FOUS11 KWBC 210737
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 AM EST Sat Dec 21 2024

Valid 12Z Sat Dec 21 2024 - 12Z Tue Dec 24 2024

...Northeast...
Day 1...

Periods of snow on the backside of a departing winter storm
heading for Nova Scotia is expected to linger over Downeast Maine through
Saturday morning before finally concluding Saturday afternoon. WPC
probabilities show low chances (10-30%) for additional snowfall
totals >2" through Saturday morning. Farther west, cyclonic flow
over the Great Lakes will keep some lake-enhanced bands lingering
through the day with additional snowfall amounts of 1-3" expected
in parts of northeast OH, near the Finger Lakes of NY, and as far
south as the central Appalachians. Snow showers will taper off by
Saturday evening as high pressure builds in from the west.

...Pacific Northwest & Northern Rockies...
Days 1-3...

A trio of Pacific storm systems will escort rounds of Pacific
moisture into the Olympics and Cascades into early next week. Some
of this moisture will spill over into the Northern Rockies. Snow
will generally be confined to the higher elevations of these
mountain ranges due to flood of Pacific air infiltrating much of
western North America and keeping any frigid Canadian air-mass
intrusions at bay. For the Olympics and Cascades, the heaviest
snowfall is likely to be located at/above 4,500ft in elevation. The
first storm system arrives Saturday morning with locally heavy
snowfall possible as far south as the tallest peaks of northern
California. The heaviest snowfall from this event for the Cascades,
Olympics, and Blue Mountains comes Saturday afternoon and tapers
off Saturday night thanks to the storm's progressive movement.
Following a brief break Sunday morning, the next round of snowfall
arrives Sunday afternoon in the Cascades with the heaviest
snowfall occurring Sunday night. Then, following another break
during the day Monday as high pressure briefly builds in, snow
returns to the Olympics and Cascades Monday night. Guidance
suggests sharper height falls aloft and a weak CAD signature in
the Columbia Basin that may result in lower elevations snow/ice
east of the Cascades. Through 12Z Tuesday, WPC probabilities show
high chances (>70%) for snowfall >8" in elevations >5,000ft in
parts of the Olympics, Cascades, and as far east as the Blue Mountains.

Some of this Pacific moisture will also result in high elevation
snow in parts of the Bitterroots, Sawtooth, Boise, Absaroka, Lewis, and
Teton Mountains. Of the ranges referenced, the Sawtooth and Teton
Ranges above 7,000ft feature moderate-to-high chances (50-70%) for
snowfall totals >8" through early Tuesday morning.


...Northern Plains to the Great Lakes...
Days 2-3...

The first in a series of Pacific Northwest storm systems will make
its way into Montana while the divergent left-exit region of a
110kt 250mb jet streak moves in over the Northern Plains. Southerly
flow at the lowest levels of atmosphere will intersect a frontal
boundary that gives way to weak vertical ascent over parts of North
Dakota and central Minnesota. The moisture source and DGZ aloft is
marginal, while 850mb temps rise >0C despite surface temps
remaining below freezing. This should lead to light icing
accumulations from northeast Montana and northern North Dakota to
central Minnesota Sunday afternoon and into Sunday night.

By Monday morning, a more consolidated surface low will form in
the Upper Mississippi Valley at the same time 925-850mb moisture
(with origins out of the Gulf of Mexico) streams into the Great
Lakes. 850mb WAA over a boundary layer air-mass that is cold and
quite dry will support wet-bulb temperatures that support mostly
snow from the Michigan U.P. to the northern half of Michigan's
Mitten. Broad 250-500mb troughing will also provide adequate lift
atop the atmosphere, giving rise to a more solid shield of
snowfall over the region. WPC probabilities show moderate chances
(40-60%) for snowfall totals >4" in the far northern portions of
Michigan's Mitten and in the eastern-most areas of Michigan's U.P.
In fact, there are some low-chance probabilities (10-30%) for
localized amounts >8" in the eastern Michigan U.P.

This same moisture source, synoptic-scale forcing, and isentropic
glide will translate over the eastern Great Lakes Monday night
with potentially 1-3" of snowfall along the Chautauqua Ridge and
along the Tug Hill. Localized amounts could top 4" along the Tug
Hill where some upslope enhancement would allow for slightly
heavier snowfall rates and thus higher totals than their neighbors
in western NY through Tuesday AM.


The probabilities for significant ice accumulations are less than
10 percent.

Mullinax
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