PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
503 PM EDT MON APR 16 2018

VALID 00Z TUE APR 17 2018 - 00Z FRI APR 20 2018


DAYS 1-3...

...GREAT LAKES/NORTHEAST...

A BROAD UPPER LOW...RESPONSIBLE FOR THE WIDESPREAD MODERATE TO
HEAVY PRECIPITATION ACROSS THE CENTRAL-EASTERN U.S. THE PAST FEW
DAYS IS EXPECTED TO GRADUALLY LIFT NORTHEAST ACROSS THE
NORTHEASTERN U.S. MON NIGHT AND TUE.  COLD AIR WRAPPING AROUND THE
BACKSIDE OF A CONSOLIDATING SURFACE LOW OVER SOUTHEASTERN CANADA
WILL SUPPORT RAIN CHANGING TO SNOW...WITH GUIDANCE SHOWING THE
POTENTIAL FOR SOME LOCALLY HEAVY ACCUMULATIONS...PARTICULARLY
ACROSS PORTIONS OF THE WESTERN U.P. OF MI...NORTHWEST PA AND
UPSTATE NY...WHERE OROGRAPHIC LIFT/LAKE-ENHANCEMENT MAY HELP TO
BOLSTER AMOUNTS.  WPC DAY 1 PROBABILITIES (ENDING 00 UTC WED) SHOW
AT LEAST A SLIGHT RISK FOR ACCUMULATIONS OF 4-INCHES OR MORE
ACROSS THESE AREAS.  AHEAD OF THE LOW...ACCUMULATING FREEZING RAIN
IS POSSIBLE...WITH DAY 1 PROBABILITIES SHOWING A SLIGHT RISK FOR
ACCUMULATIONS OF 0.25 INCH OR MORE ACROSS PORTIONS OF NORTHERN
MAINE.

...PACIFIC NORTHWEST/NORTHERN ROCKIES...

A DEEP UPPER TROUGH IS FORECAST TO MOVE ACROSS THE WESTERN U.S.
MON NIGHT AND TUE...WITH LOCALLY HEAVY MOUNTAIN SNOWS POSSIBLE IN
THE WAKE OF A WELL-DEFINED COLD FRONT TRAVERSING THE REGION. 
HEAVIEST ACCUMULATIONS ARE EXPECTED TO FALL ALONG THE NORTHERN
CASCADES...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.

INITIAL TROUGH IS FORECAST TO MOVE PROGRESSIVELY INTO THE CENTRAL
U.S. TUE NIGHT.  HOWEVER...PRECIPITATION IS EXPECTED TO RETURN TO
PORTIONS OF THE WEST BY LATE WED AS A CLOSED LOW DROPS SOUTHEAST
INTO NORTHERN CA.  WITH LIMITED MOISTURE...WIDESPREAD HEAVY SNOW
TOTALS ARE NOT EXPECTED...HOWEVER GUIDANCE DOES SHOW THE POTENTIAL
FOR SOME LOCALLY HEAVY TOTALS...WITH WPC DAY 3 PROBABILITIES
(ENDING 00 UTC FRI) SHOWING AT LEAST A SLIGHT RISK FOR
ACCUMULATIONS OF 4-INCHES OR MORE ACROSS CENTRAL INTO NORTHEAST NV.

...NORTHERN PLAINS AND UPPER MIDWEST...

THE LEADING TROUGH TRAVERSING THE WESTERN U.S. ON DAY 1 IS
EXPECTED TO ASSUME A NEGATIVE-TILT AS IT MOVES INTO THE CENTRAL
U.S....WITH AN UPPER LOW CLOSING OFF OVER THE DAKOTAS TUE NIGHT. 
IN ADDITION TO THE STRONG UPPER FORCING...LOW TO MID LEVEL
CONVERGENCE AND BAROCLINICITY 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 MODERATE
PRECIPITATION ACROSS THE DAKOTAS INTO SOUTHERN MN AND IA TUE
NIGHT.  COLD AIR DRAWN SOUTH BY THE AMPLIFYING CYCLONE IS EXPECTED
TO SUPPORT RAIN CHANGING TO SNOW ACROSS EASTERN SD AND ALONG THE
MN/IA BORDER INTO SOUTHERN WI TUE NIGHT INTO EARLY WED.  LOCALLY
HEAVY SNOW ACCUMULATIONS ARE POSSIBLE...WITH WPC DAY 2
PROBABILITIES (ENDING 00 UTC THU) SHOWING A MODERATE RISK FOR
ACCUMULATIONS OF 4-INCHES OR MORE AND A SLIGHT RISK FOR 8-INCHES
OR GREATER ACROSS THE REGION.

PEREIRA