Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
222 PM EST Tue Jan 23 2024

Valid 00Z Wed Jan 24 2024 - 00Z Sat Jan 27 2024

...Northeast...
Days 1-3...

A longwave troughing pattern over the western and central U.S.
will direct a series of upper level disturbances at the Great
Lakes and Northeast most of the work-week. This afternoon and into
tonight, sufficiently cold boundary layer temperatures anchored in
place by high pressure over Quebec will coincide with 850mb warm
air advection and 290K isentropic ascent over the Great Lakes and
Northeast. This will result in periods of snow over portions of
the Adirondacks, Green, and Berkshires that result in generally
light snowfall amounts. WPC PWPF sports high chances (>70%) for
snowfall totals >2" along some of these mountain ranges. What is
most notable are the ice accumulations as the high pressure to the
north will provide just enough of a cold air damming signature
(CAD) to favor freezing rain over snow. This will result in
hazardous ice accumulations with detrimental impacts to travel.
WPC PWPF shows moderate chances (40-60%) for ice accumulations
>0.1" in the Catskills tonight and through Wednesday morning. As
the >0C warm nose at low levels rushes north, snow will transition
over to freezing rain in the northern Appalachians late Wednesday
into Wednesday night. WPC PWPF shows moderate chances (40-60%) for
ice accumulations >0.1". Probabilities for >0.25" are generally
less than <10% for >0.25", which is why impacts will generally be
confined to travel and not infrastructure (such as power outages
or tree damage).

The second of three upper level disturbances will spawn a weak
wave of low pressure as it track north through the ArkLaTex this
afternoon and into the Middle Mississippi River Valley by
Wednesday morning. The air-mass will be marginally cold enough to
potentially cause a light glaze of ice (WPC PWPF shows
moderate-to-high chances (50-80%) for >0.01" from central Kansas
to southern Wisconsin and northern Illinois). This may lead to
some slick spots, but the strong surge of >0C air within the
boundary layer will eventually cause a switch over from freezing
rain to plain rain, helping to minimize the expected impacts in
affected areas. This storm system will bring another round of
wintry mix to northern New England Wednesday night with low
chances (10-30%) for ice accumulations >0.1" in the Green and
White mountains.

By Thursday afternoon, a passing cold front will bring about an
injection of cold Canadian air that becomes anchored yet again by
high pressure to the north. To the west, a third upper level
disturbance spawning yet another area of low pressure over the
lower Great Lakes will escort one more round of moisture and
strong 850mb warm air advection into the Northeast. With the
colder air-mass more firmly in place than Wednesday night, this
will allow for heavier snow and ice amounts to ensue over northern
New England Thursday night into Friday morning. WPC PWPF sports
low-to-moderate risks (30-50%) for >0.1" of ice accumulation in
the White Mountains and in western Maine, while far northern Maine
has moderate chances (40-60%) for snowfall totals >4". Some minor
impacts to travel are possible on Friday in affected locations.

...Western U.S....
Days 1-3...

A series of Pacific disturbances will deliver sporadic rounds of
Pacific moisture across the western third of the U.S. the
remainder of the work-week. The region that will witness the
heaviest snow over the next few days is the Pacific Northwest, and
more specifically, the Washington Cascades. They are the one
region closest to the Pacific storm systems ushering in the
greatest concentration of 850-700mb moisture through Friday.
However, there is a noticeable lack of sub-freezing temps
throughout most of the West, forcing snow levels to be higher and
SLRs to be lower. Wednesday appears to be the snowiest day in the
Pacific Northwest, then the slug of moisture will traverse the
Intermountain West late Wednesday into Thursday. To speak to the
lack cold air and duration of heavy snowfall, 72-hour WPC PWPF
shows high chances (>70%) for snowfall totals >8" in the
Washington cascades and tallest peaks of the Olympics. Otherwise,
over the next 72-hours, the tallest peaks of the Oregon Cascades,
Bitterroots, Tetons, central Sierra Nevada, and Wasatch sport low
chances (10-30%) for >8" of snowfall.

Mullinax