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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0741Z Apr 13, 2014)
 
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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
341 AM EDT SUN APR 13 2014

VALID 12Z SUN APR 13 2014 - 12Z WED APR 16 2014

..ROCKY MOUNTAINS/HIGH PLAINS...

CONTINUED UPSLOPE FLOW PARTICULARLY FOR THE EASTERN FACING SLOPES
AND PASSAGE OF THE SHORTWAVE TROUGH PRESENTLY OVER IDAHO SUPPORT
LOW AND MODERATE PROBABILITIES OF 8 INCHES OR MORE OF SNOW FOR
PORTIONS OF THE TETONS AND MUCH OF THE CENTRAL ROCKIES TODAY AND
TONIGHT...WITH LOW PROBABILITIES OF 4 INCHES EXTENDING INTO THE
HIGH PLAINS.  PREFERRED THE SLIGHTLY WARMER THERMAL PROFILES OF
THE GFS/ECMWF AWAY FROM THE HIGHER TERRAIN VERSUS THE COLDER
NAM/SREF MEAN PROFILES THAT HAVE LESS MODEL SUPPORT AND SEEM LESS
LIKELY GIVEN THE LACK OF A SNOWPACK TO PREVENT THE AIR MOVING
SOUTHWARD FROM CANADA FROM MODIFYING ABOVE FREEZING BEFORE
REACHING THE AREA.  FOR LIQUID EQUIVALENT AMOUNTS...THE MODELS
SHOWED GOOD AGREEMENT WITH THE UPSLOPE FLOW-FORCED PORTIONS BUT
DIFFERED OVER THE HIGH PLAINS...WITH A PREFERENCE TOWARD THE DRIER
SIDE OF THE GUIDANCE DUE TO THE DEPTH OF DRIER SUBSIDING AIR
BEHIND THE FRONTAL PASSAGE.

ANOTHER ROUND OF SNOW IS EXPECTED TO REACH PORTIONS OF THE
NORTHERN ROCKIES BY TUESDAY NIGHT...
WITH THE SLIGHTLY FASTER GFS/ECMWF PREFERRED OVER THE SLOWER AND
DRIER NAM...SUPPORTING MODERATE PROBABILITIES OF 4 INCHES OR MORE
ACROSS NORTHERN MONTANA.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

...NORTHERN NEW ENGLAND...

CURRENT GUIDANCE IS HAVING A DIFFICULT TIME RESOLVING THE
MESOSCALE NATURE OF UPSTREAM CONVECTION AND ASSOCIATED MID/HIGH
CLOUD COVER APPROACHING THE AREA THAT HAVE PREVENTED BL
TEMPERATURES FROM FALLING AS MUCH AS PROJECTED...WITH 03-06Z 2M
TEMPERATURES OVER MAINE RUNNING WELL ABOVE 00Z GUIDANCE...THUS
LOWERING CONFIDENCE THAT WHAT WINTRY PRECIPITATION THAT FORMS
ACROSS THE REGION TODAY AND TONIGHT WILL AMOUNT TO VERY MUCH. 
HAVE USED THESE TEMPERATURE TRENDS ALONG WITH UNCERTAINTIES IN THE
UPSTREAM CONVECTION TO JUSTIFY FORECASTING SNOW AND ICE AMOUNTS AT
OR BELOW GUIDANCE...WITH THE PROBABILITIES OF 4 INCHES OF SNOW AND
0.25 INCHES OF FREEZING RAIN GENERALLY BELOW 5 PERCENT WHICH IS
BELOW THE LOWER THRESHOLD FOR DEPICTION. 

BETTER CHANCES FOR ACCUMULATING SNOW MAY BE TUESDAY NIGHT AS THE
BASE OF THE LONGWAVE TROUGH PROGRESSES OVER THE REGION...WITH
UPSLOPE FLOW ENHANCING ASCENT ACROSS THE GREEN AND WHITE MOUNTAINS
AS WELL AS THE ADIRONDACKS AND CATSKILLS SUPPORTING LOW
PROBABILITIES OF 4 INCHES OF SNOW.  PREFERRED INTERMEDIATE VALUES
FOR P-TYPE AND LIQUID EQUIVALENTS FROM VARIOUS GUIDANCE INCLUDING
THE ECMWF/UKMET/ECMWF ENSEMBLE MEAN/GEFS MEAN...WITH LITTLE OR NO
NAM AND GFS USED.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

JAMES