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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0706Z Apr 22, 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
306 AM EDT WED APR 22 2015

VALID 12Z WED APR 22 2015 - 12Z SAT APR 25 2015


DAYS 1 THROUGH 3...


...NEW ENGLAND...

A STRONG NEGATIVELY TILTED SHORT WAVE EJECTED FROM A CLOSED MID
LEVEL LOW OVER ONTARIO CROSSES NEW ENGLAND LATE ON DAY 1 INTO DAY
2. THE SHORT WAVE SPINS UP A SURFACE LOW OVER WESTERN STERN NEW
ENGLAND DURING THIS TIME. AS THE SHORT WAVE CLOSES OFF OVER
NORTHERN NEW ENGLAND LATE ON DAY 2 INTO DAY 3...THE SURFACE LOW
MEANDERS OVER ME AND WEAKENS. THERE WAS GOOD MODEL AGREEMENT ON
THE TRACK OF THE SHORT WAVE AS IT CLOSES OFF OVER ME...BUT THERE
WAS SOME SPREAD ON THE TRACK OF THE SURFACE LOW AS IT CROSSES FROM
ME INTO QUEBEC. THE 12Z ECMWF LOOKS TO BE TOO FAR EAST
INITIALLY...BUT THE SPREAD LESSENS HEADING INTO DAY 3. IN AN
ATTEMPT TO MITIGATE SOME OF THE DIFFERENCES IN THE SURFACE LOW
POSITION (AND BOUNDARY LAYER TEMPERATURES THAT ACCOMPANY THE
DIFFERENCES IN THE LOW POSITION)...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.

THE FIRST SHOT OF MOISTURE AND LIFT ASSOCIATED WITH THE NEGATIVELY
TILTED SHORT WAVE CROSSING NORTHERN NEW ENGLAND OCCURS BETWEEN
23/06Z AND 23/18Z OVER NORTHERN NY STATE...NORTHERN VT...NORTHERN
NH AND WESTERN ME. DYNAMIC COOLING ASSOCIATED WITH THE HEIGHT
FALLS BRING SNOW LEVELS DOWN TO AROUND 2000 FEET IN THESE
LOCATIONS. THE BEST LIFT OCCURS AS THE COLUMN IS COOLING...SO THE
FIRST SHOT OF SNOWFALL WILL BE MAINLY ELEVATION DRIVEN. SURFACE
LOW PRESSURE FORMING IN RESPONSE TO THE SHORT WAVE OVER SOUTHWEST
ME TRACKS NORTH...AND THIS KEEPS SNOW LEVELS LOWER WEST OF THE LOW
TRACK (THROUGH WESTERN ME). SNOWFALL AMOUNTS ABOVE 2000/3000 FEET
COULD PRODUCE AS MUCH AS 3 TO 6 INCHES OF SNOWFALL ACROSS THE
HIGHEST PEAKS OF NORTHERN NH AND NEARBY WESTERN ME.

THESE AMOUNTS ARE WELL SUPPORTED BY MEMBERS OF BOTH THE 21Z SREF
AND LATEST ECMWF ENSEMBLE SOLUTIONS...WHICH SHOW 4+ INCH SNOWFALL
POTENTIAL IN THESE PLACES. HOWEVER...THE COLUMN ITSELF IS
BORDERLINE FOR SNOWFALL...ESPECIALLY BELOW 3000 FEET...SO THE
HIGHEST AMOUNTS WERE PLACED IN THE HIGHEST TERRAIN. VALLEY
LOCATIONS ACROSS NORTHERN NY STATE (IN THE ADIRONDACKS)...NORTHERN
VT...NORTHERN NH AND WESTERN ME MAY SEE LITTLE IF ANY SNOW
ACCUMULATION DURING THE 23/06Z TO 23/18Z TIME FRAME. THE
PROBABILITY FOR 4+ INCHES OF SNOW IS LOW OVER MUCH OF THE
ABOVEMENTIONED AREA...BUT TOPS OUT AT MODERATE FOR THE TERRAIN
ABOVE 3000 FEET IN NORTHERN NH AND WESTERN ME.

AS THE MID LEVEL SYSTEM WOBBLES ACROSS NORTHERN NEW ENGLAND DAY
3...THE LOW LEVEL WIND FLOW BECOMES FAVORABLE FOR UPSLOPE SNOWFALL
ACROSS PORTIONS OF VT (THE WHITE MOUNTAINS)...NORTHERN NH (THE
GREEN MOUNTAINS) AND WESTERN ME. MODEL SOUNDINGS SHOWED THAT SNOW
LEVELS REMAINS BETWEEN 2000 AND 3000 FEET DURING THIS TIME...SO
SNOW ACCUMULATIONS OF AN INCH OR TWO WERE PLACED ABOVE THIS LEVEL.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


...PACIFIC NORTHWEST/NORTHERN ROCKIES/GREAT BASIN...

A STRONG CLOSED MID LEVEL LOW PASSES OVER VANCOUVER ISLAND AND THE
PACIFIC NORTHWEST LATE ON DAY 2 INTO DAY 3. THE MID LEVEL SYSTEM
PUSHES A SURFACE FRONT ACROSS THE PACIFIC NORTHWEST INTO THE
NORTHERN ROCKIES ON DAY 3. THERE WAS VERY GOOD MODEL AGREEMENT
CONCERNING THE TRACK OF THE MID LEVEL LOW...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 2...
AS THE CLOSED MID LEVEL SYSTEM DROPS SOUTHEAST ACROSS VANCOUVER
ISLAND INTO PACIFIC NORTHWEST LATE ON DAY 2...FALLING HEIGHTS
ALLOW SNOW LEVELS DROP TO AS LOW AS 4500 TO 5500 FEET ACROSS THE
WA AND OR CASCADES. THE LOW LEVEL FLOW AHEAD OF THE CLOSED LOW AND
SURFACE FRONT SUPPORTS LOCAL 2 TO 4 INCH SNOWFALL AMOUNTS OVER THE
OLYMPICS AND CASCADES IN WA. MEMBERS OF THE LATEST ECMWF ENSEMBLE
OUTPUT SHOWED 4+ INCH SNOWFALL POTENTIAL IN BOTH OF THESE
LOCATIONS. HOWEVER...THE LATE TIMING OF THE MID LEVEL SYSTEM MAKES
4+ INCHES OF SNOWFALL HERE A LOW PROBABILITY EVENT.

DAY 3...
THE CLOSED LOW SLOWS AS IT CROSSES THE PACIFIC NORTHWEST DURING
DAY 3...SENDING SHORT WAVES OUT AHEAD OF IT ACROSS THE NORTHERN
ROCKIES. THE BEST MID LEVEL LIFT OCCURS ACROSS THE PACIFIC
NORTHWEST DURING THE FIRST HALF OF THE PERIOD...WHEN SNOW LEVELS
REMAIN BETWEEN 4500 AND 5500 FEET. THE LIFT AND MOISTURE WITH THE
CLOSED LOW SUPPORTS 3 TO 6 INCH SNOWFALL AMOUNTS ACROSS THE
NORTHERN WA CASCADES...AND THESE AMOUNTS ARE WELL SUPPORTED BY
MEMBERS OF THE 21Z SREF AND LATEST ECMWF ENSEMBLE OUTPUT. IN
FACT...THE ECMWF ENSEMBLE OUTPUT HAS MEMBERS SHOWING 8+ INCH
POTENTIAL...BUT MOISTURE MAKE THIS A LOW PROBABILITY EVENT AT
BEST.

ONE OF THE SHORT WAVES EJECTED FROM THE STRONG CLOSED LOW CROSSES
THE NORTHERN ROCKIES DURING THE SECOND HALF OF DAY 3...PROVIDING
AMPLE SYNOPTIC SCALE ASCENT FOR SNOWFALL. THE PROXIMITY OF THE
SURFACE FRONT SUPPLIES SURFACE CONVERGENCE...AND THE COMBINATION
OF LIFT AND SURFACE CONVERGENCE MAKES UP FOR THE LESSER AMOUNTS OF
MOISTURE IN THE COLUMN. SNOW LEVELS ACROSS THE NORTHERN ROCKIES
ARE HIGHER THAN THE PACIFIC NORTHWEST...GENERALLY AVERAGING
BETWEEN 6000 AND 8000 FEET...WHICH SUPPORTS A SWATH OF 4 TO 6
INCHES OF SNOWFALL ACROSS THE ABSAROKA MOUNTAIN RANGE IN SOUTHWEST
MT. THESE NUMBERS ARE SUPPORTED BY MEMBERS OF BOTH THE 21Z SREF
AND LATEST ECMWF ENSEMBLE OUTPUT. ALL OF THIS LEADS TO A MODERATE
PROBABILITY OF 4+ INCHES OF SNOWFALL IN A PORTION OF SOUTHWEST MT.

THE 4 TO 6 INCHES OF SNOWFALL OVER THE ABSAROKA MOUNTAINS IS
SURROUNDED BY A LARGER AREA OF 2 TO 4 INCHES OF SNOWFALL OVER THE
REMAINDER OF SOUTHWEST MT AND THE GRAND TETONS IN WY. FURTHER
SOUTH...LOCAL 2 TO 4 INCH SNOWFALL AMOUNTS ARE POSSIBLE OVER THE
UINTAS AND NORTHERN WASATCH RANGES IN UT. 


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES