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Winter Weather Forecast Discussion
 
(Latest Discussion - Issued 0824Z Jan 27, 2015)
 
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Abbreviations and acronyms used in this product
 
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
324 AM EST TUE JAN 27 2015

VALID 12Z TUE JAN 27 2015 - 12Z FRI JAN 30 2015


DAYS 1 THROUGH 3...


...MID ATLANTIC/NORTHEAST...

A MAJOR WINTER STORM PRODUCES HEAVY SNOW ACROSS PORTIONS OF
SOUTHEAST NY INTO MUCH OF NEW ENGLAND DURING DAY 1. THE 00Z ECMWF
APPEARS TO HAVE COME BACK TO THE CONSENSUS OF GUIDANCE...LIMITING
THE WESTERN EXTENT OF THE HEAVIEST SNOWFALL. THUS...THE 00Z ECMWF
WAS INCLUDED IN THE MODEL BLEND FOR THERMAL FIELDS ACROSS THE
REGION...ALONG WITH THE 00Z NAM/21Z SREF MEAN/12Z ECMWF ENSEMBLE
MEAN. THE QPF PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT
WPC QPF.

WARM AIR ADVECTION AHEAD OF A NEGATIVELY TILTED LONG WAVE TROUGH
APPROACHING THE EASTERN UNITED STATES PRODUCES LIFT FOR WINTRY
WEATHER DURING DAY 3. FOR THE MOST PART...THERE WAS GOOD MODEL
AGREEMENT ACROSS THE REGION WITH RESPECT THE LONG WAVE TROUGH...SO
THE THERMAL PORTION OF THE FORECAST WAS BASED ON A MULTI MODEL
BLEND. THE QPF PORTION OF THE FORECAST WAS BASED ON THE MOST
RECENT WPC QPF.

DAY 1..

A LONG FRONTOGENTICALLY FORCED BAND STRETCHING FROM SOUTHEAST NY
STATE ACROSS EASTERN NEW ENGLAND SLOWLY ROTATES WESTWARD DURING
THE FIRST HALF OF DAY 1. STRONG LOW LEVEL CONVERGENCE ALONG THE
BAND...AS WELL AS STRONG INFLOW (AS NOTED BY RECON FLIGHTS EAST OF
THE STORM POSITION AT 00Z)...SUPPORTS LOCALLY INTENSE SNOWFALL
RATES STRETCHING FROM EASTERN LONG ISLAND/CT/RI/MA ACROSS MUCH OF
CENTRAL AND SOUTHERN NH...AS WELL AS CENTRAL AND SOUTHERN ME. THE
HIGHEST SNOWFALL AMOUNTS (BETWEEN 12 AND 18 INCHES) ARE EXPECTED
ACROSS SOUTHWEST AND DOWNEAST ME...INTO SOUTHERN NH...AS THE
INTENSE FRONTOGENETIC BAND SITS ALONG THIS AXIS THE LONGEST. WHEN
COMBINED WITH STRONG WINDS...BLIZZARD CONDITIONS ARE
ANTICIPATED...ESPECIALLY BEFORE 28/00Z.

THE WESTERN EDGE OF THE HIGHER SNOWFALL WILL ULTIMATELY BE
DETERMINED BY HOW FAR WEST THE SNOW BANDS EARLY THIS MORNING FOR
THE MOST PART...BASED ON THE CONSENSUS OF 00Z MODEL OUTPUT...THE
WESTERN EDGE WAS MOVED EAST TO THE HUDSON RIVER IN NY STATE...AND
SOUTHEAST ACROSS CENTRAL AND SOUTHERN VT. THE WESTERN EDGE OF THE
6 INCH CONTOUR WAS EXTENDED ACROSS EASTERNMOST NY STATE INTO THE
NYC METRO AREA. MUCH OF THE SNOWFALL OUTSIDE OF THE BANDING ENDS
AS THE LONG WAVE TROUGH CLOSES OFF OVER NEW ENGLAND DURING THE
SECOND HALF OF THE PERIOD. THE STRONGEST BAND WEAKENS OVER
NORTHERN NEW ENGLAND DURING THE SECOND HALF OF THE DAY...AS THE
BEST FORCING MOVES EAST TO A POSITION SOUTH OF NOVA SCOTIA.

DAY 3...

STRONG WARM AIR ADVECTION AHEAD OF A NEGATIVELY TILTED TROUGH
INJECTS MOISTURE (IN THE FORM OF PRECIPITABLE WATER VALUES
REACHING NEAR 0.50 INCHES) FOR SNOWFALL ACROSS PORTIONS OF NY
STATE AND CENTRAL AND NORTHERN NEW ENGLAND DURING DAY 3. THE BEST
ISENTROPIC LIFT OCCURS ACROSS THIS AREA DURING THE SECOND HALF OF
THE PERIOD...RESULTING IN A LARGE AREA OF 2 TO 4 INCHES OF
SNOWFALL ACROSS MUCH OF NEW ENGLAND. LAKE EFFECT IN THE SOUTHWEST
FLOW AHEAD OF THE LONG WAVE TROUGH FAVORS LAKE ENHANCEMENT ACROSS
THE LOWER LAKES...WHERE 4 TO 6 INCHES OF SNOWFALL IS POSSIBLE.


...GREAT LAKES/MIDWEST/OH VALLEY...

A STRONG SHORT WAVE CROSSING THE GREAT LAKES LATE ON DAY 2 AND
EARLY ON DAY 3 PUSHES A SURFACE LOW ACROSS THE REGION. STRONG WARM
AIR ADVECTION AHEAD OF THE SHORT WAVE INJECTS MOISTURE INTO THE
COLUMN...RESULTING IN WINTRY WEATHER ACROSS THE GREAT LAKES AND
MIDWEST ON DAY 2...AND THE GREAT LAKES AND OH VALLEY DURING DAY 3.
THERE WAS GENERALLY GOOD MODEL AGREEMENT CONCERNING THE TRACK OF
THE SHORT WAVE AND SURFACE LOW...SO THE THERMAL FIELDS WERE BASED
ON A MULTI MODEL BLEND. THE QPF PORTION OF THE FORECAST WAS BASED
ON THE MOST RECENT WPC QPF.

THE BEST MID LEVEL LIFT ASSOCIATED WITH THE SHORT WAVE AS IT
CROSSES THE GREAT LAKES LATE ON DAY 2. STRONG WARM AIR ADVECTION
IN THE FORM OF A 25 TO 35 KNOT LOW LEVEL SOUTHWEST FLOW) ON THE I
290 ISENTROPIC SURFACE TRANSPORTS PRECIPITABLE WATER VALUES NEAR
0.50 INCHES INTO THE REGION. THE COMBINATION OF LIFT AND MOISTURE
PRODUCES A LARGE AREA OF 2 TO 4 INCHES OF SNOW ACROSS NORTHERN
MN...NORTHERN WI AND THE UP OF MI...WITH LESSER AMOUNTS TO THE
SOUTH. ACROSS PORTIONS OF SOUTHEAST WI...NORTHERN IL...NORTHERN
IN...WESTERN OH AND SOUTHWEST LOWER MI...MODEL SOUNDINGS SHOW A
WARM NOSE BETWEEN 800 AND 850 MB...WITH MAXIMUM TEMPERATURES NEAR
3 CELSIUS OVER SOUTHWEST OH...SUGGESTING THE POTENTIAL FOR
FREEZING RAIN. WIT THIS IN MIND...AN AREA OF 0.01 TO 0.10 INCHES
OF ICE WAS INTRODUCED TO THE ABOVEMENTIONED LOCATIONS.

THE SHORT WAVE STARTS ELONGATING INTO A LONG WAVE TROUGH AS IT
CROSSES THE GREAT LAKES ON ITS WAY TO THE EAST COAST. STRONG LIFT
AHEAD OF THE LONG WAVE TROUGH...COMBINED WITH 25 TO 35 KNOTS OF
LOW LEVEL WARM AIR ADVECTION...FOCUSES ON A WARM FRONT EXTENDING
FROM LOW PRESSURE CROSSING THE OH VALLEY. THE LIFT IS SUFFICIENT
TO PRODUCE A LARGE AREA OF 3 TO 6 INCHES OF SNOWFALL ACROSS
EASTERN OH INTO WESTERN PA AND WESTERN NY STATE...WHICH IS
AUGMENTED BY LAKE ENHANCEMENT OFF LAKES ERIE AND ONTARIO.  AN AREA
OF LIGHT ICING (LESS THAN 0.10 INCHES) IS EXPECTED ACROSS PORTIONS
OF SOUTHEAST LOWER MI AND OH...AS MODEL SOUNDINGS SHOW A WARM
LAYER NEAR 800 MB DURING THE HEIGHT OF THE WARM AIR ADVECTION.


...SOUTHWEST...

A MID LEVEL LOW WAVE TROUGH COMING ASHORE ACROSS SOUTHERN CA ON
DAY 2 SLOWS AS IT CLOSES OFF ON DAY 3. AHEAD OF THE CLOSED MID
LEVEL SYSTEM...A LOW LEVEL FLOW INJECTS MOISTURE INTO THE
SOUTHWEST STATES...AND THE COMBINATION OF MOISTURE AND LIFT IS
SUFFICIENT FOR SNOWFALL ACROSS THE HIGHER TERRAIN. THERE WAS
FAIRLY GOOD MODEL AGREEMENT CONCERNING THE SEQUENCE OF SYNOPTIC
EVENTS...SO THE THERMAL PORTION OF THE FORECAST WAS BASED ON A
BLEND OF THE 00Z GFS/12Z ECMWF...AND THE QPF PORTION OF THE
FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

A LOW LEVEL SOUTHERLY FLOW DEVELOPING AHEAD OF A CLOSING MID LEVEL
SYSTEM OVER SOUTHERN CA ON DAY 3 TRANSPORTS PRECIPITABLE WATER
VALUES BETWEEN 0.75 AND 1.00 INCHES ACROSS THE SOUTHWEST STATES.
THE COMBINATION OF SYNOPTIC SCALE LIFT ASSOCIATED WITH THE CLOSED
MID LEVEL SYSTEM AND MOISTURE IN THE LOW LEVEL FLOW PRODUCES A
LARGE AREA OF PRECIPITATION ACROSS MUCH OF THE SOUTHWEST STATES.
SNOW LEVELS RISE TO BETWEEN 8000 AND 9000 FEET ACROSS PORTIONS OF
NM...WHERE 2 TO 4 INCHES OF SNOWFALL ACROSS THE NORTHERN PART OF
THE STATE ARE RELEGATED TO ABOVE THOSE LEVELS. SNOW LEVELS SHOULD
BE CLOSER TO 8000 FEET ACROSS THE SOUTHERN WASATCH RANGE IN UT AND
THE SAN JUAN MOUNTAINS IN SOUTHWEST CO...WHERE LOCAL 2 TO 4 INCH
SNOWFALL AMOUNTS ARE POSSIBLE.


...CA/GREAT BASIN/NORTHERN AND CENTRAL ROCKIES...

A SHORT WAVE TRACKING FROM SOUTHERN CA CROSSES THE GREAT BASIN
INTO THE NORTHERN ROCKIES ON DAY 1...BEFORE MOVING ACROSS THE
CENTRAL ROCKIES DURING DAY 2. THERE WAS GENERALLY GOOD MODEL
AGREEMENT CONCERNING THE TRACK OF THE SHORT WAVE...SO THE THERMAL
PORTION OF THE FORECAST WAS BASED ON A 00Z GFS/12Z ECMWF BLEND.
THE QPF PORTION OF THE FORECAST WAS BASED ON THE MOST RECENT WPC
QPF.

THE BEST MOISTURE AND LIFT ASSOCIATED WITH THE SHORT WAVE CROSSES
NV INTO SOUTHERN ID AND WESTERN WY DURING DAY 1. WHILE THE
MOISTURE SOURCE FROM THE PACIFIC IS NOT AS ROBUST AS IT WAS
EARLIER...THERE SHOULD BE ENOUGH MOISTURE IN THE COLUMN FOR 3 TO 6
INCHES OF SNOWFALL ACROSS THE SIERRA NEVADA RANGE IN CA...AS WELL
AS THE GRAND TETONS IN NORTHWEST WY. SNOW LEVELS GENERALLY HOVER
NEAR 8000 FEET DURING DAY 1...SO VALLEY SNOWFALL AMOUNTS SHOULD BE
LIGHT.

AS THE SHORT WAVE CROSSES THE CENTRAL ROCKIES ON ITS WAY TO THE
CENTRAL PLAINS ON DAY 3...MOISTURE BECOMES THE LIMITING FACTOR FOR
SNOWFALL (AS PRECIPITABLE WATER VALUES TOP OUT NEAR 0.25 INCHES) .
SNOW LEVELS REMAIN NEAR 8000 FEET...SO ONLY THE HIGHER PEAKS OF
THE NORTHERN WASATCH RANGE IN UT...THE GRAND TETONS IN WY AND THE
ROCKIES OF NORTH CENTRAL CO SEE SNOWFALL AMOUNTS BETWEEN 2 AND 4
INCHES.


HAYES