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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0742Z Mar 25, 2015)
 
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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
342 AM EDT WED MAR 25 2015

VALID 12Z WED MAR 25 2015 - 12Z SAT MAR 28 2015


DAYS 1 THROUGH 3...


...GREAT LAKES/OH VALLEY/MID ATLANTIC/NORTHEAST...

A PAIR OF SHORT WAVES RESPONSIBLE FOR WINTRY WEATHER ON DAY 1
CROSS THE GREAT LAKES AND NORTHERN NEW ENGLAND. LOW PRESSURE
FORMING ON A FRONT IN THE OH VALLEY ON DAY 2 TRACKS ACROSS WESTERN
PA...NY STATE AND NEW ENGLAND...AS COLDER AIR BLEEDS INTO THE
REGION. A SHORT WAVE DIVING THROUGH THE LONG WAVE POSITION ACROSS
THE EASTERN UNITED STATES RESULTS IN SNOW SHOWER ACTIVITY FROM THE
INTERIOR MID ATLANTIC INTO THE HIGHER TERRAIN OF NORTHERN NEW
ENGLAND DURING DAY 3. FOR THE MOST PART...THERE WAS GOOD MODEL
AGREEMENT CONCERNING THE MAJOR SYNOPTIC FEATURES. THE NOTABLE
EXCEPTION WAS THE 00Z NAM (AND TO A LESSER DEGREE THE 21Z SREF
MEAN)...WHICH CONTINUE TO BE COLDER WITH THE SYSTEM ON DAY 2 FOR
NEW ENGLAND...AND FOR THIS REASON WAS NOT INCLUDED IN THE MODEL
BLEND. THE THERMAL FIELDS WERE BASED PRIMARILY ON A BLEND OF THE
00Z GFS/12Z ECMWF...WHILE THE QPF PORTION OF THE FORECAST WAS
BASED ON THE MOST RECENT WPC QPF.

DAY 1...
THE STRONG SHORT WAVE ON THE NORTHERN EDGE OF THE DEVELOPING
NEGATIVELY TILTED LONG WAVE TROUGH OVER THE NORTHERN PLAINS CLOSES
OFF JUST NORTH OF MN...BEFORE IT TRUNDLES ACROSS ONTARIO. WHILE
THE BEST LIFT AND MOISTURE WITH THE NORTHERN SYSTEM OCCURS OVER
SOUTHWEST ONTARIO...THERE SHOULD BE ENOUGH LIFT OVER NORTHERN MN
FOR 1 TO 3 INCHES OF SNOWFALL.

STRONG WARM AIR ADVECTION AHEAD OF THE DEVELOPING LONG WAVE TROUGH
PEAKS ACROSS THE GREAT LAKES DURING THE FIRST HALF OF DAY 1. THE
COLUMN AHEAD OF THE BEST LIFT IN THE WARM AIR ADVECTION RIBBON IS
COLD ENOUGH FOR SNOWFALL (BASED ON THE DRY AIR IN PLACE AS
DEPICTED ON THE 25/00Z GRB SOUNDING). MODEL SOUNDINGS SUGGEST A
PERIOD OF ISOTHERMAL CONDITIONS ACROSS THE UP OF MI...ALONG WITH
MOISTURE AND LIFT IN THE DENDRITIC GROWTH ZONE.

THESE CONDITIONS COULD SUPPORT A 2 TO 4 HOUR WINDOW OF HIGH
SNOWFALL RATES...SO A SWATH OF THE UP OF MI COULD SEE 4 TO 6
INCHES OF SNOWFALL BEFORE 26/00Z. AS THE BEST LIFT EXITS...THE
COLUMN LOSES ITS ABILITY TO MAKE SNOWFLAKES...SO A LARGE AREA
ACROSS THE UP OF MI...AS WELL AS THE NORTHERN PORTION OF THE LP OF
MI...COULD SEE LESS THAN 0.10 INCHES OF ICE ACCUMULATION AS THE
PRECIPITATION ENDS.

THE BEST LIFT AND MOISTURE IN THE WARM AIR ADVECTION CROSSES
QUEBEC THIS AFTERNOON...THEN MOVES ACROSS UPSTATE NY AND NORTHERN
NEW ENGLAND BETWEEN 26/06Z AND 26/12Z. LOW LEVEL COLD AIR WEDGED
IN NEAR THE SURFACE ACROSS THE ADIRONDACKS OF NY AND THE HIGHER
TERRAIN OF EASTERN VT...NH AND WESTERN ME COULD RESULT IN A PERIOD
OF LIGHT FREEZING RAIN ON THE NOSE OF A 40 TO 50 KNOT LOW LEVEL
JET. FOR THE MOST PART...ICE ACCUMULATIONS SHOULD BE
LIGHT...AVERAGING LESS THAN 0.10 INCHES.

DAY 2...
A SHORT WAVE RIDING NORTHEAST IN THE DEEP MID LEVEL SOUTHWEST FLOW
AHEAD OF A LONG WAVE TROUGH EXTENDING FROM THE GREAT LAKES TO THE
SOUTHERN PLAINS SPINS UP LOW PRESSURE ON A FRONTAL DRAPED ACROSS
THE OH VALLEY. COLDER AIR OVER QUEBEC AND ONTARIO STARTS BLEEDING
SOUTH BEHIND THE FRONT...RESULTING IN AN ANA FRONT PRECIPITATION
SETUP. THE COLD AIR GETS INTO NORTHERN OH AND NORTHWEST PA
FIRST...AT ABOUT THE TIME OF THE BEST MID LEVEL FORCING. THERE IS
A STRONG MODEL SIGNAL FOR 2 TO 4 INCHES OF SNOWFALL IN THIS
AREA...AND THIS IS SUPPORTED BY MEMBERS OF THE LATEST ECMWF
ENSEMBLE OUTPUT.

THE COLD AIR BLEEDS INTO WESTERN AND CENTRAL NY STATE...JUST AS
THE BEST MID LEVEL FORCING EXITS. THIS AREA WAS ALSO COVERED BY A
LARGE SWATH OF 2 TO 4 INCHES OF SNOWFALL...WHICH IS AGAIN HAS A
MULTI MODEL SUPPORT. AN AREA OF 4 TO 6 INCHES OF SNOWFALL WAS
PLACED OVER THE ADIRONDACKS OF NY...AS THE ELEVATION PLAYS A ROLE
EVEN BEFORE THE COLD AIR GETS IN BEHIND THE FRONT AND TRANSITING
SURFACE LOW. THERE WAS A MULTI MODEL SIGNAL FOR THIS AS WELL...AND
THE COLDER MODEL SOLUTIONS (00Z NAM/12Z ECMWF) SUGGEST EVEN HIGHER
AMOUNTS (CLOSER TO 8 INCHES). THE FACT THAT THE BEST MID LEVEL
LIFT IS MOVING RATHER QUICKLY MAKES THE HIGHER SNOWFALL AMOUNTS A
LOW PROBABILITY EVENT.

AS THE LOW CROSSES NORTHERN NEW ENGLAND...THE BEST LIFT OCCURS AS
THE COLD AIR SEEPS INTO THE HIGHER TERRAIN. A LARGE AREA OF 3 TO 6
INCHES OF SNOW WAS STRETCHED FROM NORTHERN VT AND NORTHERN NH
ACROSS THE MOUNTAINS OF WESTERN INTO NORTHERNMOST ME (AND THESE
AMOUNTS ARE SUPPORTED BY MEMBERS OF THE 21Z SREF AND LATEST ECMWF
ENSEMBLE OUTPUT). LOCAL 6 TO 8 INCH AMOUNTS ARE POSSIBLE ACROSS
THE HIGHER TERRAIN OF WESTERN ME...WHERE THE BEST LIFT MEETS THE
COLDER COLUMN TEMPERATURES. SNOWFALL SOUTH OF THE MOUNTAINS IN
WESTERN ME AND NORTHERN NH DROPS OFF AS THE BEST LIFT EXITS BEFORE
THE COLDER AIR IN THE COLUMN GETS TO THE COAST.

DAY 3...
A STRONG SHORT WAVE TRACKING ACROSS THE BOTTOM OF THE LONG WAVE
TROUGH OVER THE EASTERN UNITED STATES PRODUCES LIFT AND STEEPENING
MID LEVEL LAPSE RATES ACROSS PORTIONS OF THE OH VALLEY...MID
ATLANTIC AND NORTHEAST ON DAY 3. COLD AIR ADVECTION OVER THE LOWER
LAKES SETS UP CONDITIONS FAVORABLE FOR LAKE ENHANCED
SNOWFALL...MAINLY SOUTHEAST OF LAKE ONTARIO (AS LAKE ERIE IS STILL
MOSTLY FROZEN). IN THIS AREA...LOCAL 2 TO 4 INCH SNOWFALL AMOUNTS
ARE POSSIBLE.

UPSLOPE FLOW ACROSS THE NORTHERN APPALACHIAN MOUNTAINS COULD
PRODUCE AN INCH OR TWO OF SNOWFALL ACROSS PORTIONS OF SOUTH
CENTRAL PA AND EASTERN WV...WHERE BOUNDARY LAYER TEMPERATURES CAN
SUPPORT SNOW. UPSLOPE SNOWFALL IS ALSO EXPECTED IN THE FAVORED
LOCATIONS ACROSS NORTHEAST VT...NORTHERN NH AND WESTERN ME...WHERE
A COUPLE OF INCHES ARE ALSO POSSIBLE. FINALLY...STEEPENING MID
LEVEL LAPSE RATES (NEAR 7.0 C/KM) WITH THE SHORT WAVE AS IT
CROSSES THE MID ATLANTIC COULD PRODUCE SNOW SHOWERS AND LOCAL SNOW
SQUALLS WHICH COULD DEPOSIT LOCAL ONE INCH SNOWFALL AMOUNTS.


...PACIFIC NORTHWEST/ROCKIES...

A SHORT WAVE DIVING SOUTHEAST ACROSS THE ROCKIES ON DAY 1 WILL
PRODUCE SNOWFALL ACROSS THE ROCKIES IN CO AND FAR NORTHERN NM. MID
LEVEL RIDGING OVER THE WEST COAST MEANS LITTLE IN THE WAY OF
SIGNIFICANT SNOWFALL ON DAY 2. A SHORT WAVE BEGINS CHOPPING DOWN
THE TOP OF THE RIDGE CROSSES WA AND OR LATE ON DAY 3. THERE WAS
DECENT AGREEMENT WITH THE TIMING OF THE SHORT WAVE ON DAY 1...BUT
THERE IS MORE SPREAD ON THE TIMING AND STRENGTH OF THE SHORT WAVE
ON DAY 3 (AS THE 12Z CMC WAS A BIT FAST...AND THE 12Z ECMWF A BIT
SLOWER). IN AN ATTEMPT TO MITIGATE SOME OF THE DIFFERENCES...THE
THERMAL PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE 00Z
GFS/12Z ECMWF. THE QPF PORTION OF THE FORECAST WAS BASED ON THE
MOST RECENT WPC QPF.

DAY 1...
AS THE SHORT WAVE TRACKS FROM THE NORTHERN ROCKIES EARLY THIS
MORNING THROUGH THE CENTRAL AND SOUTHERN ROCKIES INTO THIS
EVENING...IT ENCOUNTERS AN INCREASINGLY DRY AIRMASS (AS
PRECIPITABLE WATER VALUES REMAINS AT OR BELOW 0.25 INCHES).
HOWEVER... UPSLOPE WINDS OVER THE CENTRAL AND SOUTHERN
ROCKIES...AS WELL AS STEEPENING MID LEVEL LAPSE RATES (AND
MARGINAL INSTABILITY)...COMBINE WITH THE SYNOPTIC SCALE ASCENT TO
PRODUCE A STRIPE OF 3 TO 6 INCHES STRETCHING FROM CENTRAL WY
THROUGH CO INTO NORTHERNMOST NM. THE HIGHEST AMOUNTS ARE EXPECTED
OVER SOUTH CENTRAL CO AND THE SANGRE DE CRISTO MOUNTAINS IN
NORTHERN NM.

DAY 3...
SHORT WAVE ENERGY STARTS CHOPPING DOWN THE MID LEVEL RIDGE OVER
THE WEST COAST DURING DAY 3. THE STRONGEST SHORT WAVE ARRIVES
ALONG THE WA/OR COASTS BETWEEN 28/06Z AND 28/12Z. IN THE WARM AIR
ADVECTION AHEAD OF THE SHORT WAVE...SNOW LEVELS RISE TO NEAR 8000
FEET. THAT MEANS 2 TO 4 INCH SNOWFALL AMOUNTS ARE LIMITED TO THE
HIGHER TERRAIN OF THE WA CASCADES.


HAYES