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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2116Z Feb 21, 2018)
 
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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
416 PM EST WED FEB 21 2018

VALID 00Z THU FEB 22 2018 - 00Z SUN FEB 25 2018


DAYS 1-3...

LINGERING TECHNICAL DIFFICULTIES CAUSED A LIMIT OF OPTIONS FOR
THERMAL FIELDS IN WWD SNOW TO THE 12Z OPERATIONAL GFS AND NAM.
THESE WERE IN FAIRLY GOOD AGREEMENT EXCEPT FOR SURFACE
TEMPERATURES AND ASSOCIATED ICE ISSUES IN THE NORTHEAST ON DAY 2
WHICH IS NOTED BELOW.


...CENTRAL ROCKIES ACROSS PLAINS TO MIDWEST...

A SHORTWAVE TROUGH EJECTS NORTHEAST FROM THE DESERT SOUTHWEST
TONIGHT. DEEP MOISTURE FROM A PLUME CONNECTED TO THE TROPICAL EAST
PACIFIC ALLOWS DECENT SNOW TO SOUTHERN CO TONIGHT WHERE SOUTHERLY
850MB FLOW BRINGS IN 0.25IN PW.

MEANWHILE...A LOW LEVEL INVERTED TROUGH DEVELOPS FROM SOUTH TO
NORTH UP THE GREAT PLAINS FROM OK TO IA WITH A WINTRY MIX EXPECTED
GIVEN THE SHALLOW COLD AIR IN PLACE AT THE SURFACE. FREEZING RAIN
IS THE PREFERRED P-TYPE WHERE THE 850MB TEMP GETS ABOVE 0C. RISK
FOR A TENTH INCH OF ICE THROUGH MAIN CORE OF THIS TROUGH PRIMARILY
FROM CENTRAL OK TO CENTRAL IA THROUGH THURSDAY. SOUTHERLY LOW
LEVEL FLOW IS ENHANCED AHEAD OF THE EJECTING SHORTWAVE FROM THE
SOUTHWEST...SETTING UP A RAIN TO FREEZING RAIN TO SLEET TO SNOW
PATTERN ACROSS THE MID-CONUS BOTH TONIGHT AND THURSDAY.

ALSO...LOW LEVEL SOUTHERLY FLOW OVERRUNNING OVER THE SURFACE COLD
FRONT OVER THE MIDWEST WILL ALSO LEAD TO A SWATH OF FREEZING RAIN
CENTERED OVER NORTHERN OH WHERE A TENTH INCH ACCUM IS LIKELY
TONIGHT.

A STRONGER WAVE IN THE WESTERN TROUGH EJECTS NORTHEAST FROM THE
DESERT SOUTHWEST FRIDAY NIGHT. SURFACE LOW DEVELOPMENT IS EXPECTED
NEAR THE EASTERN CO/NM BORDER WITH A TRACK TO THE UP SATURDAY
NIGHT. GULF MOISTURE IS TAPPED INTO AN WRAPPED AROUND THE SYSTEM
INTO A DEVELOPING DEFORMATION ZONE LATE FRIDAY NIGHT MEANING THE
SOUTHWESTERN GREAT PLAINS WHICH ARE FACING QUITE A DROUGHT LOOK TO
REMAIN DRY DESPITE A SURFACE LOW PASSAGE OVERHEAD. IN FACT...MOST
OF THE HEAVY SNOW LATE ON DAY 2 IS OVER UT AND WESTERN CO...WEST
OF THE CONTINENTAL DIVIDE.

A SWATH OF HEAVY SNOW IS LIKELY TO FORM ALONG THE NORTH SIDE OF
THE LOW STARTING IN NEBRASKA. AS DAY 3 ENDS AT 00Z SATURDAY THIS
IS AT THE END OF THE WWD FORECAST PERIOD...BUT AS OF NOW THE 12Z
GFS MAINTAINS THAT HEAVY SWATH ACROSS MN AS THE LOW CENTER
APPROACHES THE UP. STRENGTH AND TRACK UNCERTAINTY IS PARTICULARLY
HIGH IN THIS SUITE OF PRODUCTS GIVEN THE LACK OF FOREIGN MODEL
AVAILABILITY. 


...NORTHEAST...

OVERRUNNING GULF AND TROPICAL PACIFIC MOISTURE OVER THE COLD FRONT
WILL GREAT A WINTRY MIX SWATH FROM NORTHERN OH/PA/NY TO SOUTHERN
NEW ENGLAND THURSDAY. RATHER MILD AIR IS CURRENTLY ACROSS THIS
AREA...BUT THE COLD FRONT WILL MOVE THROUGH TONIGHT...ALLOWING
COLD AIR TO BE IN PLACE BY THURSDAY. A QUICK TRANSITION FROM RAIN
TO FREEZING RAIN TO SLEET TO SNOW WILL TAKE PLACE ALONG THIS
FRONT. THE SWATH OF HEAVY OVERRUNNING SNOW ALONG THE PA/NY BORDER
INTO NY.

A SECOND SWATH OF WINTRY MIX CROSSES THE ENTIRE NORTHEAST FRIDAY
AS A SHORTWAVE TRACKS OVER THE NORTHERN GREAT LAKES WITH CONTINUED
SOUTHERLY FLOW OVER THE SURFACE COLD AIR. THE 1040MB HIGH SHIFTS
EAST FROM NORTHERN NEW ENGLAND THURSDAY NIGHT WITH A COLD AIR
DAMMING WEDGE EXPECTED TO PERSIST BACK ACROSS THE NORTHERN
MID-ATLANTIC. MODELS ARE NOTORIOUS FOR ERODING COLD AIR
PREMATURELY ON WINTER NIGHTS WITH SOUTHERLY FLOW. ATTEMPTED TO
BLEND THE COLDER/ICIER 12Z NAM WITH THE WARMER 12Z GFS.

ACCUMULATING SNOW LOOKS LIMITED IN THE NORTHEAST DAY 2/3 TO
NORTHERN MAINE FRIDAY EVENING.


...PACIFIC NORTHWEST TO GREAT BASIN...

A STRONG SHORTWAVE TROUGH DROPS SOUTH FROM THE PAC NW THIS
EVENING. HOWEVER...PACIFIC MOISTURE WILL BE CONTAINED OFFSHORE
FROM NORTH WINDS. THEREFORE...LIGHT TO LOCALLY MODERATE SNOWS ARE
EXPECTED ACROSS OR/SOUTHERN ID/NORTHERN NV/AND HIGHER ELEVATIONS
OF NORTHERN CA. OF NOTE IS THE LOWERING HEIGHTS WHICH WILL ALLOW
INTERIOR VALLEYS IN OR TO RECEIVE A SECOND ROUND OF SNOW THIS
WEEK...INCLUDING PORTLAND WHERE AROUND AN INCH IS FORECAST.

A SECOND TROUGH WILL PUSH SOUTHEAST INTO THE PAC NW LATE FRIDAY
INTO SATURDAY. NORTHWESTERLY ONSHORE FLOW WILL ALLOW MUCH HEAVIER
PRECIPITATION THAN THE PREVIOUS SHORTWAVE WITH HEAVY SNOW EXPECTED
IN THE HIGHER ELEVATIONS AND LOWERING HEIGHTS AGAIN CAUSING VERY
LOW SNOW ELEVATIONS POTENTIALLY IMPACTING SEATTLE AND PORTLAND
FRIDAY NIGHT.

THE PROBABILITY FOR IMPACTFUL ICING IS LOW ACROSS THIS AREA
THROUGH THE FORECAST PERIOD.


JACKSON