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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0729Z Oct 26, 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
329 AM EDT SUN OCT 26 2014

VALID 12Z SUN OCT 26 2014 - 12Z WED OCT 29 2014


DAYS 1 THROUGH 3...


...PACIFIC NORTHWEST/NORTHERN AND CENTRAL ROCKIES...

A DEVELOPING MID LEVEL LONG WAVE TROUGH CROSSING THE PACIFIC
NORTHWEST THIS MORNING REACHES THE NORTHERN ROCKIES DURING THE
SECOND HALF OF DAY 1...BEFORE EXITING THROUGH THE NORTHERN
PLAINS/GREAT LAKES ON DAY 3. THIS SYSTEM WILL BE PRIMARILY
RESPONSIBLE FOR QPF AND SNOWFALL ACROSS THE PACIFIC NORTHWEST AND
NORTHERN ROCKIES THROUGH DAY 2. A WEAKENING MID LEVEL SYSTEM AND
ITS SURFACE REFLECTION APPROACHES THE PACIFIC NORTHWEST AND
BRITISH COLUMBIA DURING DAY 3. FOR THE MOST PART...THE QPF PORTION
OF THE FORECAST WAS BASED ON A BLEND OF THE 21Z SREF MEAN...12Z
ECMWF AND THE MOST RECENT WPC QPF...WHILE A MULTI MODEL APPROACH
WAS USED FOR THE THERMAL PORTION OF THE FORECAST.

A STRONG SHORT WAVE CROSSING THE WA/OR COAST EARLY THIS MORNING
WILL BEGIN TO ELONGATE INTO A LONG WAVE TROUGH AS IT CROSSES THE
PACIFIC NORTHWEST AND NORTHERN ROCKIES ON DAY 1. THE BEST LIFT IN
THE WARM AIR ADVECTION AHEAD OF THE SHORT WAVE OCCURS DURING THE
FIRST HALF OF THE PERIOD ACROSS WA/OR. SNOW LEVELS WILL BE ON THE
RISE EARLY...REACHING BETWEEN 8000 AND 10000 FEET ACROSS THE
NORTHERN CASCADES. THE RISING SNOW LEVELS AND BEST LIFT OCCURRING
EARLY SHOULD LIMIT SNOWFALL AMOUNTS ACROSS THE NORTHERN CASCADES
TO 2 TO 5 INCHES...MAINLY ABOVE 9000 FEET.

AS THE BEST LIFT/MOISTURE WITH THE DEVELOPING LONG WAVE TROUGH
REACHES THE NORTHERN AND CENTRAL ROCKIES DURING THE SECOND HALF OF
DAY 1... SNOW LEVELS SHOULD STILL RANGE BETWEEN 6000 AND 8000
FEET. THE COMBINATION OF LOWER SNOW LEVELS AND EXPECTED QPF SHOULD
PRODUCE STRIPES OF 4 TO 8 INCHES OF SNOWFALL ACROSS THE BITTEROOTS
OF ID AND THE GRAND TETONS OF WY. THESE SNOWFALL VALUES ARE WELL
SUPPORTED BY THE LATEST ECMWF ENSEMBLE OUTPUT...WITH A FEW MEMBERS
INDICATING THE POTENTIAL FOR 8+ INCHES OF SNOW ACROSS CENTRAL ID
AND NORTHWEST WY. THE LACK OF DEEP MOISTURE AND SPEED OF THE MID
LEVEL AND SURFACE SYSTEMS SHOULD RESULT IN A LOW PROBABILITY OF 8+
INCHES OF SNOW IN THESE LOCATIONS.

LESSER AMOUNTS ARE EXPECTED ACROSS THE THE ROCKY MOUNTAIN FRONT
RANGE IN MT AND THE ROCKIES OF NORTH CENTRAL CO. BEHIND THE LONG
WAVE TROUGH AND ITS ATTENDANT COLD FRONT...A BROAD COLD AIR
ADVECTION PATTERN DEVELOPS OVER THE ENTIRE REGION...AND THE
UPSLOPE FLOW SHOULD MAKE THE MOST OF THE AVAILABLE MOISTURE TO
PRODUCE LIGHT UPSLOPE SNOWFALL ACROSS THE NORTHERN ROCKIES LATE IN
THE PERIOD.

THE REGION IS EXPECTED TO BE IN BETWEEN SYSTEMS ON DAY 2 AS THE
LONG WAVE TROUGH EXITS THROUGH THE NORTHERN PLAINS AND GREAT
LAKES...AND A STRONG SHORT WAVE APPROACHES FROM WELL WEST OF THE
PACIFIC NORTHWEST COAST. A BROAD COLD AIR ADVECTION PATTERN
REMAINS IN PLACE FOR MUCH OF THE PERIOD...AND THE RESULTANT
UPSLOPE SNOW COULD RESULT IN AN INCH OR TWO OF SNOWFALL ACROSS THE
HIGHER TERRAIN OF THE NORTHERN CASCADES IN WA AND THE GRAND TETONS
IN WY.

THE STRONG SHORT WAVE APPROACHING FROM THE WEST ON DAY 3 WILL
WEAKEN AS IT AND ITS SURFACE SYSTEM APPROACH THE PACIFIC NORTHWEST
AND SOUTHERN BRITISH COLUMBIA. THE MAJORITY OF THE SHORT WAVE
ENERGY AND MOISTURE IS EXPECTED TO PASS NORTH OF WA DURING DAY 3.
IN THE BROAD WARM AIR ADVECTION PATTERN AHEAD OF THE SHORT
WAVE...SNOW LEVELS RISE FROM BETWEEN 6000 AND 8000 FEET EARLY TO
NEAR 10000 FEET ACROSS THE NORTHERN CASCADES OF WA. THE BEST
MOISTURE AND LIFT CROSS THE NORTHERN CASCADES DURING THE TIME OF
SNOW LEVEL RISES...SUPPORTING A SWATH OF 3 TO 6 INCHES OF SNOW
HERE.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES