THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


PROBABILISTIC HEAVY SNOW AND ICE DISCUSSION




PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
304 PM EST TUE JAN 16 2018

VALID 00Z WED JAN 17 2018 - 00Z SAT JAN 20 2018

DAYS 1 TO 3...

...CENTRAL GULF COAST STATES TO THE MID-ATLANTIC AND NORTHEAST...

STRONG COLD AIR ADVECTION BEHIND AN ARCTIC FRONT SETTLING
SOUTHEASTWARD ACROSS THE GULF COAST WILL BE ADVANCING EAST ACROSS
THE MID-ATLANTIC AND NORTHEAST TONIGHT THROUGH WEDNESDAY AND THIS
WILL SET THE STAGE FOR AN EXPANSIVE AREA OF WINTRY PRECIPITATION
AS A POSITIVELY TILTED TROUGH SPLITS INTO DISTINCT NORTHERN AND
SOUTHERN STREAM SHORTWAVE COMPONENTS OVER THE EASTERN U.S. THIS
WILL RESULT IN THE DEVELOPMENT OF MULTIPLE WAVES OF LOW PRESSURE
DEVELOPING ALONG THE FRONT.

THE NORTHERN STREAM ENERGY WILL TRAVERSE THE NORTHERN MID-ATLANTIC
AND NORTHEAST TONIGHT THROUGH WEDNESDAY AND SHOULD ALLOW A WAVE OF
LOW PRESSURE TO DEVELOP EAST OF THE DELMARVA TONIGHT WHICH WILL
RIDE NORTH-NORTHEAST UP ACROSS COASTAL AREAS OF SOUTHEAST NEW
ENGLAND WEDNESDAY BEFORE THEN CROSSING THE GULF OF ME BY WEDNESDAY
EVENING. A STRENGTHENING AXIS OF DEEPER LAYER FRONTOGENETIC
FORCING AND DIVERGENT FLOW ALOFT AROUND THE EASTERN FLANK OF THE
UPPER TROUGH WILL FOSTER A SOUTHWEST/NORTHEAST AXIS OF LIGHT TO
MODERATE SNOW FROM AREAS NEAR THE MASON-DIXON LINE NORTHEAST
ACROSS NORTHEAST PA/NORTHERN NJ AND UP ACROSS MUCH OF SOUTHEAST
NY/HUDSON VALLEY AND THROUGH SOUTHERN AND EASTERN NEW ENGLAND.
THERMAL PROFILES IN THE BOUNDARY LAYER WILL BE FAIRLY MARGINAL
FROM PARTS OF THE FAR LOWER HUDSON VALLEY AND MORE SO UP ACROSS
SOUTHEAST NEW ENGLAND INVOLVING MOST OF SOUTHEAST MA AND THE CAPE.
THIS WILL GREATLY MITIGATE THE SNOWFALL POTENTIAL ACROSS THESE
AREAS WITH SOME AREAS OF RAIN OR A MIX OF RAIN/SNOW INITIALLY
BEFORE LOW LEVEL COLD AIR ADVECTION SUPPORTS A CHANGEOVER TO ALL
SNOW. HOWEVER...FARTHER NORTH AND WEST...THE MODELS SUGGEST A
NARROW SOUTHWEST/NORTHEAST AXIS OF DEEP LAYER FRONTOGENESIS AND
LOCALLY NEGATIVE EPV VALUES THAT WILL BE CONDUCIVE FOR SOME BANDED
HEAVY SNOWFALL FROM NEAR THE POCONOS UP OVER THE NORTHERN AREAS OF
THE LOWER HUDSON VALLEY AND UP THROUGH NORTHWEST CT/WESTERN
MA...SOUTHERN NH AND SOUTHERN/EASTERN ME. AN AXIS OF 4 TO 8+
INCHES IS EXPECTED ACROSS THESE AREAS BEFORE THE SNOW TAPERS DOWN
LATER WEDNESDAY. THIS AXIS OF HEAVIER SNOW WILL BE MAINLY NORTH
AND WEST OF THE I-95 CORRIDOR AND THUS SITUATED OVER THE NORTHERN
AND WESTERN SUBURBS OF THE URBAN CORRIDOR FROM NEW YORK CITY TO
BOSTON.

MEANWHILE...FARTHER SOUTH OVER THE CENTRAL MID-ATLANTIC REGION
INCLUDING D.C...BALTIMORE AND PHILADELPHIA...A RELATIVELY BRIEF
PERIOD OF FRONTOGENETICALLY ENHANCED SNOWFALL IS EXPECTED LATER
TONIGHT AND EARLY WEDNESDAY...BUT THIS AREA WILL TEND TO BE IN
BETWEEN NORTHERN AND SOUTHERN STREAM FORCING AND SHOULD SEE A
RELATIVE MIN IN ACCUMULATIONS WITH ONLY 1 TO 2 INCHES AT
BEST...AND MANY AREAS SEEING LESS THAN 1 INCH.

THE SOUTHERN STREAM PORTION OF THE TROUGH THOUGH IS EXPECTED TO
BECOME MORE OF A FACTOR FOR LOCALLY HEAVY SNOWFALL OVER AREAS OF
SOUTHERN VA AND DOWN THROUGH CENTRAL NC AND TO SOME EXTENT EVEN
FAR NORTHERN SC. MUCH MORE DYNAMICALLY ENHANCED SNOWFALL IS
EXPECTED HERE WITH THE APPROACH OF WHAT SHOULD BE AN UPPER LOW
CLOSING OFF IN THE 500/700MB LAYER AND ALSO THE STRENGTHENING OF A
200/300 MB LAYER JET MAX. THIS ENERGY WILL FOSTER DEVELOPMENT OF A
SECONDARY AREA OF LOW PRESSURE ALONG THE EVOLVING ARCTIC COLD
FRONT AND SHOULD HELP FOCUS AN AREA OF GREATER MOISTURE TRANSPORT
AND FORCING FOR ACCUMULATING SNOW. SOME RAIN IS EXPECTED INITIALLY
OVER AREAS OF THE CENTRAL CAROLINAS AND SOUTHEAST VA WITH
INITIALLY MILD BOUNDARY LAYER TEMPERATURES BEFORE THEN QUICKLY
TRANSITIONING TO A LOCALLY HEAVY WET SNOW AS COLDER AIR FILTERS
IN. SEVERAL INCHES OF SNOW ARE EXPECTED WITH THE HEAVIEST TOTALS
FROM CENTRAL NC UP INTO SOUTH-CENTRAL VA.

THE ONLY OTHER AREA OF CONCERN FOR FROZEN PRECIPITATION WILL BE
OVER AREAS OF THE CENTRAL GULF COAST AND INTERIOR SOUTHEAST WHERE
THE LOW LEVEL COLD AIR UNDERCUTTING THE RELATIVELY WARMER AIR
ALOFT WILL FOSTER THE POTENTIAL FOR SOME LIGHT ICING/FREEZING RAIN
OVERNIGHT. LOCALLY EVEN A LITTLE BIT OF SNOW WILL BE EARLY
WEDNESDAY BEFORE THE PRECIPITATION LIFTS OFF TO THE NORTHEAST.


...WESTERN U.S...

SNOW COVERAGE WILL INCREASE OVER THE HIGHER TERRAIN OF THE WESTERN
U.S. AND ESPECIALLY THE PACIFIC NORTHWEST THROUGH THE MIDDLE TO
LATTER PART OF THE WEEK. THIS WILL BE OWING TO STRONG HEIGHT FALLS
ALOFT AND MULTIPLE PACIFIC SHORTWAVES APPROACHING THE WEST COAST
WHICH WILL GIVE WAY TO A LARGER SCALE TROUGH MOVING INLAND BY THE
END OF THE WEEK. THE WETTEST CONDITIONS SHOULD BEGIN TO ARRIVE IN
EARNEST BY WEDNESDAY NIGHT 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.

ORRISON





Last Updated: 304 PM EST TUE JAN 16 2018