BECOMES THE LIMITING FACTOR
FOR SNOWFALL WITH THIS FEATURE...AND MUCH OF THE AVAILABLE
GUIDANCE SUGGESTS A SMALL SWATH OF 2 TO 4 INCHES OF SNOWFALL
ACROSS PORTIONS OF NORTH CENTRAL IA. BY DAY 3...THE BEST LIFT
FOCUSES ON THE GREAT LAKES...WHERE AN AREA OF 2 TO 4 INCHES OF
SNOWFALL IS EXPECTED ACROSS THE UP OF MI.


...PACIFIC NORTHWEST/NORTHERN ROCKIES...

A SERIES OF SHORT WAVES TOPPING THE MID LEVEL RIDGE OVER THE WEST
COAST WILL AFFECT THE PACIFIC NORTHWEST AND NORTHERN ROCKIES
DURING EACH OF THE NEXT THREE DAYS. EACH SHORT WAVE WILL HAVE
ACCESS TO A PACIFIC PLUME OF MOISTURE...RESULTING IN THE POTENTIAL
FOR HEAVY SNOW ACROSS THE HIGHER TERRAIN. THE 12Z GFS/UKMET HAD
THE BEST HANDLE ON BRINGING THE SHORT WAVE ENERGY UP AND OVER THE
MID LEVEL RIDGE...AND WAS USED AS A BLEND (ALONG WITH THE LATEST
WPC QPF) FOR THE QPF PORTION OF THE FORECAST. THE THERMAL PORTION
OF THE FORECAST WAS BASED ON A BLEND OF THE 12Z GFS/09Z SREF MEAN.

THE FIRST SHORT WAVE IN THE CHAIN CROSSES THE PACIFIC NORTHWEST
AND BRITISH COLUMBIA DURING DAY 1. THE LATEST WATER VAPOR LOOPS
SHOWED PACIFIC MOISTURE COMING ASHORE WITH THE SHORT WAVE...AND
THAT MOISTURE COINCIDES WITH THE BEST LIFT ACROSS THE NORTHERN
ROCKIES DURING THE SECOND HALF OF DAY 1. SURFACE LOW PRESSURE
DEVELOPING ON A LOW LEVEL BAROCLINIC ZONE OVER THE NORTHERN
ROCKIES AND NORTHERN PLAINS WILL FOCUS UPSLOPE FLOW ACROSS THE
NORTHERN ROCKIES...AUGMENTING SYNOPTIC SCALE LIFT AHEAD OF A 150
KNOT JET STREAK. THE UPSLOPE FAVORS THE HIGHER TERRAIN...AND
SWATHS OF 6 TO 12 INCHES OF SNOW WERE PLACED OVER THE NORTHERN
CASCADES OF WA...THE BITTERROOTS OF ID...THE ROCKY MOUNTAIN FRONT
RANGE IN MT...AND THE GRAND TETONS IN WY.

AS THE MID LEVEL RIDGING REMAINS MORE OR LESS IN PLACE ACROSS THE
WEST COAST...THE SHORT WAVE ENERGY BECOMES DISPLACED NORTHWARD
ACROSS THE NORTHERN ROCKIES ON DAY 2. THE MOISTURE CONVERGENCE
ALONG A FRONTAL BOUNDARY EXTENDING FROM NORTHERN ID ACROSS WESTERN
AND CENTRAL MT WILL AUGMENT SYNOPTIC SCALE LIFT AND UPSLOPE FLOW
TO PRODUCE HEAVY SNOW ACROSS THE ROCKY MOUNTAIN FRONT RANGE IN MT
AND THE GRAND TETONS IN WY.
BROAD WARM AIR ADVECTION AHEAD OF THE SHORT WAVE WILL RESULT IN
RISING SNOW LEVELS ACROSS THE NORTHERN ROCKIES...AND THIS SHOULD
LIMIT SNOWFALL ACCUMULATIONS BELOW 6000 FEET.

BY DAY 3...THE MID LEVEL FLOW STEERS THE SHORT WAVES (AND LIFT)
ACROSS SOUTHWEST CANADA...WITH MOST OF THE MOISTURE IN THE FORM OF
BROAD WARM AIR ADVECTION FOCUSED ON THE HIGHER TERRAIN. SNOW
LEVELS RISE TO ABOVE 8000 FEET ACROSS THE NORTHERN ROCKIES...AND
SNOWFALL AMOUNTS BETWEEN 4 AND 8 INCHES ARE LIMITED TO THE
NORTHERN CASCADES IN WA AND THE ROCKY MOUNTAIN FRONT RANGE IN MT.


HAYES