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Winter Weather Forecast Discussion
 
(Latest Discussion - Issued 0724Z Oct 30, 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
324 AM EDT THU OCT 30 2014

VALID 12Z THU OCT 30 2014 - 12Z SUN NOV 02 2014

CALIFORNIA/INTERMOUNTAIN WEST/NORTHERN ROCKIES...

SNOW IS EXPECTED TO SPREAD INTO THE SIERRAS FRIDAY NIGHT WITH
ELEVATIONS ABOVE 5000 FT PRIMARILY IMPACTED...BEFORE THE
ASSOCIATED SHORTWAVE TROUGH MOVES INTO THE GREAT BASIN TOWARD THE
NORTHERN ROCKIES.  THE TROUGH IS QUITE DEEP...WITH LOW PRESSURE
FORMING IN THE LEE OF THE SIERRAS THEN REDEVELOPING NORTHEASTWARD
WITH THE MOTION OF THE TROUGH.  PREFERRED THE INTERMEDIATE AND
MOST STEADY GFS FOR BEGINNING THE PROBABILITY PROCESS WITH THE
ECMWF COMPRISING THE REMAINING 40 PERCENT OF THE BLEND IN CREATING
THE DETERMINISTIC AMOUNTS...WHILE THE NAM RECEIVED LITTLE OR NO
WEIGHTING DUE TO ITS WEST AND SOUTHWARD POSITIONS WITHIN THE
GUIDANCE ENVELOPE.  BY SATURDAY AND SATURDAY NIGHT...EXPECT THE
TROUGH TO REACH THE NORTHERN ROCKIES...WITH PROBABILITIES OF 4
INFLUENCING THE HIGHER ELEVATIONS OF CENTRAL AND NORTHERN NEVADA
BEFORE THE TROUGH REACHES THE NORTHERN ROCKIES.  GIVEN FALLING
SNOW LEVELS TO 3000 TO 4000 THOUSAND FEET AND STRONG AND
PERSISTENT UPSLOPE FLOW OCCURRING IN MULTIPLE DIRECTIONS OF THE
NORTHERN ROCKIES...PORTIONS OF THE SAWTOOTH RANGES...AS WELL AS
THE BITTERROOTS/TETONS/ABSAROKA RANGES...COULD RECEIVE UPWARDS OF
6 TO 12 INCHES BY SUNDAY MORNING.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

UPPER GREAT LAKES...

LAKE ENHANCED AND LAKE EFFECT SNOW WILL OVERSPREAD THE UPPER
PENINSULA OF MICHIGAN AND PORTIONS OF NORTHERN MICHIGAN TONIGHT
INTO FRIDAY BEFORE WANING.  MODELS ARE WELL CLUSTERED IN DEPICTING
STEEP LOWER LEVEL LAPSE RATES...LONG FETCH...AND SUFFICIENTLY HIGH
THERMAL INVERSIONS TO PROMOTE MODERATE TO OCCASIONALLY HEAVY
SNOW...PARTICULARLY FOR THE UPPER PENINSULA...WHERE 4 TO 8 INCHES
OF SNOW WILL BE POSSIBLE...ESPECIALLY INLAND AND AWAY FROM THE
WARM INFLUENCE OF THE LAKES.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

CENTRAL/SOUTHERN APPALACHIANS...

DESPITE SOME REMAINING SPREAD TO RESOLVE THAT WILL HELP TO
DETERMINE THE FINAL LOCATIONS AND TIMING FOR SNOW...MODELS REMAIN
VERY CONSISTENT IN PROMOTING A SYNOPTIC/MESOSCALE PATTERN FOR
WINDWARD SNOWS FOR PORTIONS OF THE CENTRAL/SOUTHERN APPALACHIANS
INTO THE SMOKIES BEGINNING FRIDAY NIGHT AND CONTINUING INTO
SATURDAY NIGHT.  THE GFS HAS REMAINED THE MOST CONSISTENT FOR A
COUPLE DAYS NOW...WITH THE NAM GENERALLY HAVING BEEN TOO SLOW WITH
THE APPROACHING TROUGH...AND ECMWF OCCASIONALLY TOO QUICK. 
THUS...THE MODEL BLEND WAS PROPORTIONED ACCORDINGLY WITH A
PREDOMINANT GFS BLEND AND SMALLER PERCENTAGES OF THE ECMWF USED. 
THE SREF MEAN HAS ALSO BEEN CONSISTENTLY NEAR THE CONSENSUS AND IT
TOO WAS USED IN DETERMINING THE FINAL SNOW TOTALS.  EXPECT
ELEVATIONS ABOVE 2500 FT IN WEST VIRGINA/SOUTHWEST
VIRGINIA/EASTERN TENNESSEE/WESTERN NORTH CAROLINA TO RECEIVE
VARIOUS DURATIONS AND INTENSITY OF OROGRAPHIC ASCENT WHICH WILL
CONTRIBUTE TO ADDITIONAL SNOW TOTALS.  THE EXACT TRAJECTORY OF THE
MID/UPPER LEVEL CENTER WILL BE QUITE IMPORTANT IN PROMOTING THE
MOST FAVORABLE DENDRITIC GROWTH CONDITIONS FOR HIGHER
TOTALS...WITH THE CURRENT SPREAD STILL MODEST ENOUGH TO CONSIDER A
RANGE OF POSSIBLE OUTCOMES...WITH THE GFS TRACK SLIGHTLY MORE
FAVORED OVER OTHER GUIDANCE AT THIS TIME...ALTHOUGH THE NEW ECMWF
IS QUITE CLOSE AND CONSIDERED NEARLY EQUALLY PROBABLE. 

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

MAINE...

LOW PROBABILITIES FOR 4 INCHES OF SNOW ARE DEPICTED FOR PORTIONS
OF THE MAINE FOR SATURDAY NIGHT AS THE OFFSHORE CYCLONE CONTINUES
TO DEEPEN AND PIVOT NORTHWARD.  USED A COMBINATION OF THE
GFS/ECMWF TO DERIVE THE SNOW AMOUNTS WHICH THEN FORM THE BASIS FOR
THE SNOW PROBABILITIES...WHILE THE NAM CONSIDERED TOO FAR FROM THE
CONSENSUS TO BE USED AT ALL.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

JAMES