PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
440 PM EDT SUN APR 29 2018

VALID 00Z MON APR 30 2018 - 00Z THU MAY 03 2018

...ROCKY MOUNTAINS AND RANGES OF THE GREAT BASIN...

ON SUNDAY NIGHT-MONDAY...AN AMPLIFYING UPPER LEVEL TROUGH CROSSING
ID AND NV INTO UT
PROVIDES  UPPER LEVEL DIVERGENCE MAXIMA THAT INDUCES SYNOPTIC
SCALE ASCENT AHEAD OF THE THE UPPER TROUGH AXIS THAT PRODUCES SNOW
IN THE RANGES OVER SOUTHWEST MT/SOUTHERN ID...WHERE SEVERAL INCHES
MAY OCCUR. 

A SHORTWAVE DISTURBANCE TRACKING AROUND THE AFOREMENTIONED UPPER
LOW IS EXPECTED TO RESULT IN A CLOSED 700 MB LOW DEVELOPING OVER
NORTHERN UTAH AND INTO WYOMING BY MONDAY MORNING.  THE COMBINATION
OF STRONG 700MB FRONTOGENETICAL FORCING, LEFT EXIT JET DYNAMICS,
AND BOUNDARY LAYER UPSLOPE FLOW IS EXPECTED TO RESULT IN AREAS OF
MODERATE TO HEAVY LATE SEASON SNOW FROM THE NORTHERN WASATCH AND
UINTA MOUNTAINS OF UTAH INTO THE ABSAROKA, WIND RIVER, AND BIG
HORN MOUNTAINS OF WYOMING.  THE POTENTIAL EXISTS FOR ACCUMULATIONS
OF 12-18 INCHES MONDAY THROUGH TUESDAY.  THE SNOW TAPERS AS THE
700 MB LOW OR WAVE EJECTS NORTHEAST ACROSS THE NORTHERN PLAINS TUE.

LATER TUESDAY A CLOSED LOW IS FORECAST TO DEVELOP AT THE BASE OF
THE TROUGH OVER THE SOUTHWEST US.  DEVELOPMENT OF COUPLED UPPER
DIVERGENCE/LOWER CONVERGENCE MAXIMA IN THE VICINITY OF THE LOW-MID
LEVEL FRONT AS IT CROSSES THE CA SIERRA NEVADA MOUNTAINS TUE
CONTINUE DOWNSTREAM IN TANDEM WITH THE FRONT ACROSS NV AND THEN
THE UT WASATCH MOUNTAINS.
THE 12Z NAM SHOW POTENTIAL FOR AN INCH LIQUID EQUIVALENT IN THE
FORM OF SNOW IN THE UT UINTAS AND ISOLATED PEAKS OF THE
WASATCH...INDICATING POTENTIAL FOR ANOTHER HEAVY SNOW EVENT TO
DEVELOP. BOTH THE GFS AND ECMWF SHOW 300 MB DIVERGENCE DEVELOPING
IN NORTHERN CO WED AFTERNOON IN NORTHERN CO IN THE LEFT EXIT
REGION OF THE UPPER JET IN SOUTHEAST CO...SO SNOW SHOULD BE
INCREASING IN CONVERGE/INTENSITY THEN.

THE QPF AND THERMAL PROFILES WERE BASED ON A BLEND OF THE 12Z
GFS/NAM/ECMWF/UKMET.

...NEW YORK/VERMONT...

A CLOSED UPPER LOW OVER NORTHERN NEW YORK AND SHOULD CROSSING NEW
ENGLAND TONIGHT, BEFORE MOVING OFFSHORE BY MONDAY EVENING.  SNOW
HAS BEEN FALLING IN THE NEW YORK ADIRONDACK MOUNTAINS TODAY INTO
SARANAC LAKE NY AND SNOW SHOWERS SHOULD CONTINUE WITHIN THE MID
LEVEL DEFORMATION ZONE TONIGHT INTO MONDAY AND 700 MB VERTICAL
VELOCITY MAXIMA CONTINUING...WITH LOCALLY SEVERAL INCHES POSSIBLE.
 A FEW INCHES OF ACCUMULATION ARE LIKELY TONIGHT INTO EARLY MONDAY
IN THE SOUTHERN GREEN MOUNTAINS OF VT AND ALSO THE CATSKILL
MOUNTAINS OF NY AS PERSISTENT DEEP MOISTURE TO AROUND 500 MB
REMAINS IN PLACE WITH NORTHWEST LOW LEVEL WINDS PROVIDING
LOCALIZED UPSLOPE FLOW IN WINDWARD TERRAIN.

COVERAGE IS FORECAST TO DROP QUICKLY MON AS DRIER AIR ALOFT
ADVECTS AROUND THE LOW AND THE DEPARTURE OF THE LOW REMOVES THE
LIFT. 

DECENT AGREEMENT ON THE TIMING OF THE MID-LEVEL LOW LED TO A BLEND
OF THE ECMWF/GFS/NAM FOR BOTH QPF AND THE THERMAL PROFILES.

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

PETERSEN