Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 PM EDT Sun Apr 26 2020

Valid 00Z Mon Apr 27 2020 - 00Z Thu Apr 30 2020

...Northeast...
Days 1-2...

The primary mid-level low is currently over eastern West Virginia
per GOES-16 water vapor imagery. The surface reflection of this
wave was over the Delmarva Peninsula as of the 18Z WPC analysis.
This low will develop in an increasingly coupled jet structure
over the Northeast and slowly track northeast to a point east of
Cape Cod through Sunday night where it will stall Monday before
shifting east of Nova Scotia Monday night. North and west of this
low, an area of moderate to heavy snow is likely, especially
tonight in the terrain from the Catskills to the Greens of VT and
White of NH/ME with light to moderate slow lingering Monday into
Monday night over eastern New England.

Rain with very high elevation snow has overspread the Northeast
today in warm air advection ahead of the low. Low-level
frontogenesis increases this evening as the low tracks northeast
along the Mid-Atlantic coast with high pressure over eastern
Quebec. Dynamic cooling of the column, aided by wet-bulb cooling
and ageostrophic cooling from the displaced high to the north will
promote wet snow mainly above 1500ft tonight. Lower level snow is
only expected in the heaviest mesoscale banding where most dynamic
cooling will occur. This is expected to be over interior central
New England east across the Lakes Region of NH and toward the
coast of Maine on the west side of a dry slot pivoting across
parts of the region during the time of most pronounced forcing.
Day 1 WPC probabilities for 4 or more inches are moderate for
terrain, the Adirondacks, Greens, and Whites of NH and ME with
lower probabilities for the Catskills and Monadnocks. Moderately
high probabilities for 8 or more inches are over the Presidential
Range of the Whites in NH.

As the negatively tilted low re-closes east from the Gulf of Maine
Monday night, wrap around snow due to continued deformation,
continued cold air drainage, and modest upslope ascent should
produce additional light to possibly moderate snow. There is
likely to be a long duration of snow/snow showers on Monday as the
low gets caught in the newly closed mid-level circulation, and
stalls east of Cape Cod for a time before finally exiting to the
northeast Tuesday. However, much of the forcing at this point has
been lost, and drying should occur from the top down, including at
least part of the DGZ. Still, with some potential lingering modest
deformation and cooler temperatures, additional snow accumulations
into Monday night are likely, but generally confined to where the
rates are heaviest during daylight under the intense late April
sun. Day 2 WPC probabilities are low for 2 or more inches over
much of interior Maine and the Presidential Range in NH.


...Pacific Northwest...
Day 1...

A frontal system from a Gulf of Alaska low will push into the
WA/northern OR coast tonight and progress through northern Rockies
through Monday. The mid-level trough axis will cross Vancouver
Island Monday morning ahead of an intensifying Pacific jet streak
arcing into Washington state. The combination of height falls,
PVA, and left exit diffluence of the jet will drive ascent in a
column moistened by a subtropical plume from the Pacific. Snow
levels will rise tonight and be generally above 6000ft for the
heaviest precip making this mostly a rain maker. Heavy snow will
be limited to the higher WA/northern OR Cascades with Day 1
probabilities for 8 or more inches limited above 7000ft (with
mountaintop snows likely with 1 to 1.5 inch QPF). With best
mid-level forcing north of the border, precip rates will generally
be moderate for the Northern Rockies with Day 1.5 probabilities
for 4 or more inches and into the Northern Rockies. WPC
probabilities for 4 inches are moderate on days 1.5-2 for MT
ranges including around Glacier NP and the northern Absarokas.


For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Jackson