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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1006Z Dec 15, 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
506 AM EST MON DEC 15 2014

VALID 12Z MON DEC 15 2014 - 12Z THU DEC 18 2014


DAYS 1-3...

...NORTHERN-CENTRAL PLAINS/UPPER MIDWEST...

A MID/UPPER LEVEL LOW CLOSING OFF WITHIN A WELL DEFINED NEGATIVELY
TILTED UPPER TROF WHICH LIFTS NORTHEAST ACROSS THE NORTHERN AND
CENTRAL PLAINS WILL BRING A STRIPE OF SNOW AND/OR GENERALLY LIGHT
FREEZING RAIN TO PORTIONS OF THE AREA.  EARLY MORNING RADAR
IMAGERY SHOWS A WELL DEFINED CIRCULATION CENTER WITH MOISTURE
WRAPPING BACK INTO THE COLD AIR ACROSS PARTS OF NE/SD. 

MODELS STILL DIFFER ON COLD SECTOR PRECIPITATION TYPES AND AMOUNTS
OVER PARTS OF NEBR/SD. 
FREEZING AS WELL AS FROZEN PRECIPITATION TYPES ARE EXPECTED INVOF
THE DEFORMATION ZONE PCPN THIS MORNING BEFORE ENOUGH COLD AIR
COMES IN FROM THE NORTHWEST AND CHANGES ALL PRECIPITATION OVER TO
SNOW LATE THIS MORNING OR EARLY AFTERNOON.

THE NAM...NAM CONEST...GFS AND GFS PARALLEL ALL SHOW ENOUGH
PRECIPITATION FALLING AS FREEZING RAIN TO WARRANT A SLIGHT RISK OF
0.25 INCHES OF FREEZING PCPN TODAY IN PARTS OF SOUTHEAST SD OR
SOUTHWEST MN.  EVEN AWAY FROM THE DEFORMATION ZONE...FREEZING
DRIZZLE OR VERY LIGHT FREEZING RAIN WILL BE POSSIBLE ACROSS THE
EASTERN DAKOTAS INTO THE MORNING GIVEN SATURATED CONDITIONS WITHIN
THE FAVORED ICE REGION ALTHOUGH VERTICAL MOTION IN THIS LAYER WILL
BE QUITE WEAK. 

IN TERMS OF SNOWFALL AMOUNTS...THERE WAS PRETTY GOOD AGREEMENT
AMONG THE 00Z MODELS FOR AN AREA OF 4 OR MORE INCHES NEAR THE
SD/NEB BORDER TODAY.  NOTED THAT THE NAM RIME FACTOR...WHICH
ATTEMPTS TO ACCOUNT FOR THE DEGREE OF RIMING ON
HYDROMETEORS...YIELDED MORE THAN 8 INCHES OF SNOW IN NORTHEAST NE.
 THIS WAS SOUTH OF WHERE THE GFS OR NAM PLACED THEIR MAX WHEN
APPLYING THE ROEBBER TECHNIQUE TO THOSE MODELS.  IN ADDITION...THE
NAM RIME SNOWFALL MAX WAS HEAVIER THAN THE AMOUNTS DERIVED BY
USING THE ROEBBER TECHNIQUE. ON THE OTHER HAND...HI RES GUIDANCE
USING ARW CORE ALSO FOCUSED THE HEAVIEST SNOWFALL AMOUNTS JUST
SOUTH OF THE NE/SD BORDER.

CONSEQUENTLY...FELT CONFIDENT ENOUGH TO PLACE A HI RISK OF 4 INCH
SNOWFALL AMOUNT NEAR THE STATE LINE NEAR THE DEFORMATION ZONE BUT
WENT WITH A SLIGHTLY LARGER THAN USUAL SLIGHT RISK FOR 8 INCHES TO
ACCOUNT FOR UNCERTAINY IN PLACEMENT OF THE MAX.

AS THE SYSTEM CONTINUES TO MOVE NORTHEAST TOWARDS THE MN
ARROWHEAD/NW WISC AND THE WESTERN UPPER PENINSULA...THE RISK OF
LIGHT FREEZING PCPN WILL CONTINUE IN THE WARM SECTOR FOLLOWED BY
AN EVENTUAL CHANGE OVER TO SNOW FOLLOWING A WIND SHIFT AND
SUBSEQUENT ONSET OF COLD ADVECTION.  BUT THE SYSTEM WILL BE MOVING
INTO AN ENVIRONMENT LESS FAVORABLE FOR HEAVY SNOW AND/OR
SIGNIFICANT ICING. A FAIRLY NARROW SLIGHT RISK OF 4 INCHES WAS
INDICATED HERE.  

...SOUTHERN OREGON/CALIFORNIA/SOUTHWEST...

THE HEAVY SNOW THREAT IS EXPECTED TO RETURN TO THE
REGION...ESPECIALLY TO THE SOUTHERN CASCADES SOUTHWARD ALONG MUCH
OF THE SIERRA NEVADA RANGE...AS A COMPLEX MID-UPPER TROUGH
GENERATES A PAIR OF SHORTWAVES THAT PROGRESS ACROSS OR AND CA. 
THE FIRST SHORTWAVE IS EXPECTED TO SWING ACROSS THE REGION FROM
LATER TODAY INTO TUESDAY AS A COUPLED 300 MB JET IN CA LEADS TO
DIVERGENCE MAXIMA TO AID UPSLOPE FLOW IN PRODUCING SNOW. 

THE SECOND SYSTEM IS EXPECTED TO REACH THE AREA TUE
NIGHT...PRODUCING ADDITIONAL ROUNDS OF SNOW AS UPPER DIVERGENCE
MAXIMA AND MOIST SOUTHWEST FLOW PERSIST.  THERE IS EVEN POTENTIAL
FOR ANOTHER SHORTWAVE TO DEVELOP WITHIN THE LONGWAVE TO PRODUCE
ONE LAST ROUND OF SNOWS IN THE SIERRA WED. ONCE THE TROUGH MOVES
ONSHORE...THE UPPER DIVERGENCE MAXIMA SHIFTS INLAND INTO SOUTHERN
NV AND THEN NORTHERN AZ AND SOUTHERN UT...SO SNOW SHOWERS SHOULD
DEVELOP AT HIGHER ELEVATIONS TUE NIGHT THROUGH WED.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT DAY
2-3.

BANN