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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0819Z Apr 16, 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
419 AM EDT MON APR 16 2018

VALID 12Z MON APR 16 2018 - 12Z THU APR 19 2018


DAYS 1-3...

...PACIFIC NORTHWEST AND CALIFORNIA TO THE NORTHERN AND CENTRAL
ROCKIES...

WIDESPREAD SNOWFALL IS EXPECTED WITH AN UPPER TROUGH MOVING INLAND
OVER THE NORTHWESTERN U.S.  MODEL GUIDANCE REMAINED IN AGREEMENT
THAT LOWERING HEIGHTS AND SNOW LEVELS WITH THE TROUGH WILL SUPPORT
PERIODS OF MODERATE TO HEAVY SNOW ALONG THE WA-OR CASCADES, SIERRA
NEVADA, AND SAWTOOTH RANGE TODAY, WITH ACTIVITY INTO THE
NORTHERN/CENTRAL ROCKIES MONDAY NIGHT INTO TUESDAY AS THE
ASSOCIATED COLD FRONT PROGRESSES EASTWARD OUT OF THE GREAT BASIN
AND UPPER DIVERGENCE MAXIMA MOVING NORTH ACROSS ID AND WESTERN MT.
 THE HEAVIEST SNOW IS FORECAST TO BE 1- FEET OVER THE WASHINGTON
CASCADES. 
SEVERAL INCHES ARE EXPECTED ON TUE IN THE RANGES OF WY AS THE AREA
EXPERIENCES PRE-FRONTAL CONVERGENCE AS THE 700 MB FRONT APPROACHES
AND THEN ARRIVES. A BREAK COMES WED AS THE UPPER TROUGH MOVES EAST
ON TO THE PLAINS.

SNOWS MAY REDEVELOP AS THE NEXT UPPER TROUGH FROM THE EASTERN
PACIFIC APPROACHES WESTERN OR AND CA.  THERE IS A SPREAD IN TROUGH
TIMING/AMPLITUDE AND THUS PRECIP AND SNOW AMOUNTS TO BE RESOLVED
LATER.  CONSEQUENTLY A LOW RISK OF HEAVY SNOW IS INDICATED IN THE
PROBABILITIES.  

...CENTRAL AND NORTHERN PLAINS TO THE UPPER MISSISSIPPI VALLEY...

AN AREA OF SNOWFALL WILL BE POSSIBLE OVER PORTIONS OF THE
CENTRAL/NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY TUESDAY NIGHT
INTO WEDNESDAY AS A VIGOROUS UPPER TROUGH EJECTS OUT OF THE
ROCKIES AND A CLOSED OFF MOVES FROM KANSAS TO NEAR THE
NEBRASKA/IOWA/MISSOURI BORDER INTO IOWA.  SNOW WILL BE POSSIBLE
WITHIN AN AXIS OF MOISTURE AND CONVERGENCE ALIGNING WITHIN A
DEFORMATION ZONE SETTING UP TO NORTH/NORTHWEST OF THE DEEPENING
SURFACE CYCLONE.  THERE CONTINUES TO BE A FAIR AMOUNT OF MODEL
SPREAD WITH THE TIMING AND EVOLUTION OF THE SYSTEM, WHICH IS
IMPACTING BOTH THE AXIS/ORIENTATION AND POTENTIAL AMOUNTS. 
GREATER WEIGHTING WAS GIVEN TO THE NAM/UKMET/ECMWF AND ECMWF
ENSEMBLE MEAN SOLUTIONS...WITH THE GFS SHOWING FASTER MOVEMENT AND
MORE NORTHERN AXIS OF THE FORECAST SNOWFALL.  THE PREFERRED
CLUSTER PROVIDES POTENTIAL FOR 6-10 INCHES OF SNOW.    

...UPPER GREAT LAKES...

THE AREA OF HEAVY SNOW OCCURRING IN AREAS ADJACENT TO LAKE
SUPERIOR IS EXPECTED TO PERSIST MONDAY INTO MONDAY NIGHT INTO
MONDAY AS THE LOW LEVEL CYCLONE DRIFTS SLOWLY
NORTHEAST....ALLOWING AN EXTENDED PERIOD OF LEE AREA CONVERGENCE
TO OCCUR IN THE UP OF MI. 
SEVERAL ADDITIONAL INCHES OF SNOW IS EXPECTED TODAY BEFORE
TAPERING TONIGHT AS THE LOW MOVES ACROSS THE BORDER INTO CANADA. 
AS THE LOW DEPARTS...NORTHWEST FLOW DEVELOPS IN THE UP OF MI
ACROSS NORTHERN LAKE MICHIGAN....WITH ENHANCED MOISTURE AND LEE
SHORE CONVERGENCE RESULTING IN SEVERAL INCHES OF SNOW IN NORTHWEST
LOWER MICHIGAN.

...NORTHEAST...

A MESSY MIX OF SLEET AND FREEZING RAIN IS EXPECTED WITHIN THE
NORTHERN EDGE OF PRECIPITATION SPREADING NORTHWARD THROUGH MONDAY
ACROSS NORTHERN NEW YORK AND NORTHERN NEW ENGLAND.   DESPITE A
MELTING LAYER ALOFT, MODEL GUIDANCE IS IN AGREEMENT THAT COLD AIR
IN THE LOWER LEVELS SHOULD REMAIN DAMMED IN LONG ENOUGH TO ALLOW
FOR ACCUMULATING ICE OVER MUCH OF INTERIOR NORTHERN NEW ENGLAND. 
WPC PROBABILITIES SUGGEST THAT THERE IS A SLIGHT TO MODERATE RISK
FOR ICE ACCUMULATIONS TO EXCEED 0.25" ACROSS INTERIOR MAINE AND NH.

THE THREAT FOR SNOW AND ICE WITH THIS STORM SHOULD FINALLY COME TO
AN END LATE MONDAY/EARLY TUESDAY AS THE GREATEST MOISTURE AND
FORCING LIFT INTO CANADA.  HOWEVER, BROAD CYCLONIC FLOW ALOFT
SHOULD HELP SUPPORT SNOW SHOWERS DOWNWIND OF THE LAKES INTO THE
CENTRAL APPALACHIANS.
THROUGH TUESDAY.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT DAY 2
(TUE) and 3 (WED).

PETERSEN