Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
501 PM EDT Fri Apr 02 2021

Valid 00Z Sat Apr 03 2021 - 00Z Tue Apr 06 2021

Day 1...
The probability of significant snow and/or ice is less than 10
percent.

Pacific Northwest...
Day 2-3...

An upper low near the southern Alaska Panhandle will move
south-southeastward along the British Columbia coast Saturday,
with its attendant cold front approaching northwestern Washington.
Heights will start to fall over the Cascades Saturday evening as a
narrowing corridor of PW values between 0.50-0.75" bisect the
Olympics, spreading light snow to the higher elevations. Snow
levels will lower as colder air filters in but overall QPF amounts
will likely be light to perhaps modest over the higher peaks as
the available moisture decreases with time. 12Z GFS lagged behind
the consensus near the 12Z NAM/ECMWF/Canadian (UKMET was the
quickest) and was therefore much lighter with QPF/snowfall.
Utilized the better clustering around the NAM/ECMWF as the system
moves across the Cascades late Sun/early Mon and into the Snake
River Valley by Monday late afternoon. With the cold front sinking
southward through Montana, upslope and modest convergence will
yield more coverage of light to modest amounts especially in the
higher east/northeast-facing terrain. Probabilities of at least 8
inches of snow are about 20-40 percent in the highest peaks of the
Washington Cascades but about 10-20 percent over Idaho into SW
Montana.

The probability of significant ice is less than 10 percent.


Northern New England...
Day 3...

A stretched upper low will split off its southern extent just off
southeastern New England on Saturday and become negatively tilted
as it pivots near 60W. Late Sunday it will loop back to the west
just south of Nova Scotia and wrap back some light snowfall (or
mixed rain/snow as temperatures moderate) to portions of Maine and
into far northern VT/NH. Aside from the much farther NW 12Z UKMET,
the GFS/NAM/Canadian clustered together just east of the ECMWF
which was a bit farther west. Preferred to remain conservative
given the uncertain evolution of the cutoff system which yielded
only light snow amounts. Due to the varied ensemble spread,
probabilities of at least 4 inches of snow were non-zero (about
10-20 percent) over far northern/northwestern Maine.

Fracasso