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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0727Z Apr 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
327 AM EDT TUE APR 17 2018

VALID 12Z TUE APR 17 2018 - 12Z FRI APR 20 2018


DAYS 1-3...

...PACIFIC NORTHWEST/NORTHERN TO CENTRAL ROCKIES/GREAT BASIN...

COVERAGE AND INTENSITY OF SNOW IS LIMITED ON DAY 1 AS THE UPPER
TROUGH DEAMPLIFIES OVER THE NORTHWEST US AND A MID-UPPER RIDGE
APPROACHES FROM THE EASTERN PACIFIC.  SEVERAL INCHES OF SNOW ARE
POSSIBLE IN THE WA CASCADES TUE AS DEEP WESTERLY FLOW AND HIGH
LAYER RELATIVE HUMIDITY INITIALLY NEAR 90 PERCENT GET LIFTED IN
WINDWARD TERRAIN WITH SNOW SHOWERS FOCUSED ON THE EARLY PART OF
TUE BEFORE DRIER AIR ALOFT MOVES INTO THE REGION.  FURTHER
INLAND...
MOUNTAIN SNOWS POSSIBLE IN THE WAKE OF A WELL-DEFINED COLD FRONT
MOVING EAST OUT OF THE ROCKIES ON TO THE NORTHERN PLAINS.  SEVERAL
INCHES ARE POSSIBLE IN THE
RANGES OF NORTHERN IDAHO...WESTERN MONTANA AND NORTHWESTERN
WYOMING RANGES...WITH WPC DAY 1 PROBABILITIES SHOWING THE
POTENTIAL FOR 8-INCHES OR MORE ACROSS SOME OF THE HIGHER TERRAIN.

LIGHT SNOW DEVELOPS IN THE RANGES OF SOUTHWEST OR AND NORTHWEST CA
ON WED AS THE NEXT EASTERN PACIFIC TROUGH MOVES EAST SOUTHEAST AND
PRODUCES COUPLED LOW-MID LEVEL CONVERGENCE AND UPPER DIVERGENCE
WED NIGHT AS THE TROUGH COMES ONSHORE.
WITH LIMITED MOISTURE...WIDESPREAD HEAVY SNOW TOTALS ARE NOT
EXPECTED.

ON DAY 3 LOW LEVEL CONVERGENCE IN THE VICINITY OF THE 700 MB LOW
AS IT CROSSES NV LEADS TO DEVELOPMENT OF HIGHER ELEVATION
SNOWS....WITH WPC DAY 3 PROBABILITIES SHOWING AT LEAST A SLIGHT
RISK FOR ACCUMULATIONS OF 4-INCHES OR MORE ACROSS CENTRAL INTO
NORTHEAST NV.
AS THE LOW MOVES EAST TOWARDS UT...MOIST SOUTHERLY DEEP LAYERED
FLOW DEVELOPS ACROSS THE RANGES OF SOUTHWEST CO...WITH LIFT
ACCOMPANYING THE WARM/MOISTURE ADVECTION IN TO THE SAN JUAN MNTNS.
 SEVERAL INCHES ARE POSSIBLE FOLLOWING AN EARLY DRY PERIOD A
SUSTAINED MOISTURE AND LIFT PERSISTS LATE THU NIGHT TO FRI MORNING.

...NORTHERN TO CENTRAL PLAINS AND UPPER MIDWEST...

A MID-UPPER TROUGH ON DAY 1 IS EXPECTED TO ASSUME A NEGATIVE-TILT
AS IT MOVES INTO THE CENTRAL U.S....WITH A 700 MB LOW CLOSING OFF
OVER SOUTH DAKOTA TO SOMEWHERE EITHER IN IOWA OR MN WED BEFORE
MERGING WITH THE NORTHERN STREAM WAVE AS IT CROSSES THE UPPER
GREAT LAKES ON WED NIGHT-EARLY THU.  IN ADDITION TO THE STRONG
UPPER FORCING...LOW TO MID LEVEL CONVERGENCE AND FRONTOGENESIS
ALONG A TROUGH EXTENDING NORTHWEST OF THE ASSOCIATED SURFACE
CYCLONE LIFTING NORTHEAST INTO THE MID MS VALLEY IS EXPECTED TO
SUPPORT A BROADENING AREA OF PRECIPITATION ACROSS SOUTH DAKOTA
INTO SOUTHERN MN AND NORTHERN IA TUE NIGHT INTO WI AND IL WED. 
LIGHT AMOUNTS ARE ALSO EXPECTED IN ADVANCE OF THE FRONT IN NORTH
DAKOTA ON TUE.

COLD AIR ADVECTED SOUTH BY THE AMPLIFYING CYCLONE IS EXPECTED TO
SUPPORT RAIN CHANGING TO SNOW ACROSS EASTERN SD AND POSSIBLY
ADJACENT NORTHEAST NE TO NORTHERN IOWA/SOUTHERN MN...CONTINUING
INTO WI WED.  LOCALLY HEAVY SNOW ACCUMULATIONS ARE POSSIBLE...AS
THE 00Z NAM SHOWS COMBINED STRONG 700 MB CONVERGENCE AND THETA-E
ADVECTION IN NORTHERN IA TO ADJACENT SOUTHERN MN WITH WPC DAY 2
PROBABILITIES SHOWING A MODERATE RISK FOR ACCUMULATIONS OF 8
INCHES OR MORE IN THESE AREAS. 


...GREAT LAKES/NORTHEAST...

COLD AIR WRAPPING AROUND THE BACKSIDE OF A SURFACE LOW OVER
SOUTHEASTERN CANADA WILL SUPPORT SNOW...WITH GUIDANCE SHOWING THE
POTENTIAL FOR SOME LOCALLY HEAVY ACCUMULATIONS ACROSS PORTIONS OF
THE TUG HILL PLATEAU OF NY...WHERE OROGRAPHIC LIFT AND LAKE
ENHANCEMENT SHOULD HELP TO ENHANCE AMOUNTS ON TUE.  LAKE ENHANCED
SNOW ALSO IS EXPECTED DOWNWIND FROM LAKE ERIE TO NORTHWEST PA TO
ADJACENT OH/FAR SOUTHWEST NY.
AS THE LOW IN CANADA MOVES FURTHER EAST...THE SNOW ON DAY 2 IS
FORECAST TO BE LIGHT AND PRIMARY IN WINDWARD TERRAIN OF THE
NORTHERN GREEN MNTNS OF VT AND ACROSS PORTIONS OF WESTERN MAINE.

ON DAY 3...THE MODELS SHOW THE DEVELOPMENT OF A TRIPLE POINT LOW
IN NEW YORK AND NEW ENGLAND...CONTINUING INTO THE GULF OF MAINE. 
NORTH OF THE LOW...SNOW IS EXPECTED ACROSS NORTHERN NEW YORK AND
NORTHERN NEW ENGLAND.  IF THE LOW IS A BIT STRONGER LIKE THE GFS
AND/OR 00Z ECMWF...THERE IS POTENTIAL FOR SEVERAL INCHES IN
NORTHERN NEW ENGLAND.
LOW LEVEL COLD ADVECTION ACROSS THE LAKES WITH ENHANCED MOISTURE
FLUXES IS EXPECTED TO PRODUCE LAKE ENHANCEMENTS OFF LK ONTARIO
INTO AREAS DOWNSTREAM ALONG THE SOUTHEAST SHORE LINE OF LK ONTARIO
AND LK ERIE THU NIGHT.

THE PROBABILITY OF FREEZING RAIN GREATER THAN 0.25 INCHES IS LESS
THAN 10 PERCENT DAYS 1-3.

PETERSEN