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Winter Weather Forecast Discussion
 
(Latest Discussion - Issued 2019Z Dec 17, 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
319 PM EST WED DEC 17 2014

VALID 00Z THU DEC 18 2014 - 00Z SUN DEC 21 2014


...WESTERN U.S....

SNOWS ARE EXPECTED TO DEVELOP IN HIGHER ELEVATIONS OF THE PACIFIC
NORTHWEST AND NORTHERN ROCKIES ON DAY2 WITH A WELL DEFINED
UPPER TROUGH APPROACHING AND ASCENT GENERATED FROM AN UPPER LEVEL
JET STREAK IN ADVANCE OF THE LOW...WHICH FORCES UPPER
DIVERGENCE/LOWER CONVERGENCE MAXIMA PEAKING OVER WASHINGTON BEFORE
WEAKENING AS IT MOVES INLAND OVER THE NORTHERN ROCKIES...RESULTING
IN LOWER SNOW TOTALS THERE. 
LIKEWISE...SNOWS OCCURS OVER HIGHER ELEVATIONS OF THE OR/NORTHERN
CA RANGES AS A JET MAX ROUNDS THE BASE OF THE TROUGH AND UPPER
DIVERGENCE AIDS UPSLOPE FLOW IN PRODUCING ASCENT...LIKELY
RESULTING IN SEVERAL INCHES AT HIGHER ELEVATIONS.

ON DAY 3...THE MODELS/ENSEMBLES SHOW A WELL DEFINED UPPER JET
MAXIMA MOVING WEST TO EAST FROM THE NORTHEAST PACIFIC AND
EVENTUALLY INTO OR SAT.
HIGHER AMOUNTS ARE EXPECTED IN THE WA CASCADES AS WELL DEFINED
UPPER DIVERGENCE MAXIMA OCCUR INT HE LEFT EXIT REGION OF THE UPPER
JET.
850-700 MB MOISTURE FLUXES INCREASE TO 3-4 STANDARD DEVIATIONS
ABOVE NORMAL IN WESTERN OR...SHOWING HEAVY PRECIPITATION
POTENTIAL. THE UNCERTAINTY RELATES TO QPF AMOUNTS WHILE IN THE
FORM OF SNOW.  THE MODELS SHOW RAPID WARMING AS STRONG LOW LEVEL
SOUTHWEST FLOW PRODUCES WARMING TEMPERATURES...WITH RAPIDLY RISING
SNOW LEVELS AS A RESULT.  MANUAL PROGS USED A BLEND OF THE 09Z
SREF MEAN/00Z ECMWF/12Z GFS QPF AND TEMPERATURE PROFILES.

...CENTRAL PLAINS DAY 1...

A 700 MB WAVE WITH POTENTIAL EMBEDDED CLOSED CIRCULATION
PROGRESSES ACROSS NEBRASKA AND ADJACENT KS.  700 MB CONVERGENCE
OCCURS NEAR THE WARM FRONT ACROSS NORTHEAST KS AND NORTHERN MO. 
THIS LEADS TO A PERIOD OF SNOW LATE TONIGHT AND EARLY THU...WITH
THE EVENT WINDING DOWN WITH THE WAVE DEPARTURE THU AFTERNOON...WIT
DRYING ALOFT CUTTING DOWN ON SNOW GROWTH IN THE CLOUD.  
UNCERTAINTY EXISTS FURTHER SOUTH WITH A PRECIP TYPE TRANSITION
ZONE SETS UP ACROSS SOUTHERN KS AND SOUTHERN MO.
THE MODELS SHOW A QUARTER TO FOUR TENTHS OF AN INCH LIQUID
EQUIVALENT PRECIP.  WITH FORECAST SNOW RATIOS IN THE MID TEENS
OVER NORTHEAST KS...A SLIGHT TO MODERATE RISK OF 4 INCHES HAS BEEN
INDICATED. MANUAL PROGS USED A BLEND OF THE 09Z SREF MEAN/00Z
ECMWF/12Z GFS QPF AND TEMPERATURE PROFILES.

...NORTHEAST DAY 1...

A CLOSED MID LEVEL LOW MOVES FROM MAINE TO THE ADJACENT CANADIAN
MARITIMES.  LOW LEVEL FLOW TURNS TOWARDS THE NORTH
TONIGHT...HELPING HOLD IN COLD AIR OVER NORTHERN MAINE AND INDUCE
COOLING FURTHER SOUTH WHERE TEMPS THIS AFTERNOON ARE TOO WARM FOR
SNOW.  DEEP LAYER ASCENT OCCURS TONIGHT IN THE MID LEVEL
DEFORMATION/FRONTOGENESIS ZONE.  THE SLOW MOVEMENT OF THE LOW
LEADS TO A PROLONGED SNOW EVENT OVERNIGHT INTO EARLY THU.
THE HIGHEST RISK FOR HEAVY SNOW IS WHERE THE HIGHEST MULTI-MODEL
ENSEMBLE QPF IS WHERE TEMPS REMAIN COLD ENOUGH FOR ALL SNOW IN
NORTHEAST MAINE.
THERE WILL ALSO BE SOME POTENTIAL FOR BRIEF ICING IN THE PRECIP
TYPE TRANSITION ZONE FURTHER SOUTH IN MAINE BUT DUE TO SHORT
DURATION SHOULD NOT ACCUMULATE MUCH. LIGHT SNOW IS EXPECTED
FURTHER WEST ACROSS PARTS OF NORTHERN NH/VT/NY IN REGIONS WHERE
ASCENT IS AIDED BY OROGRAPHY IN UPSLOPE AREAS. MANUAL PROGS USED A
BLEND OF THE 09Z SREF MEAN/00Z ECMWF/12Z GFS QPF AND TEMPERATURE
PROFILES.

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

PETERSEN