PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
240 AM EDT SAT APR 28 2018

VALID 12Z SAT APR 28 2018 - 12Z TUE MAY 1 2018

NORTHERN ROCKIES

THE MAIN WINTER WEATHER MAKER OVER THE NORTHWESTERN QUADRANT OF
THE NATION WILL BE A CLOSED UPPER LEVEL LOW OVER THE PACIFIC
NORTHWEST SATURDAY THAT WILL BE PROGRESSING SLOWLY EASTWARD.  THIS
IS FORECAST TO EVOLVE INTO A LONG WAVE TROUGH OVER THE
INTERMOUNTAIN WEST BY SUNDAY NIGHT AND INTO MONDAY.  DEEP LAYER
ASCENT AHEAD OF THE LOW, LEFT EXIT JET DYNAMICS, AND LOWERING
HEIGHTS ALOFT WITH A SHORTWAVE PASSING THROUGH WILL RESULT IN
LOCALLY HEAVY SNOW OVER THE HIGHER MOUNTAINS OF IDAHO, WESTERN
MONTANA, AND WESTERN AND CENTRAL WYOMING.  THE HEAVIEST
ACCUMULATIONS ARE EXPECTED FROM SUNDAY NIGHT THROUGH MONDAY, WITH
MANY ENSEMBLE MEMBERS INDICATING THE POTENTIAL FOR AT LEAST 4
INCHES OF SNOW, AND SOME ARE SHOWING OVER 8 INCHES, PARTICULARLY
FOR THE ABSAROKA, WIND RIVER, AND BIG HORN MOUNTAINS.  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 OVER THE NORTHERN GREAT LAKES
ON SATURDAY IS FORECAST TO EVOLVE INTO A CLOSED UPPER LOW OVER NEW
ENGLAND BY SUNDAY, AND THEN MOVING OFFSHORE BY MONDAY NIGHT. AS
THE LOW MOVES EAST, 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 MONDAY AS
POST-FRONTAL NORTHWEST WINDS PROVIDE UPSLOPE FLOW OVER WINDWARD
FACING TERRAIN, ESPECIALLY ABOVE 3000 FEET.  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