FOUS11 KWBC 250801
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Sun Feb 25 2024

Valid 12Z Sun Feb 25 2024 - 12Z Wed Feb 28 2024

...Pacific Northwest and California to the northern and central Rockies...
Days 1-3...

A strong winter storm and cold front will begin to impact the
northwestern U.S. later today, before progressing further
southeast Monday and Tuesday -- producing widespread heavy
mountain snow and dangerous travel conditions across the region.

A dynamic shortwave trough, currently positioned over the Gulf of
Alaska, is forecast to dive southeast -- carving out an amplified
trough across British Columbia into the Pacific Northwest later
today and through the overnight. Increasing onshore flow ahead of
the wave will support heavy snow developing initially over the
Olympics, northern Cascades, and the northern Rockies, before
spreading south along with a sharp cold front that will begin to
plunge southeast across the Northwest by this evening. Snow is
expected to quickly accumulate, with guidance continuing to show
snowfall rates of 1-2 in/hr developing over the northern Cascades,
and portions of the northern Idaho and the northwestern Montana
Rockies by the afternoon, with these rates continuing as the snow
edges south during the evening and overnight hours. By early
Monday, WPC PWPF continues to indicate that accumulations of a
foot or more will be common across the northern Cascades and the
northern Idaho and northwestern Montana ranges -- likely impacting
travel as snow levels drop below pass level.

The upper trough will continue to amplify and dig southeast --
pushing its associated cold front into Northern California, the
Great Basin, and the central Rockies by late Monday. This will
bring areas of snow through the Oregon Cascades and into the
Sierra Nevada, as well as across the remainder of the northern and
into central Rockies. WPC PWPF shows that widespread snow
accumulations of 6 inches or more are likely across these areas on
Monday into early Tuesday. Supported by strong onshore flow and
favorable upper forcing, portions of the Oregon Cascades are
likely to see over a foot of snow during this period. The WPC
PWPF also indicates that amounts of a foot more are also likely
along the Tetons, as right-entrance upper jet forcing helps to
increase ascent across western Wyoming late Monday.

As it plunges southeast across the region, snow squalls are likely
along the path of the cold front, especially over the northern
Great Basin and Rockies on Monday. Brief, but intense snowfall,
along with strong gusty winds may rapidly reduce visibility and
contribute to dangerous travel conditions.

While amounts will be less than the previous day, persistent
onshore flow, along with trailing energy dropping into the base of
the broader scale trough, will support additional snowfall from
western Washington State to the northern Rockies. Snow levels,
which are already expected to be below 500 ft across much of the
region early Monday, will dip even further, bringing the potential
for accumulating snow to the coast and onto the valley floors from
Washington State through the Intermountain West Monday into
Tuesday.

On Tuesday, drier conditions will spread from west to east across
California and the Great Basin as the upper trough begins to pivot
east and is followed by a shortwave ridge that will move across
the West beginning late in the day. However, heavy snow will
continue into late Tuesday for portions the central Rockies.
Additional accumulations of 8 inches or more are likely for many
of the western Colorado ranges and into far north-central New
Mexico along the San Juans before ending Wednesday morning.

By early Wednesday, the ridge in the Northwest will begin to give
way to the next approaching upper trough, with warm advection
precipitation and the threat for heavy snow returning to the
Olympics and the northern Cascades. Although snow levels will be
rising across western Washington, the Cascade passes are likely to
be impacted by several more inches of snow late Tuesday into
Wednesday.

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

On Monday, light snow will spread east of the Rockies, with a band
of potentially heavier snow setting up across eastern North Dakota
into northern Minnesota Monday night into Tuesday. While the
general model consensus does not indicate widespread heavy amounts
at this point, there is good signal for a potential narrow band of
heavy snow, supported by an upper jet couplet and low level
frontogenesis, setting up across the region.

This initial band is expected to be followed by a second band,
setting up a little further to the east across northern Wisconsin
and the U.P. of Michigan as a secondary wave developing along the
trailing cold front, becomes the primary low and lifts across the
region Tuesday night. WPC PWPF shows high probabilities for
accumulations of 4 inches or more extending from northwestern
Wisconsin into the western U.P.

Pereira

*** Key Messages for Significant Western U.S. Storm ***

--A strong winter storm and cold front will move into the
Northwest later today and progress southeastward on Monday into
the Northern Rockies.

--Heavy mountain snow over the Cascades will impact the passes by
late tonight, where there is >80% chance of at least a foot of
snow above 1500 ft through early Tuesday.

--Snowfall will become heavy at times with rates of 1-2"/hr along
with strong winds, creating areas of blowing and drifting snow as
well as significantly reduced visibility.

--Snow squalls are likely along the path of the cold front on
Monday over the northern Great Basin/Rockies, which could create a
rapid drop in visibility and a flash freeze on roads leading to
dangerous travel.

--Much colder air behind the strong cold front will drop
temperatures into the teens and colder by Tuesday morning.


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