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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0745Z Apr 29, 2018)
 
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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
345 AM EDT SUN APR 29 2018

VALID 12Z SUN APR 29 2018 - 12Z WED MAY 02 2018

ROCKY MOUNTAINS

THE MAIN WINTER WEATHER MAKER OVER THE NORTHWESTERN QUADRANT OF
THE NATION WILL BE AN AMPLIFYING UPPER LEVEL TROUGH WITH MULTIPLE
SHORTWAVE PERTURBATIONS WITHIN IT THROUGH TUESDAY NIGHT.  THE
FIRST FEATURE OF INTEREST IS A CLOSED UPPER LEVEL LOW OVER THE
PACIFIC NORTHWEST SUNDAY MORNING THAT IS FORECAST TO EVOLVE INTO A
LONG WAVE TROUGH OVER THE INTERMOUNTAIN WEST BY MONDAY MORNING. 
SYNOPTIC SCALE ASCENT AHEAD OF THIS FEATURE AND LOWERING HEIGHTS
ALOFT IS EXPECTED TO RESULT IN LIGHT TO MODERATE SNOW SHOWERS OVER
THE HIGHER MOUNTAINS OF CENTRAL IDAHO AND SOUTHWESTERN MONTANA ON
DAY 1 (SUNDAY INTO SUNDAY NIGHT), AND ALSO THE HIGHEST ELEVATIONS
OF THE CASCADES. 

A SHORTWAVE DISTURBANCE TRACKING AROUND THE AFOREMENTIONED UPPER
LOW IS EXPECTED TO RESULT IN A CLOSED 700MB LOW DEVELOPING OVER
NORTHERN UTAH AND INTO WYOMING BY THE BEGINNING OF THE DAY 2
PERIOD MONDAY MORNING, WITH THE 00Z GFS INDICATING THE GREATEST
REFLECTION OF THIS FEATURE.  THE COMBINATION OF STRONG 700MB
FRONTOGENETICAL FORCING, LEFT EXIT JET DYNAMICS, AND BOUNDARY
LAYER UPSLOPE FLOW IS EXPECTED TO RESULT IN SIGNIFICANT LATE
SEASON SNOW FROM THE NORTHERN WASATCH AND UINTA MOUNTAINS OF UTAH,
AND PARTICULARLY THE ABSAROKA, WIND RIVER, AND BIG HORN MOUNTAINS
OF WYOMING.  THE POTENTIAL EXISTS FOR ACCUMULATIONS IN EXCESS OF
12 INCHES, AND SOME ENSEMBLE MEMBERS ARE INDICATING THIS SCENARIO.
 THE THERMAL PROFILE IS BASED ON A BLEND OF THE 00Z GFS/NAM AND
12Z ECMWF WITH LOCAL ADJUSTMENTS FOR TERRAIN.


NORTHEAST U.S.

A PRONOUNCED NORTHERN STREAM TROUGH IS IN THE PROCESS OF EVOLVING
INTO A CLOSED UPPER LOW OVER UPSTATE NEW YORK AND SHOULD DEVELOP
FURTHER WHILE CROSSING NEW ENGLAND SUNDAY NIGHT, BEFORE MOVING
OFFSHORE BY MONDAY EVENING.  FALLING HEIGHTS AND LOWERING FREEZING
LEVELS WILL ALLOW PRECIPITATION TO CHANGE OVER TO SNOW FOR THE NEW
YORK ADIRONDACK MOUNTAINS, AND PROBABLY FOR SOME OF THE CATSKILL
MOUNTAINS AND NORTHERN VERMONT AND NEW HAMPSHIRE AS WELL.  A FEW
INCHES OF ACCUMULATION ARE LIKELY SUNDAY INTO EARLY MONDAY AS
POST-FRONTAL NORTHWEST WINDS PROVIDE UPSLOPE FLOW OVER WINDWARD
FACING TERRAIN, ESPECIALLY ABOVE 3000 FEET.  THERE IS SOME
ENSEMBLE SUPPORT FOR LOCAL 4+ INCH AMOUNTS.  DECENT AGREEMENT ON
THE TIMING OF THE MID-LEVEL LOW JUSTIFIED A BLEND OF THE
ECMWF/GFS/NAM FOR THE THERMAL PROFILES.

THE PROBABILITY OF ICING GREATER THAN 0.25 INCHES IS LESS THAN 10
PERCENT FOR DAY 1 THROUGH 3.

D. HAMRICK