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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0940Z Dec 17, 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
440 AM EST WED DEC 17 2014

VALID 12Z WED DEC 17 2014 - 12Z SAT DEC 20 2014


DAYS 1 THROUGH 3...

...WESTERN U.S....

A SERIES OF VORT CENTERS OVER THE ERN NOPAC AND ALONG THE WEST
COAST OF NORTH AMERICA WILL CONTINUE TO HELP CARVE OUT A LARGE
SCALE TROF OVER THE WESTERN STATES OVER THE NEXT COUPLE OF DAYS. 
SOME VORT ENERGY STAYS IN THE NORTHERN STREAM WHILE OTHER VORT
ENERGY DROPS SOUTH ACROSS THE SOUTHERN GREAT BASIN AND ADJACENT
PORTIONS OF MEXICO.  THE LEAD VORTS SHOULD DAMPEN AS THEY MAKE
THEIR WAY INLAND ON WEDNESDAY WHILE SUBSEQUENT VORTS RELOAD THE
TROF.  IN TERMS OF WINTER WEATHER...NOT EXPECTING LARGE AMOUNTS OF
SNOW WITH A SLIGHT RISK AT THE 4 AND 8 INCH THRESHOLD GENERALLY
COVERING THE BASES.  WHILE THE ONSHORE FLOW WILL BE TAPPING
SUFFICIENT MOISTURE...THE PROBLEM WILL BE FAIRLY WARM TEMPS/HIGH
THICKNESSES AND LACK OF STRONG COLD ADVECTION OR DYNAMIC COOLING
THROUGH 20/12Z.

...CENTRAL PLAINS...

ONE OF THE LEAD SHORTWAVES ROUNDING THE BASE OF THE LARGER SCALE
TROF WILL BE MAKING ITS WAY NORTHEAST FROM THE SOUTHERN PLAINS TO
THE MID MS VALLEY.  AS IT MOVES ACROSS THE CENTRAL PLAINS...THE
ASSOCIATED JET DYNAMICS WILL ARRIVE TO INTERACT WITH INCREASING
AMOUNTS OF MOISTURE BEING DRAWN NORTHWARD FROM THE GULF OF MEXICO.
 AS THE SHIELD OF PCPN EXPANDS NORTH...IT WILL EVENTUALLY
ENCOUNTER COLDER AIR OVER KS/MO WITH PTYPES TRANSITIONING FROM
RAIN TO SNOW.  GIVEN HIGH PRESSURE TO THE NORTH/NORTHEAST OF THE
AREA FEEDING DRY AIR OVER THIS PART OF THE COUNTRY TODAY...THERE
IS SOME CONCERN OVER HOW MUCH MOISTURE WILL BE NEEDED TO SATURATE
THE COLUMN BEFORE SNOWS ACCUMULATE. CURRENT THINKING IS THAT
INITIAL ACCUMULATIONS WILL BE OVER SOUTHERN PARTS OF KS/MO LATE
THIS AFTN OR EARLY EVENING WHILE DRY AIR OVER NORTHERN KS INTO
CENTRAL MO WILL DELAY THE ONSET OF SNOW A BIT WHICH IMPACTS
CHANCES FOR ACCUMULATON.  AS A RESULT...ONLY A SLIGHT RISK OF 4
INCHES SHOULD BE SUFFICIENT.  THERE IS ALSO SOME POTENTIAL A
LITTLE BIT OF FREEZING DRIZZLE IN THE TRANSITION ZONE EARLY ON BUT
NO ACCUMULATIONS SHOULD COME CLOSE TO A QUARTER OF AN INCH. 

...NORTHEAST UNITED STATES...

SURFACE LOW PRESSURE MOVING NORTHEAST FROM THE GREAT LAKES WILL BE
WEAKENING TODAY AS A VORT IN THE NORTHERN STREAM BECOMES SHEARED
APART.  HOWEVER A VORT IN THE SOUTHERN STREAM SHOULD INDUCE THE
FORMATION OF A COASTAL LOW SOUTH OF NEW ENGLAND LATER
TODAY...WHICH MOVES NORTHEAST THROUGH TONIGHT.  THE HIGHEST RISK
FOR 8/12 INCH AMOUNTS ARE IN THE ME MOUNTAINS WHICH SHOULD ROUGHLY
BE COINCIDENT WITH MESOSCALE FORCING IN THE FORM OF DEEP LAYER AND
SLOPED FGEN.  THERE WILL ALSO BE SOME POTENTIAL FOR ICING ON DAY 1
WITH PLUME DIAGRAMS FROM THE SREF POINTING TO A SLIGHT RISK THAT
AMOUNTS MAY EXCEED 0.25 INCHES IN PARTS OF NORTHERN/EASTERN ME. 
SOME LIGHT SNOW IS POSSIBLE FURTHER TO THE SOUTH ACROSS PARTS OF
NORTHERN NH/VT/NY IN REGIONS WERE NORTHWESTERLY SURFACE LOW IS
FORCED TO RISE BY OROGRAPHY...BUT NOT ENOUGH SUPPORT FROM THE
OPERATIONAL MODELS OR THEIR ENSEMBLES TO WARRANT A SLIGHT RISK OF
4 INCHES HERE.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT ON
DAYS 2 AND 3.

BANN