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

Valid 12Z Sun Apr 26 2020 - 12Z Wed Apr 29 2020

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

Surface low pressure developing in response to an increasingly
coupled jet structure and mid-level height falls/PVA will lift
from VA this morning into the Gulf of Maine Monday, before slowing
temporarily, and then ejecting into SE Canada Tuesday. North and
west of this low, an area of moderate to heavy snow is likely,
especially in the terrain from the Catskills and points northeast
into Maine.

As the low begins to lift northward, precipitation will spread
into New York and New England on WAA and 290/295K isentropic
ascent atop a cold surface high pressure. Initially, precipitation
will likely be in the form of rain nearly everywhere as the
thermal structure of the column is too warm to support snow in all
but the highest peaks of NH/ME. However, as WAA intensifies
causing increased frontogenesis, this will dynamically cool the
column, aided by wet-bulb cooling and ageostrophic cooling from
the displaced high to the north. During Sunday night, it is likely
that much of the terrain will transition from rain to snow. What
is more difficult to discern is where even lower-levels will
changeover due in large part to a robust deformation axis likely
to stretch e/ne across central New England. Where this sets up is
still in question, but the intense ascent associated with the
deformation band should be strong enough to overcome marginal low
level temperatures and transition even elevation <1000 ft over to
snow. This appears to be most likely in eastern NH and along the
coast of Maine where the fgen occurs concurrently with the
deformation. However, guidance has trended more aggressive with a
dry slot pivoting across parts of the region during the time of
most pronounced forcing. There will likely be some overlap of
these features, so confidence is less than usual for where the
heaviest snow will occur outside of the terrain. WPC probabilities
on D1 are moderate for 4 inches in the Adirondacks, Greens, and
Whites, highest within the Presidential Range where more than 8
inches is possible.

As the negatively tilted low re-closes east from the Gulf of Maine
Monday/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 by D3. 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 through Monday are likely, but
generally confined to where the rates are heaviest as the nearly
May sun angle will offset snow accumulation otherwise. WPC
probabilities on D2 are moderate for 4 inches across the White
Mountains and into central Maine, with low probabilities for 2
inches across most of central New England and into the Adirondacks.


...Pacific Northwest...
Day 2...

Confluent flow into British Columbia will support several fast
moving shortwaves lifting onshore over the next several days. The
most potent of these will race along the international border
Monday morning ahead of an intensifying Pacific jet streak arcing
into Washington state. The combination of height falls, PVA, and
LFQ diffluence will drive ascent in a column moistened by a
subtropical plume from the Pacific. Snow levels will be generally
high, above 5000 ft, and the system is transient so forcing lacks
any significant duration. However, snowfall is still likely to be
moderate to heavy across the Washington Cascades and into the
Northern Rockies. WPC probabilities for 4 inches are moderate on
days 1.5-2, with locally up to 12 inches in the highest peaks.

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

Weiss