Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 212117
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
516 PM EDT TUE APR 21 2015

VALID 00Z WED APR 22 2015 - 00Z SAT APR 25 2015


DAYS 1 THROUGH 3...


...WESTERN/NORTHERN NEW ENGLAND...

A SHORT WAVE RIDING AROUND THE PERIPHERY OF A STRONG CLOSED MID
LEVEL LOW OVER ONTARIO IS FORECAST TO SCOOT ACROSS NORTHERN NEW
ENGLAND MAINLY WEDNESDAY NIGHT INTO THU MORNING.  THE SHORT WAVE
INDUCES CYCLOGENESIS OVER THE WESTERN GULF OF ME DURING THIS TIME.
THE LOW THEN TRACKS OVER ME DURING DAY 2. A WARM CONVEYOR BELT
SHOULD RESULT IN PCPN STARTING OUT AS ALL RAIN.  BUT AS THE DAY
PROGRESSES AND THE DEVELOPING CYCLONE CAN TAP THE COLDER AIR OVER
CANADA...COMBINED WITH DYNAMICAL COOLING DUE TO THE DEVELOPING
SYSTEM...PCPN SHOULD BECOME MIXED WITH OR CHANGE OVER TO SNOW.

ONCE AGAIN...THE GFS WAS DISMISSED GIVEN HOW IT HANDLES THE UPPER
LEVEL FEATURE.  OF THE REMAINING MODELS...THE ECMWF AND THE NAM
WERE IN BEST AGREEMENT AND HAD THE BEST RUN TO RUN CONSISTENCY.
SO THE THERMAL PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE
21/12Z NAM AND THE 21/00Z ECMWF PLUS A SMALL AMOUNT OF THE 21/09Z
SREF...WHILE THE WPC QPF WAS USED FOR QPF AMOUNTS.  THIS RESULTS
IN A SNOWFALL FORECAST WHICH IS DEPENDENT UPON ELEVATION.  THIS IS
INDICATED BY THE SLIGHT RISK OF 4 INCHES OF SNOW ON DAY 2 BEING
CONFINED TO THE HIGHER ELEVATIONS.  WHILE SOME ADDITIONAL
ACCUMULATIONS ARE EXPECTED ON DAY 3...PROBABILITIES OF AN
ADDITIONAL 4 OR MORE INCHES IS LOW.  EVEN SO...SOME SPOTS MAY END
UP WITH 2 DAY TOTALS IN THE 3 TO 6 INCH RANGE.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

BANN

$$





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