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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0918Z Dec 18, 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
418 AM EST THU DEC 18 2014

VALID 12Z THU DEC 18 2014 - 12Z SUN DEC 21 2014


...WESTERN U.S....

SNOWS ARE EXPECTED TO DEVELOP IN HIGHER ELEVATIONS OF THE PACIFIC
NORTHWEST AND NORTHERN ROCKIES LATER TONIGHT AND EARLY FRIDAY AS A
WELL DEFINED
UPPER TROUGH APPROACHES AND ASCENT GETS GENERATED BY AN UPPER
LEVEL JET STREAK IN ADVANCE OF THE LOW.  THIS INDUCES UPPER
DIVERGENCE/LOWER CONVERGENCE MAXIMA PEAKING OVER WASHINGTON BEFORE
WEAKENING AS IT MOVES INLAND OVER THE NORTHERN ROCKIES. 
LIKEWISE...SNOWS OCCURS OVER HIGHER ELEVATIONS OF THE OR/NORTHERN
CA RANGES AS A JET MAX ROUNDS THE BASE OF THE TROUGH AND UPPER
DIVERGENCE AIDS UPSLOPE FLOW IN PRODUCING ASCENT...LIKELY
RESULTING IN SEVERAL INCHES AT HIGHER ELEVATIONS.

FROM LATE FRIDAY THROUGH SATURDAY...THE MODELS/ENSEMBLES SHOW A
WELL DEFINED UPPER JET MAXIMA MOVING WEST TO EAST FROM THE
NORTHEAST PACIFIC AND EVENTUALLY INTO OR SAT.  HIGHER AMOUNTS ARE
EXPECTED IN THE WA CASCADES AS WELL DEFINED UPPER DIVERGENCE
MAXIMA OCCUR INT HE LEFT EXIT REGION OF THE UPPER JET.  THE GFS
AND EC STILL DEPICT 850-700 MB MOISTURE FLUXES INCREASING TO 3-4
STANDARD DEVIATIONS ABOVE NORMAL IN WESTERN OR.

THE UNCERTAINTY RELATES TO QPF AMOUNTS WHILE IN THE FORM OF SNOW. 
THE MODELS SHOW RAPID WARMING AS STRONG LOW LEVEL SOUTHWEST FLOW
PRODUCES WARMING TEMPERATURES...WITH RAPIDLY RISING SNOW LEVELS AS
A RESULT. BY SATURDAY...THOUGH...HEIGHTS COME DOWN ENOUGH WHERE
THE AREAL COVERAGE OF SNOW AND AMOUNTS INCREASE QUITE A BIT WHEN
COMPARED WITH DAYS 1/2 ACROSS PARTS OF THE PAC NW. 

...CENTRAL PLAINS DAY 1...

A 700 MB WAVE WITH A POSSIBLE CLOSED CIRCULATION WILL CONTINUE TO
MOVE FROM NEB/KS EASTWARD INTO PARTS OF MO.  WHILE SOME ADDITIONAL
LIGHT SNOWFALL AMOUNTS ARE POSSIBLE IN THIS AREA AFTER 12Z THIS
MORNING...WITH A FEW PLACES IN NORTHEAST KS AND ADJOINING AREAS
MAY SEE ANOTHER INCH OR TWO...THE WINDOW OF OPPORTUNITY TO
ACCUMULATE AN ADDITIONAL 4 INCHES OF SNOW IS QUITE SHORT. 
FREEZING PCPN WILL BE ISOLATED IN NATURE AND QUITE LIGHT. 
CONSEQUENTLY...NO AREAS FOR SNOW OR ICING DEPICTED HERE. 

...NORTHEAST DAY 1...

A LOW AT THE SURFACE AS WELL AS IN THE LOW/MID LEVELS WILL
CONTINUE TO PULL AWAY FROM NEW ENGLAND.  DESPITE WEAKENING
MESOSCALE FORCING...SOME LIGHT SNOW SHOULD CONTINUE ACROSS PARTS
OF NORTHERN/WESTERN ME TODAY AS NORTHERLY LOW LEVEL WINDS FORCES
COLD AIR WITH HI RH UPSLOPES.  THERE WAS ENOUGH ENSEMBLE AND
OPERATIONAL MODEL SUPPORT FOR A SLIGHT RISK OF 4 INCHES TO
CONTINUE INTO THE DAY TODAY ACROSS NORTHERN ME.  THIS ALSO HAPPENS
TO BE THE AREA WHERE TEMPERATURES REMAIN COLD ENOUGH TO SNOW ALL
DAY . 

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

BANN