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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2109Z Nov 18, 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
409 PM EST TUE NOV 18 2014

VALID 00Z WED NOV 19 2014 - 00Z SAT NOV 22 2014

DAYS 1 THROUGH 3...

...GREAT LAKES...
HEAVY LAKE EFFECT SNOWS THIS EVENING OFF LAKES ERIE AND ONTARIO
SHOULD QUICKLY WIND DOWN AS LOW PRESSURE DEVELOPS IN THE UPPER
LAKES AND THE BNDRY LAYER WINDS BACK TO A SOUTHERLY DIRECTION ON
WED...WITH PEAK BNDRY LAYER MOISTURE CONVERGENCE ON THE CANADIAN
SIDE. WHILE LAKE EFFECT SNOW WILL BE WIDESPREAD...THE STEADILY
CHANGING WIND DIRECTION AND TRAJECTORIES MAKE IT DIFFICULT TO NAIL
DOWN WHO GETS A LONGER DURATION HEAVY SNOW BAND.  THE SAME APPLIES
TO THE UPPER LAKES...AND THE BACKING FLOW LATE TONIGHT-EARLY WED
DECREASES SNOW SHOWER COVERAGE OFF LK MICHIGAN INTO SOUTHWEST
LOWER MI.  A BAND OF LOW LEVEL CONVERGENCE DEVELOPS NEAR THE FRONT
AND SPREAD ONSHORE FROM LK MICHIGAN IN BOTH NORTHWEST AND
SOUTHWEST LOWER MI BY 00Z THU.  POST FRONTAL LEE SHORE CONVERGENCE
IN THE UP OF MI LEADS TO DEVELOPMENT OF SNOW SQUALLS IN THE
WESTERN U OF MI.

ON WED NIGHT...THE COLD FRONT PROGRESSES ACROSS THE LOWER LAKES. 
POST FRONT VEERING RESULTS IN WINDS MOVING ALONG THE MAJOR AXIS OF
LK ERIE...SO HEAVY SNOW BANDS ARE EXPECTED TO REDEVELOP INTO
SOUTHWEST NY LATE WED NIGHT-THU MORNING. GRADUAL VEERING SHOULD
RESULT IN THE BAND BECOMING LESS SOUTHWEST TO NORTHEAST AND MOVE
TOWARDS A WEST SOUTHWEST TO EAST NORTHEAST ORIENTATION...WITH THE
HEAVIEST RATES SHIFTING SOUTH IN THE LEE SHORE AREAS OF SOUTHWEST
NY. ONCE THE FRONT PROGRESSES ACROSS LK ONTARIO THE LK EFFECT
BANDS WILL MOVE FROM THE CANADIAN SIDE TO THE US SIDE NEAR THE
NORTHEAST CORNER OF THE LAKE.
UPSTREAM...LONG DURATION WEST-NORTHWEST FLOW SETS UP SUSTAINED LEE
SHORE CONVERGENCE IN THE WESTERN AND EASTERN UP OF MI AND IN
NORTHWEST LOWER MI. 

ON THU NIGHT...RIDGING IN THE UPPER MIDWEST BUILDS INTO THE UPPER
LAKES...WITH DRYING RESULTING IN A DECREASE IN SNOW COVERAGE AND
INTENSITY FIRST OFF LK SUPERIOR....THEN LK MICHIGAN AND POSSIBLY
BY THE END OF THE DAY LK ERIE. OTHERWISE...VEERING WINDS DIRECTION
DOWNSTREAM FROM LAKES ERIE/ONTARIO RESULT IN SHIFT BAND
AXIS/ORIENTATION.
MANUAL PROGS USED A MODEL/ENSEMBLE CONSENSUS FOR MASS FIELDS AND
TEMPERATURES BUT RELIED MORE ON THE HIGHER RESOLUTION FORECASTS
FOR QPF.


...PACIFIC NORTHWEST/NORTHERN ROCKIES.

LIGHT SNOW OCCURS ON DAY 2 AS A WEAK PROGRESSIVE 850-700 MB WAVE
STEADILY MOVES ACROSS THE PACIFIC NORTHWEST AND DEAMPLIFIES MOVING
INLAND ACROSS THE NORTHERN ROCKIES.  ON DAY 3...A 700 MB WAVE WITH
A STRONGER PRE-WAVE 700 MB CONVERGENCE AND WARM/MOIST ADVECTION
APPROACHES THE COAST.
SNOW DEVELOPS WITH THE APPROACHING WAVE...WITH ASCENT AIDED BY
UPPER LEVEL DIVERGENCE MAXIMA INT HE LEFT EXIT REGION OF THE 300
MB JET
THE PRIMARY UNCERTAINTY THE TIMING OF THE WAVE (I.E. SIGNIFICANT
SNOWS COULD WAIT FOR FRI EVENING IF THE WAVE SLOWS DOWN). WPC
GENERALLY FOLLOWED A BLEND OF THE GLOBAL MODELS AND 12Z NAM/09Z
SREF MEAN ON MASS FIELDS AND SNOW FORECASTS. 

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT DAYS
1-3.

PETERSEN