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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0916Z Dec 16, 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
416 AM EST TUE DEC 16 2014

VALID 12Z TUE DEC 16 2014 - 12Z FRI DEC 19 2014


DAYS 1 THROUGH 3...

...CA/GT BASIN/ROCKIES/SOUTHWEST...

A SERIES OF SHORT WAVES COMING ASHORE ACROSS NORTHERN AND CENTRAL
CA DURING DAYS 1 AND 2 CARVE OUT A LONG WAVE TROUGH ACROSS THE
GREAT BASIN AND SOUTHWEST STATES ON DAY 3. THERE WAS DECENT MODEL
AGREEMENT DURING DAYS 1 AND 2 AS THE SHORT WAVES COME ACROSS CA
AND AFFECT THE GREAT BASIN...ALTHOUGH THE MODEL AGREEMENT TENDS TO
FALL APART ON DAY 3.  IN AN ATTEMPT TO MITIGATE SOME OF THE
DIFFERENCES...THE QPF PORTION OF THE FORECAST WAS BASED PRIMARILY
ON THE 15/12Z ECMWF AND THE 16/00Z UKMET.  THE PREFERRED SOLNS
TEND TO HAVE SOMEWHAT LESSER SNOWFALL AMOUNTS FROM THE PREVIOUS
FORECAST BUT THE OVERALL THINKING HAS NOT CHANGED SIGNIFICANTLY. 

THE SHORT WAVES CROSSING CA AND THE GREAT BASIN CARVE OUT A LONG
WAVE TROUGH ACROSS THE THE SOUTHWEST STATES DURING DAY 3. AHEAD OF
THE LONG WAVE TROUGH...MOISTURE IS DRAWN INTO THE SOUTHWEST STATES
ON AN INCREASING LOW LEVEL SOUTHEAST FLOW. SNOW LEVELS DROP TO
BETWEEN 5500 AND 6500 FEET ACROSS THE AZ/NM AND THE SAN JUAN RANGE
OF SOUTHWEST CO. THE COMBINATION OF SYNOPTIC SCALE LIFT WITH THE
LING WAVE TROUGH AND INCREASED MOISTURE SETS THE STAGE FOR A SWATH
OF MDT TO HEAVY SNOWS ACROSS THE DRAGOON AND CHIRICHUA MOUNTAINS
IN AZ.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


...CENTRAL PLAINS/MID MS VALLEY...

ISENTROPIC LIFT DEVELOPING AHEAD OF A LONG WAVE TROUGH OVER THE
SOUTHWEST STATES ON DAY 3 WILL SPREAD MOISTURE NORTH OF A FRONTAL
BOUNDARY EXTENDING ACROSS THE SOUTHERN PLAINS. THE MOISTURE
ENCOUNTERS COLD AIR BLEEDING SOUTH FROM STRONG HIGH PRESSURE OVER
THE UPPER MS VALLEY AND MIDWEST...RESULTING IN LIGHT WINTRY
PRECIPITATION DURING DAY 3. THERE IS A FAIR AMOUNT OF MODEL SPREAD
CONCERNING THE PLACEMENT OF THE SWATH OF WINTRY PRECIPITATION.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


...UPPER MS VALLEY/GREAT LAKES...

A STRONG MID LEVEL SHORT WAVE OVER WESTERN ONTARIO WAS IN THE
PROCESS OF CLOSING OFF AS IT APPROACHES THE UPPER MS VALLEY.  OVER
TIME THE CLOSED LOW AND A VORT MAX OUT OF AHEAD OF THE LOW WILL
INTERACT...RESULTING IN A BROAD/FLAT CIRCULATION CENTER OVER THE
GREAT LAKES BY EARLY DAY 2.  THE 00Z MODELS IN GENERAL TENDED TO
USHER THE COMPLEX LOW OUT TO SEA A BIT EARLIER THAN EARLIER RUNS. 

THE SPECTER FOR HEAVY SNOWFALL LINGERS ACROSS PARTS OF THE GREAT
LAKES...ESPECIALLY THE UPPER PENINSULA OF MI THANKS TO A
PERSISTENT EAST TO WEST ORIENTED SURFACE TROF...A MID LEVEL SHEAR
AXIS AND A FETCH OF COLD AIR MOVING OVER LAKE MI.  FOR THE MOST
PART...THERE WAS FAIRLY GOOD MODEL AGREEMENT WITH RESPECT TO THE
TRACK OF THE MID LEVELS AND SURFACE SYSTEMS...SO THE QPF PORTION
OF THE FORECAST WAS BASED ON THE WPC FORECASTS/

CONDITIONS BECOME MORE FAVORABLE ACROSS THE LOWER LAKES ON DAY 2
AND EARLY DAY 3 . THE LOW LEVEL FLOW BECOMES PARALLEL TO THE LONG
AXIS OF THE LAKES...BUT MODEL SOUNDINGS SHOWED CAPPING INVERSION
GENERALLY BELOW 850 MB. THE LOW LEVEL FLOW IS STRONGER EAST OF
LAKE ONTARIO...WHICH WAS WHERE A SLIGHT AND A SMALL MDT RISK OF 4
INCH AMOUNTS WAS DEPICTED.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


...NEW ENGLAND...

SURFACE LOW PRESSURE DEVELOPING ALONG THE SOUTHEAST NEW ENGLAND
COAST ON TUE IS EXPECTED TO DEEPEN AS IT CROSSES THE GULF OF ME ON
ITS WAY INTO THE MARITIMES BY EARLY THU.  THIS TRACK PRESENTS THE
POTENTIAL FOR HEAVY SNOW ACROSS PORTIONS OF NORTHERN MAINE...AS
WELL AS THE WESTERN MOUNTAINS. THE ECMWF AND GFS WERE IN BEST
AGREEMENT HERE IN TERMS OF THEIR MASS FIELDS AND QPF INTO DAY 2. 
ON DAY 3...THE TRACK OF THE LOW AND ASSOCIATED MID/UPPER LEVEL
FEATURES BECOMES MORE UNCERTAIN.   EVEN SO...ONE COMMON THEME FROM
THE MID/UPPER LEVEL FEATURE SEEMS TO MOVE EASTWARD MORE QUICKLY
THAN IN EARLY MODEL RUNS.  DESPITE THE SMALLER WINDOW OF
OPPORTUNITY...THE POTENTIAL STILL EXISTS FOR A FOOT OR MORE OF
SNOW IN PARTS OF ME.  GIVEN THE TRACK OF THE LOW...ISENTROPIC LIFT
AHEAD OF THE SURFACE LOW DRAWING MOISTURE UP AND OVER THE COLD AIR
IN PLACE NEAR THE MOUNTAINS...THERE IS ALSO A SLIGHT RISK FOR MORE
THAN A QUARTER OF AN INCH OF FREEZING RAIN ON WEDNESDAY.


BANN