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Winter Weather Forecast Discussion
 
(Latest Discussion - Issued 0801Z Dec 22, 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
301 AM EST MON DEC 22 2014

VALID 12Z MON DEC 22 2014 - 12Z THU DEC 25 2014

DAY 1

ROCKIES AND NORTHERN PLAINS...

SLIGHT AND MODERATE PROBABILITIES OF 4 AND 8 INCHES OF SNOW WILL
ACCOMPANY THE CLOSELY-SPACED AND SLIGHTLY INTERACTING SHORTWAVE
TROUGHS CROSSING THE NORTHERN ROCKIES TODAY AND PLAINS TONIGHT. 
THE LIQUID EQUIVALENT VALUES AND THERMAL PROFILES OF THE 00Z
NAM/00Z GFS ARE PREFERRED OVER THE 12Z ECMWF ACROSS THE
DAKOTAS...WHILE THE 12Z ECMWF IS MORE PREFERRED OVER THE CENTRAL
ROCKIES INTO THE CENTRAL PLAINS DUE TO GREATER CONCERNS WITH THE
STRUCTURE AND DEPTH OF THE NAM/GFS...PARTICULARLY THE GFS WHICH IS
DEEPER ALOFT AND THUS COLDER RESULTING IN MORE SNOW FARTHER SOUTH.


THE PROBABILITY OF ICE GREATER THAN 0.25 INCHES IS LESS THAN 10
PERCENT.

DAYS 2 AND 3

UPPER GREAT LAKES...

PREFER THE LOW TRACK OF THE 00Z NAM REACHING THE UPPER PENINSULA
OF MICHIGAN BY EARLY WEDNESDAY...WHILE THE 00Z GFS MAY BE TOO
FAST...AND 12Z ECMWF TOO FAR WEST.  THE NAM'S LOW TRACK PRODUCES
PREFERRED THERMAL PROFILES AS WELL...ALTHOUGH ITS ASSOCIATED
LIQUID EQUIVALENT AMOUNTS MAY BE TOO GENEROUS.  FOR
AMOUNTS...PREFER A SOLUTION ABOUT MIDWAY BETWEEN THE 00Z GFS AND
00Z ECMWF.  THE RESULT IS A NARROW SWATH OF MODERATE PROBABILITIES
OF 4 INCHES FOR NORTHERN WISCONSIN.  FOR THE LOW MOVING NORTHWARD
THROUGH THE OHIO VALLEY WEDNESDAY NIGHT INTO THURSDAY...PREFER A
SOLUTION CLOSE TO THE 00Z ECMWF WHICH IS SUPPORT BY THE 00Z GFS. 
THE TIMING AND THERMAL PROFILES ARE THE LARGEST CHALLENGES WITH
THIS LOW...AS MOISTURE SHOULD BE ABUNDANT.  THE NORTHERN
TRAJECTORY SHOULD BE ACCOMPANIED BY WARM THERMAL PROFILES THAT
REDUCE THE LIKELIHOOD FOR ACCUMULATING SNOWS ACROSS THE OHIO
VALLEY...WITH A NARROW WINDOW OF OPPORTUNITY FOR NORTHERN MICHIGAN
EARLY THURSDAY...WHERE MAINLY LOW PROBABILITIES FOR 4 INCHES OF
SNOW EXIST.

THE PROBABILITY OF ICE GREATER THAN 0.25 INCHES IS LESS THAN 10
PERCENT.

PACIFIC NORTHWEST/NORTHERN ROCKIES...

THE SOLUTION SPREAD REGARDING THE SIGNIFICANT SHORTWAVE TROUGH
CROSSING THE PACIFIC NORTHWEST LATE WEDNESDAY INTO THURSDAY
REMAINS QUITE LARGE...WITH A SOLUTION ABOUT MIDWAY BETWEEN THE
WEAKER 00Z NAM AND STRONGER 00Z ECMWF PREFERRED...WHICH IS
SUPPORTED BY THE 00Z PARALLEL GFS.  THIS APPROACH ALSO DISCOUNTS
THE SLOWER 00Z GFS.  THE DEPTH OF THE TROUGH AND ASSOCIATED STRONG
WESTERLY FLOW FAVOR MODERATE OR HIGH PROBABILITIES OF 8 AND 12
INCHES OF SNOW FOR THE CASCADES...AS WELL AS THE
BITTERROOTS/TETONS...INTO EARLY THURSDAY.  SNOW LEVELS SHOULD ALSO
FALL APPRECIABLY AND QUICKLY ONCE THE TROUGH PASSES.

THE PROBABILITY OF ICE GREATER THAN 0.25 INCHES IS LESS THAN 10
PERCENT.

JAMES