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Winter Weather Forecast Discussion
 
(Latest Discussion - Issued 0855Z Jan 17, 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
355 AM EST WED JAN 17 2018

VALID 12Z WED JAN 17 2018 - 12Z SAT JAN 20 2018

DAYS 1 TO 3...

...SOUTHEAST...MID-ATLANTIC...NORTHEAST...

A POSITIVELY TILTED TROUGH STRETCHING FROM THE EASTERN GREAT LAKES
TO THE MID-SOUTH STATES WILL SHIFT TO THE EASTERN SEABOARD THROUGH
TONIGHT. AS IT DOES...A SPLIT INTO DISTINCT NORTHERN AND SOUTHERN
STREAM SHORTWAVE COMPONENTS WITH SURFACE LOWS WILL DEVELOP.

NORTHERN PORTION: SURFACE LOW PRESSURE WILL DEVELOP OFF THE
SOUTHERN NEW ENGLAND COAST THIS MORNING AND REACH NOVA SCOTIA THIS
EVENING. SNOW WILL BE TAPERING OFF IN SOUTHERN NY/NYC AROUND THE
START OF DAY 1 (12Z TODAY). A RAIN/SNOW LINE WILL DEVELOP OVER
LOWER SOUTHEASTERN NEW ENGLAND WITH ALL SNOW IN INTERIOR NEW
ENGLAND...ENDING AS ALL SNOW ON THE WRAP AROUND. FOUR INCH OR MORE
SNOW IS EXPECTED TO BE LIMITED TO DOWN EAST MAINE GIVEN SLOW LOW
DEVELOPMENT AND THE PROGRESSIVE NATURE OF THE SYSTEM.

SOUTHERN PORTION: THE UPPER TROUGH WILL CLOSE INTO A LOW AT 500MB
OVER KY/TN THIS MORNING. THIS WILL FURTHER CUT OFF THE AREA
BETWEEN THE TWO PORTIONS/MOST OF THE MID-ATLANTIC AND FOCUS SNOW
OVER VA/NC/SC. A WINTRY MIX IS ONGOING OVER GA/AL/NORTHERN FL WITH
GENERALLY LIGHT ACCUMULATIONS AFTER 12Z. MUCH MORE DYNAMICALLY
ENHANCED SNOWFALL IS EXPECTED FROM SOUTHERN VA ACROSS NC WITH THE
APPROACH OF THE CLOSED UPPER AND THE STRENGTHENING OF A 200/300 MB
LAYER SWLY JET MAX. THIS ENERGY WILL FOSTER DEVELOPMENT OF A
SECONDARY AREA OF LOW PRESSURE ALONG THE EVOLVING ARCTIC COLD
FRONT AND HELP FOCUS AN AREA OF GREATER MOISTURE TRANSPORT AND
FORCING FOR ACCUMULATING SNOW. RAIN CURRENTLY OVER AREAS OF THE
CENTRAL CAROLINAS WILL QUICKLY TRANSITION TO A LOCALLY HEAVY WET
SNOW AS COLDER AIR FILTERS IN. UP TO AROUND SIX INCHES OF SNOW ARE
EXPECTED WITH THE HEAVIEST TOTALS FROM CENTRAL NC UP INTO
SOUTH-CENTRAL VA.


...WESTERN U.S...

SNOW COVERAGE WILL INCREASE AND SNOW LEVELS WILL DECREASE OVER THE
HIGHER TERRAIN OF THE WESTERN U.S. AND ESPECIALLY THE PACIFIC
NORTHWEST WITH THE ARRIVAL OF A COLD FRONT FROM A LOW OFF BC TODAY
THEN INTO THE WEEKEND. STRONG HEIGHT FALLS ALOFT AND MULTIPLE
PACIFIC SHORTWAVES CROSSING THE WEST COAST WHICH WILL GIVE WAY TO
A LARGER SCALE TROUGH MOVING INLAND BY FRIDAY. THE WETTEST
CONDITIONS SHOULD BEGIN TO ARRIVE IN EARNEST BY TONIGHT AND
THURSDAY FOR THE PACIFIC NORTHWEST WITH LOWER SNOW LEVELS AND
STRONGER FORCING AIDED BY UPSLOPE FLOW AND DEEP LAYER ASCENT FOR
HEAVY SNOWFALL ACROSS THE CASCADES. BY FRIDAY...ROBUST HEIGHT
FALLS SPREADING INLAND WILL FOSTER HEAVY ACCUMULATIONS OF SNOW FOR
THE NORTHERN ROCKIES...HIGHER TERRAIN OF THE PACIFIC NORTHWEST AND
AS FAR SOUTH AS THE NORTHERN SIERRA-NEVADA. MULTI-DAY SNOWFALL
ACCUMULATIONS OF LOCALLY OVER 2 FEET IN THE WA CASCADES ARE
EXPECTED ALONG WITH PORTIONS OF THE SIERRA-NEVADA. GENERALLY AS
MUCH AS 12 TO 18 INCHES ARE EXPECTED OVER THE NORTHERN ROCKIES AND
MAINLY FOCUSED ON THE BLUE MOUNTAINS...SAWTOOTH...BITTERROOTS AND
TETONS WHERE A FOCUSED AREA OF MOIST MID LEVEL FLOW ASSOCIATED
WITH THE UPPER TROUGH CAN BE EXPECTED LATER THURSDAY AND
ESPECIALLY FRIDAY.


THE PROBABILITY OF SIGNIFICANT ICING (0.25 INCH OR MORE) IS LESS
THAN 10 PERCENT.

JACKSON