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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0759Z Dec 20, 2014)
 
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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
259 AM EST SAT DEC 20 2014

VALID 12Z SAT DEC 20 2014 - 12Z TUE DEC 23 2014


DAYS 1-3...

...PACIFIC NORTHWEST TO THE NORTHERN AND CENTRAL ROCKIES...

A POWERFUL PACIFIC JET WILL BE ASSOCIATED WITH SEVERAL SIGNIFICANT
SHORTWAVE SYSTEMS THAT WILL DRIVE MOISTURE INLAND ACROSS THE
PACIFIC NORTHWEST BEFORE GENERATING AN AMPLIFYING LONGER
WAVELENGTH TROUGH ACROSS THE ROCKIES INTO THE CENTRAL UNITED
STATES BY EARLY IN THE WEEK.  BUILDING HEIGHTS OVER THE EASTERN
PACIFIC WILL BE ASSOCIATED WITH THE DOWNSTREAM AMPLIFICATION OF
THE TROUGH AS THE SHORTWAVES PROGRESS SOUTH AND EASTWARD ACROSS
THE WESTERN UNITED STATES.  WHILE SNOW LEVELS WILL RISE INITIALLY
IN RESPONSE THE WARM AIR ADVECTION AHEAD OF THESE SYSTEMS...COLDER
AIR WILL DROP SOUTH AND EASTWARD ACROSS THE INTERMOUNTAIN WEST AND
ROCKIES...AGAIN DROPPING SNOW LEVELS ON MONDAY.

ON DAY 1/SATURDAY MORNING TO SUNDAY MORNING...EVEN THOUGH SNOW
LEVELS WILL BE RISING...SIGNIFICANT ACCUMULATIONS WILL STILL OCCUR
AS QPF TOTALS ESTIMATED AT 1 TO GREATER THAN 4 INCHES WILL RESULT
IN VERY HEAVY HIGH ELEVATION SNOW ESPECIALLY ACROSS THE WASHINGTON
CASCADES..WITH THE HIGHEST PEAKS WITH HIGH PROBABILITIES OF MORE
THAN A FOOT OF SNOW.  HEAVY SNOW WILL ALSO OCCUR IN THE HIGHER
ELEVATIONS OF THE OREGON CASCADES...THE SISKIYOU IN NORTHERN
CALIFORNIA AND ESPECIALLY ACROSS THE BLUE MOUNTAINS IN NORTHEAST
OREGON...AND THE BITTERROOTS/SALMON RIVER AND SAWTOOTH RANGES IN
IDAHO.  SIGNIFICANT SNOW WILL ALSO FALL IN THE HIGHER PEAKS OF
NORTHERN NEVADA...THE ABSAROKA/TETONS AND WIND RIVER MOUNTAINS IN
NW WYOMING...AND START TO SNOW IN THE COLORADO ROCKIES. A MULTI
MODEL BLEND WAS USED ON DAY 1 WHILE A COMBINATION OF ECMWF/GFS WAS
FAVORED ON DAYS 2/3.

ON DAY 2...SUNDAY MORNING TO MONDAY MORNING...SNOW REMAINS HEAVY
IN THE WASHINGTON CASCADES THOUGH AMOUNTS SHOULD DIMINISH COMPARED
TO DAY 1.  HOWEVER...HEAVY SNOW WILL CONTINUE ACROSS THE MOUNTAINS
IN IDAHO AND WILL BE ON THE INCREASE ACROSS THE MOUNTAINS OF
WESTERN WYOMING/THE WASATCH/UINTA IN UTAH AND THE COLORADO
ROCKIES...WHERE MORE THAN A FOOT OF SNOW CAN BE EXPECTED WITH AT
LEAST A MODERATE LIKELIHOOD.  BY DAY 3...SNOWFALL AMOUNTS SHOULD
BE DIMINISHING ACROSS MUCH OF THE WEST THOUGH MORE MODEST AMOUNTS
ARE STILL LIKELY.  THE ONE EXCEPTION WILL BE IN THE COLORADO
ROCKIES...WHERE MODERATE TO HIGH PROBABILITIES OF 4 TO 8 INCHES
ARE EXPECTED.

DAYS 2 TO 3...

...NORTHERN PLAINS TO UPPER MISSISSIPPI VALLEY/MIDWEST...

THE DEEPENING TROUGH OVER THE CENTER OF THE COUNTRY ON EARLY
MONDAY INTO EARLY TUESDAY IS EXPECTED TO PRODUCE THE DEVELOPMENT
OF A COMPLEX AREA OF LOW PRESSURE AS SOUTHERLY FLOW TAPS INTO GULF
MOISTURE FOR AN AREA OF LOW PRESSURE OVER THE PLAINS STATES. 
INITIALLY...THIS SYSTEM IS NOT EXPECTED TO DEEPEN
RAPIDLY...ALTHOUGH IT HAS BEEN WELL ADVERTISED TO DEEPEN MORE
SIGNIFICANTLY BUT AFTER THE FORECAST PERIOD.  IN THE MEANTIME...A
BROAD AREA OF MOSTLY LIGHT SNOW IS EXPECTED TO DEVELOP FROM THE
DAKOTAS ACROSS MINNESOTA INTO WISCONSIN MOSTLY ON DAY 3...WITH A
LOW TO PERHAPS MODERATE PROBABILITY OF SOME AREAS GETTING 4 OR
MORE INCHES OF SNOW.  WITH SOME HIGH PRESSURE BANKED TO THE
NORTHEAST OF NEW ENGLAND...SOME LIGHT FREEZING RAIN OR DRIZZLE IS
POSSIBLE ACROSS THE INTERIOR PORTIONS OF THE MIDDLE ATLANTIC
STATES BUT AMOUNTS ARE EXPECTED TO BE LIGHT. 

DAYS 1 TO 3...

...EASTERN NEW ENGLAND...

WHILE THERE IS AT MOST A LOW PROBABILITY OF GREATER THAN 4 INCHES
OF SNOW...A SLOW MOVING UPPER TROUGH AND ASSOCIATED SURFACE
INVERTED TROUGH OVER EASTERN NEW ENGLAND BE GENERATING SOME
MESOSCALE AREAS OF SNOWFALL ACROSS COASTAL MASSACHUSETTS/RHODE
ISLAND/NEW HAMPSHIRE AND MAINE WITH MOST MODELS INDICATING LIGHT
ACCUMULATIONS.  HOWEVER...THESE KINDS OF INSTABILITY/COASTAL
SYSTEMS OFTEN CAN GENERATE LOCAL AREAS OF SIGNIFICANT SNOW EVERY
NOW AND THEN SO THIS ONE WILL BE WATCHED. FOR NOW...SIGNIFICANT
SNOW IS STILL AN OUTSIDE CHANCE.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

KOCIN