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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0939Z Feb 20, 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
439 AM EST TUE FEB 20 2018

VALID 12Z TUE FEB 20 2018 - 12Z FRI FEB 23 2018


DAYS 1-3...

...WEST...

ALTHOUGH THE THREAT FOR WIDESPREAD HEAVY SNOWFALL ACROSS THE WEST
HAS DIMINISHED...WELL BELOW NORMAL TEMPERATURES WITH AN ANOMALOUS
LONGWAVE TROUGH WILL BRING ADDITIONAL SNOWFALL TO PORTIONS OF THE
WESTERN U.S. OVER THE NEXT SEVERAL DAYS.  THE GREATEST CHANCE FOR
SNOW WILL BE WITH MULTIPLE SHORTWAVES DROPPING SOUTHWARD ALONG THE
WEST COAST.  ENHANCED LIFT AND A SURGE OF MOISTURE WITH THE FIRST
SHORTWAVE SHOULD BRING LIGHT TO MODERATE SNOW TO MUCH OF THE
PACIFIC NORTHWEST ON TUESDAY, WHILE THE SECOND SHORTWAVE IS
EXPECTED TO HAVE A GREATER IMPACT FARTHER SOUTH OVER THE SIERRA
NEVADA THURSDAY INTO FRIDAY.  ALSO, AS THE ENERGY ROUNDS THE BASE
OF THE LONGWAVE TROUGH, ADDITIONAL SNOWFALL WILL BE POSSIBLE
ACROSS THE CENTRAL GREAT BASIN/FOUR CORNERS STATES.


...PLAINS AND MISSISSIPPI VALLEY INTO GREAT LAKES, OHIO VALLEY,
AND NORTHEAST...

ANOMALOUS GULF MOISTURE FEEDING NORTHWARD BETWEEN A WESTERN U.S.
TROUGH AND EASTERN U.S. RIDGE FUEL HEAVY PRECIPITATION FROM THE
SOUTHERN PLAINS TO THE MISSISSIPPI AND OHIO VALLEYS OVER THE NEXT
SEVERAL DAYS.  WHILE EXCESSIVE RAINFALL WILL BE THE MOST
WIDESPREAD THREAT, AN INVADING ARCTIC AIRMASS WILL MAKE SNOW AND
ESPECIALLY FREEZING RAIN POSSIBLE WITHIN THE WESTERN/NORTHERN
FRINGE OF PRECIPITATION.

TUESDAY INTO WEDNESDAY, THE GREATEST CHANCE FOR WINTRY
PRECIPITATION WILL BE WITH A SHORTWAVE AND SURFACE LOW LIFTING OUT
OF THE UPPER GREAT LAKES REGION.  A SWATH OF LIGHT SNOW
ACCUMULATIONS WILL BE POSSIBLE ACROSS NORTHERN MINNESOTA, AND AN
AXIS OF FREEZING RAIN IS EXPECTED WITHIN WARM ADVECTION
PRECIPITATION DEVELOPING OVER THE U.P. OF MICHIGAN.  HOWEVER, THE
BIGGER CONCERN AND FORECAST CHALLENGE CONTINUES TO BE THE
POTENTIAL FOR SIGNIFICANT ICE ACCUMULATIONS BEHIND THE TRAILING
COLD FRONT PRESSING THROUGH THE MIDDLE MISSISSIPPI TUESDAY NIGHT. 
THERE REMAINED SOME MODEL SPREAD WITH THE ARRIVAL OF ARCTIC AIR,
WHICH WILL PLAY A BIG ROLE IN HOW MUCH ICE WILL ACCUMULATE.  THE
00Z GFS MADE A SHIFT TOWARDS THE NAM/ECMWF AND IS A BIT FASTER
THAN ITS PREVIOUS RUN WITH THE ARRIVAL OF ARCTIC AIR, WHICH
SUPPORTS A GREATER ICE POTENTIAL.  PLUS, THE LATEST WPC
PROBABILITIES ARE SHOWING A DAY 1 MODERATE RISK OF ICE
ACCUMULATIONS EXCEEDING 0.25-INCHES FROM EAST CENTRAL MISSOURI
INTO CENTRAL ILLINOIS.  THE FORECAST SHOULD CONTINUE TO BE
MONITORED CLOSELY.   

WEDNESDAY INTO THURSDAY, SHORTWAVE ENERGY EJECTING OUT OF THE
WESTERN U.S. TROUGH COULD HELP SPREAD PRECIPITATION INTO THE COLD
SECTOR WHILE LIFTING FROM THE SOUTHERN PLAINS TO THE UPPER GREAT
LAKES.  A SWATH OF LIGHT SNOW AND ICE ACCUMULATIONS WILL BE
POSSIBLE WITH THIS FEATURE, BUT THERE REMAINS SOME SPREAD WITH THE
EXACT TIMING/AMPLITUDE OF THE SHORTWAVE.  FOR NOW, WPC
PROBABILITIES ARE SHOWING THE GREATEST POTENTIAL FOR SNOWFALL
ACCUMULATIONS GREATER THAN 4-INCHES WILL BE ACROSS THE UPPER
MISSISSIPPI VALLEY. 

FARTHER EAST, MODEL GUIDANCE AND WPC PROBABILITIES ARE STARTING TO
SHOW A SIGNAL THAT ACCUMULATING SNOW AND SIGNIFICANT ICE
ACCUMULATIONS WILL BE POSSIBLE ACROSS PORTIONS OF CENTRAL
PENNSYLVANIA AND NEW YORK ON THURSDAY WHILE WARM ADVECTION
PRECIPITATION LIFTS THROUGH THE REGION AND A STRONG SURFACE RIDGE
BUILDS TO THE NORTH.


GERHARDT