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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0815Z Apr 22, 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
415 AM EDT TUE APR 22 2014

VALID 12Z TUE APR 22 2014 - 12Z FRI APR 25 2014


DAYS 1 THROUGH 3...


...PACIFIC NORTHWEST/NORTHERN ROCKIES...

A SERIES OF SHORT WAVES CROSSING THE PACIFIC NORTHWEST AND
NORTHERN ROCKIES WILL PROVIDE SUFFICIENT LIFT FOR SNOW EACH OF THE
NEXT THREE DAYS. THERE WAS GOOD MODEL AGREEMENT WITH RESPECT TO
THE EVOLUTION OF THE LONG WAVE TROUGH ACROSS THE WEST...AND WITH
THE TIMING OF THE SHORT WAVES WITHIN THE LONG WAVE TROUGH. DUE TO
THE GOOD MODEL AGREEMENT...A CONSENSUS APPROACH WAS USED WITH
RESPECT TO THE QPF PORTION OF THE FORECAST...LEANING MORE HEAVILY
TOWARD A 00Z GFS/12Z ECMWF BLEND. SOME 00Z NAM WAS USED TO BETTER
DELINEATE HIGHER ELEVATION QPF AND SNOWFALL MAXIMA. THE THERMAL
PORTION OF THE FORECAST WAS BASED PRIMARILY ON A BLEND OF THE 00Z
NAM/GFS/21Z SREF MEAN.

THE LONG WAVE TROUGH MOVES INLAND ACROSS THE WEST COAST ON DAY
1...WEAKENING AS IT REACHES THE NORTHERN ROCKIES BY THE END OF THE
PERIOD. AHEAD OF THE LONG WAVE TROUGH...FREEZING LEVELS RISE IN
THE MID LEVEL SOUTHWEST FLOW TO ABOUT 4000 FEET ACROSS THE HIGHER
TERRAIN OF WA...OR AND ID. THE QPF CAN SUPPORT 4 TO 8 INCHES OF
SNOW ACROSS THE CASCADES OF WA AND OR...AND A FEW OF THE LATEST
ECMWF ENSEMBLE MEMBERS SUGGEST 8+ INCH AMOUNTS IN THE WA CASCADES.
FURTHER EAST... UPSLOPE FLOW BEHIND THE TROUGH SHOULD FOCUS THE
AVAILABLE MOISTURE IN THE HIGHER TERRAIN...AND THE QPF COULD
SUPPORT 6 TO 10 INCHES OF SNOW ACROSS THE BITTERROOTS OF ID AND
GRAND TETONS IN WY. THESE AMOUNTS ARE SUPPORTED BY A FEW MEMBERS
OF THE LATEST ECMWF ENSEMBLE OUTPUT.

THE NEXT SHORT WAVE APPROACHES THE PACIFIC NORTHWEST AND NORTHERN
ROCKIES DURING THE SECOND HALF OF DAY 2. FREEZING LEVELS RISE ONCE
AGAIN AHEAD OF THE SHORT WAVE...REACHING 6000 FEET OR HIGHER
ACROSS THE CASCADES AND BITTERROOTS. THE UPSLOPE FLOW ACROSS THE
CASCADES COULD SUPPORT 4 TO 6 INCHES OF SNOW ACROSS THE WA
CASCADES...WITH LESSER AMOUNTS ACROSS THE OR CASCADES. FURTHER
EAST...THE MOISTURE IS NOT AS ROBUST...BUT THE UPSLOPE FLOW COULD
RESULT IN 4 TO 8 INCHES OF SNOW ACROSS THE BITTERROOTS.

THE FINAL SHORT WAVE IN THE TRAIN CROSSES THE PACIFIC NORTHWEST
COAST EARLY ON DAY 3. FREEZING LEVELS REMAIN ABOVE 6000 FEET AS
THE BEST LIFT/MOISTURE CROSS THE REGION...LIMITING THE WINTER
WEATHER IMPACTS. THIS SHOULD RESULT IN 2 TO 4 INCHES OF SNOW ABOVE
6000 FEET ACROSS THE CASCADES OF WA AND OR...AND AS MUCH AS 4 TO 6
INCHES ACROSS THE BITTEROOTS OF ID.


...UPPER MS VALLEY/GREAT LAKES...

A CLOSED MID LEVEL SYSTEM AFFECTING THE UPPER MS VALLEY AND GREAT
LAKES LATE ON DAY 2 AND DAY 3 COULD PRODUCE A SIGNIFICANT LATE
SEASON SNOWFALL FOR A PORTION OF THE AREA. THERE WAS DECENT
AGREEMENT AMONG THE 00Z MODEL SUITE MEMBERS CONCERNING THE OVERALL
SYNOPTIC SETUP AND MASS FIELD PLACEMENT IN CONJUNCTION WITH THE
CLOSED MID LEVEL SYSTEM. BECAUSE OF THIS...THE QPF PORTION OF THE
FORECAST WAS BASED ON A MULTI MODEL BLEND...WHICH LEANED MORE
HEAVILY TOWARD THE 00Z GFS/12Z ECMWF. THERE WERE HOWEVER...SOME
IMPORTANT THERMAL DIFFERENCES...AS THE 00Z GFS AND GEFS APPEARED
TO BE TOO WARM IN THE LOWER LEVELS...ESPECIALLY ACROSS NORTHERN MN
AND NORTHERN WI ON DAY 3. THEREFORE...THE THERMAL PORTION OF THE
FORECAST LEANED MORE TOWARD AS 12Z ECMWF/00Z NAM BLEND.

THE MORE SIGNIFICANT LIFT APPROACHES THE REGION LATE ON DAY 2...AS
SURFACE LOW PRESSURE DEVELOPS ON A LOW LEVEL BAROCLINIC ZONE OVER
THE NORTHERN PLAINS. THE PATTERN OF LIFT TAKES ON A
NORTHWEST-SOUTHEAST ORIENTATION ACROSS MN AND WI.
INITIALLY...MODEL SOUNDINGS SUGGEST THAT THE COLUMN IS TOO WARM IN
THE BOUNDARY LAYER TO SUPPORT SNOW. HOWEVER...AS THE 850 MB LOW
DEVELOPS AND DEEPENS ACROSS ND AND EASTERN MT COLDER AIR FILTERS
SOUTH INTO NORTHERN MN... NORTHERN WI AND THE UP OF MI. THERE IS A
MULTI MODEL SIGNAL THAT A TROWAL DEVELOPS NORTH OF THE SURFACE AND
850 MB LOWS...WITH THE BEST LIFT ACROSS THE ABOVEMENTIONED AREAS
INTO SOUTHERN QUEBEC.

THE BEST LIFT OCCURS BETWEEN 18Z THU AND 06Z FRI. BY THIS
TIME...ENOUGH COLD AIR FILTERS SOUTH TO SUPPORT ALL SNOW SOUNDING
ACROSS NORTHEAST MN...NORTHERNMOST WI AND THE UP OF MI. THE LIFT
IN THE DEFORMATION ZONE THAT FORMS NORTH OF THE 850 MB LOW APPEARS
TO BE COINCIDENT WITH THE DEEPEST MOISTURE...AND MODEL SOUNDINGS
SHOW A PERIOD OF MAXIMUM DENDRITIC GROWTH...CENTERED AROUND 00Z
FRI. THE COMBINATION OF LIFT AND MOISTURE IN THE DENDRITIC GROWTH
ZONE SUPPORTS NEAR ISOTHERMAL CONDITIONS IN THE
COLUMN...INDICATIVES OF HEAVY SNOW.

THE ABOVE SUPPORTS A SWATH OF 4 TO 8 INCHES OF SNOW ACROSS THE
ARROWHEAD OF MN...NORTHERNMOST WI...AND NORTHERN SECTIONS OF THE
UP OF MI. THE QPF SUPPORTS A SMALL AREA OF 8 TO 10 INCHES OF SNOW
ACROSS THE NORTHEAST TIP OF MN INTO THE NORTHERNMOST TIP OF THE UP
OF MI. THESE AMOUNTS ARE STRONGLY SUPPORTED BY MOST MEMBERS OF THE
LATEST ECMWF ENSEMBLE OUTPUT (AND THERE EVEN A FEW 12+ INCH
ENSEMBLE MEMBERS IN THIS AREA). THE UNCERTAINTY IN THE BOUNDARY
LAYER CONDITIONS EARLY ON DAY 3 SUGGEST THAT 12+ INCH AMOUNTS
WOULD HAVE NO HIGHER THAN LOW CHANCE PROBABILITIES.


...NORTHERN NEW ENGLAND...

MOISTURE WRAPPING AROUND A CLOSING MID LEVEL SYSTEM AND ITS
ASSOCIATED SURFACE LOW COULD ENCOUNTER ENOUGH COLD AIR FOR SNOW
ACROSS NORTHERN PORTIONS OF ME DURING DAY 2. THERE WAS DECENT 00Z
MODEL SUITE AGREEMENT WITH RESPECT TO OVERALL SYNOPTIC
SETUP...THOUGH THERE WERE SOME SUBTLE TEMPERATURE DIFFERENCES IN
THE COLUMN THAT COULD AFFECT PRECIPITATION TYPE. THE QPF PORTION
OF THE FORECAST WAS DERIVED FROM A MULTI MODEL BLEND. THE THERMAL
PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE 00Z GFS/18Z
GEFS MEAN/12Z ECMWF...AS THE 00Z NAM APPEARS TO BE TOO AGGRESSIVE
WITH THE COLD AIR WRAPPING INTO THE MOISTURE.

AS THE CLOSED MID LEVEL LOW PIVOTS PAST NORTHERN ME...MOISTURE IS
WRAPPED BACK INTO THE DEFORMATION ZONE NORTHWEST OF THE
LOW...WHICH WITH TIME TAKES ON A NORTH TO SOUTH ORIENTATION. IN
THE BAND OF LIFT THAT DEVELOPS...MOISTURE IS DEEP ENOUGH TO ENTER
THE DENDRITIC GROWTH ZONE...AND COULD SUPPORT BANDING OF SNOW
ACROSS NORTHERNMOST ME AS IT MOVES EAST BETWEEN 18Z THU AND 06Z
FRI. IN THIS BAND...2 TO 4 INCHES OF SNOW IS EXPECTED ACROSS THE
ROOFTOP OF ME... WITH HIGHER AMOUNTS EXPECTED IN EASTERN QUEBEC
AND WESTERNMOST NEW BRUNSWICK. IT SHOULD BE NOTED THAT THE 1Z
ECMWF AND 18Z GEFS MEAN ACTUALLY SUPPORTED HIGHER AMOUNTS (IN THE
4 TO 6 INCH RANGE...WHICH ARE SUPPORTED BY A FEW ECMWF ENSEMBLE
MEMBERS). THE PROGRESSIVE NATURE OF THE SYSTEM SHOULD KEEP THESE
PROBABILITIES LOW.


HAYES