Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
229 PM EST Thu Nov 25 2021

Valid 00Z Fri Nov 26 2021 - 00Z Mon Nov 29 2021


...Pacific Northwest and the Northern Rockies...
Day 1...

Confluent mid-level flow will transport moisture and a shortwave
onshore the WA Coast, flattening the downstream ridge and
producing heavy precipitation across western WA and spilling
eastward into the Northern Rockies. The shortwave providing the
most intense ascent will race eastward towards the Northern Plains
by the end of D1, so the heavy precipitation is likely to end by
Friday night. Snow levels during the period of heaviest
precipitation will be around 7000 ft in the Olympics and Cascades,
and 4000 ft in the Northern Rockies. This will confine the
heaviest snow above these levels, and WPC probabilities are
moderate for 4 inches in the WA Cascades and Northern Rockies. At
pass level the precipitation is likely to be mostly rain, but some
light accumulations are possible at both Lookout Pass and Marias
Pass late D1.


...Great Lakes through the Northeast...
Days 1-3...

Two separate storm systems will bring periods of moderate to heavy
snow through the weekend.

The first is associated with a rapidly amplifying shortwave which
will drop across the Great Lakes/Ohio Valley tonight before
sharpening into a closed low over Upstate New York Friday morning
and then lift into the Gulf of Maine Friday night. This feature
will become blocked by an amplified downstream ridge, forcing it
to crawl northward into the Canadian Maritimes Saturday. As this
happens, a subtropical jet streak will extend eastward from the
Four Corners into New England, with amplification and poleward
extension occurring as the mid-level trough deepens. This will
provide robust upper diffluence to promote deep layer ascent, and
as a primary surface low weakens across Ontario beneath the upper
low, secondary redevelopment is likely to occur near the coast of
Maine and then lift slowly northward as it becomes vertically
stacked. This evolution will spread widespread precipitation
across the eastern Great Lakes and Northeast Friday into Saturday.

As the primary low shifts eastward from near Lake Huron to the
Adirondacks and fills, it will drag a cold front eastward to the
New England coast. WAA ahead of this front will transport moisture
and aid in local ascent, producing an expanding area of
precipitation through D1. Behind the front, CAA across the still
warm Great Lakes will lead to a ramp up of LES in the favored W/NW
snow belts downwind of Lake Erie and Ontario. Although the most
intense ascent within the LES is progged to remain below the DGZ,
this should drive parcels upward into a deepening inversion as
lapse rates steepen, and a period of heavy LES is likely Friday
aftn into Saturday morning /D2/. The heaviest snow is forecast
near Erie, PA where upwind connection from Lake Huron will add
moisture, and WPC probabilities are high for 4+ inches, with
locally 10 inches possible. Elsewhere, WPC probabilities for LES
snowfall exceeding 4 inches are high in the Tug Hill Plateau and
moderate near Syracuse, NY.

Downstream of the low, moist advection in the presence of the deep
layer ascent will spread precipitation across Upstate New York and
New England. As the secondary low develops in the Gulf of Maine,
forcing for ascent will intensify and a weak TROWAL will attempt
to pivot into Maine before the entire system lifts northeast and
crawls across Canada. While the most intense synoptic lift will
displace eastward, strong upslope flow into the terrain of the
Adirondacks, Greens, and Whites will produce a prolonged period of
heavy snowfall within which rates will likely exceed 1"/hr. It is
this upslope that should produce the heaviest snowfall, and WPC
probabilities for 4+ inches are high in the NW Adirondacks, the
Greens, the NE Kingdom of VT, and the Whites of NH/ME, with
locally more than 12 inches likely in the highest terrain. Heavy
snow is also likely across northern Maine which will be in closer
proximity to the low itself, and WPC probabilities are high for 4
inches with more than 8 inches likely. Additionally, as the
secondary low strengthens quickly near the coast of Maine, the
guidance indicates that rapid cooling of the column within a
pivoting deformation axis may produce a burst of snow from SE NH
into eastern ME, much closer to the coast. This is low confidence
at this time, but if this occurs it could produce a quick few
inches of snowfall as snow rates become impressive in the presence
of folded theta-e lapse rates which could impact the I-95 corridor
north of Boston, MA.

As this first system pulls away Saturday, a second shortwave will
dig out of Saskatchewan and race southeast towards the Great Lakes
late Saturday and Sunday. This will be accompanied by a robust
upper jet streak, and the overlap of the LFQ and this sharpening
shortwave will likely produce a swath of precipitation with bands
of heavier snowfall possible. There remains considerable
uncertainty into the exact timing and placement of this feature,
but a swath of snowfall is likely from northern MN on D2 southeast
through the eastern Great Lakes on D3 where WPC probabilities for
2 inches are more than 30%. This system will likely be a fast
mover, but the setup appears favorable for a narrow band of
heavier snowfall, which is echoed by the WSO 90th percentile
showing pockets of 4-6".

For Days 1-3, the probability of significant icing is less than 10
percent.

Weiss