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FOUS11 KWBC 272037
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
437 PM EDT Thu Mar 27 2025

Valid 12Z Thu Mar 27 2025 - 12Z Sun Mar 30 2025


...West Coast through Northern/Central Rockies...
Days 1-3...

A deep, slow-moving low centered off of the Northwest coast will
gradually fill as it lifts to the north over the next couple of
days. However, even as the system lifts out, snow levels across the
Northwest will continue to drop Friday into early Saturday as a
series of shortwave troughs digging south of the low move inland.
This will bring levels down to around 3000-4000ft in the Washington
and Oregon Cascades on Friday, before dropping below 3000ft with
the passage of a well-defined trough Saturday morning. Further to
the south, snow levels starting above 5000ft in the Shasta-
Siskiyou region and the northern Sierra Nevada will drop below
4000ft in many locations before precipitation ends. Although snow
levels will be on the decline, so will the onshore moisture
advection, with guidance showing IVTs dropping off after the start
of the period. The heaviest precipitation is expected to center
across southern Oregon and far northern California, where the best
moisture and forcing is expected to focus later today. By early
Saturday, precipitation is expected to diminish from south to north
as an upper ridge moves inland in the wake of the previously-noted
trough. Probabilities for 72-hr accumulations exceeding 8in are
over 50 percent for parts of the Olympics, Cascades, Klamath
Mountains, and the northern Sierra Nevada.

Meanwhile, moisture spreading inland will begin to interact with a
developing low-to-mid level baroclinic zone and periods of
enhanced forcing associated with the inland moving troughs to
produce light-to-moderate precipitation, including mountain snow
from the northern to the central Rockies. Precipitation coverage is
expected to increase Friday night, with unsettled weather
continuing through the weekend. Snow levels beginning above 5000ft
in the northern Rockies Friday night are forecast to dip below
4000ft in some location by late Saturday. In the central Rockies,
snow levels will drop to 7000-8000ft in the Colorado ranges and
around 5000-6000ft across northern Utah. Probabilities for 72-hr
accumulations exceeding 8in are above 50 percent across portions of
the northern and central Rockies from western Montana to
northwestern and north-central Wyoming and from south-central
Wyoming to north-central Colorado.

...Northern High Plains and Black Hills...
Days 2/3...

A shortwave trough moving into the Southwest on Friday is expected
to lift northeast from the Four Corners across the central Rockies
and into the High Plains Saturday morning. Moisture fed into an
area of strong ascent generated in part by favorable upper jet
forcing will support precipitation north of an associated surface
low that will develop and move east of the Colorado Rockies. Rain
changing to snow is expected from southweastern Montana and
northeastern Wyoming into parts of western to central South Dakota
and Nebraska. The heaviest amounts are forecast to fall across the
Black Hills, where totals exceeding 4in are likely.

...Upper Midwest/Northern Great Lakes/Northeast...
Days 1-3...

A longwave upper trough in the West working in tandem with the
right-entrance region of a 250mb jet streak Friday afternoon will
foster large-scale ascent aloft throughout the Upper Midwest and
Great Lakes. The upper trough and a strong low-level ridge of high
pressure off the Southeast coast supports a strengthening SWrly
low-mid level flow regime that results in exceptional warm-air
advection (WAA) for the regions reference above. The WAA from the
Upper Midwest and Great Lakes to interior New England will clash
with a cold Canadian air-mass anchored by a 1030mb+ high pressure
system over Ontario and Quebec. The result is 925-700mb FGEN that
produces a narrow band of disruptive snow and ice starting Friday
afternoon from northern Minnesota to the Upper Great Lakes,
followed by interior New England`s start time of Friday night. The
strong WAA over sub- freezing boundary layer temps is a favorable
setup for freezing rain/sleet that will make for hazardous travel
conditions.

By Saturday morning, a brief break in the action arrives from
northern Minnesota to the Upper Great Lakes, but periods of
mountain snow and valley ice will linger throughout much of the day
in New England. WPC probabilities for this event show moderate
chance probabilities (40-60%) for >0.25" of ice accumulation over
the tip of Michigan`s Mitten and the far east section of Michigan`s
U.P.. Some instances of scatter power outages and tree damage
cannot be ruled out here. Meanwhile, there is an expansive area of
moderate- to-high chance probabilities (50-70%) for >0.1" of ice
from northeast Michigan, northern Wisconsin, and for the rest of
northern Michigan. Snow will be the primary hazard Friday night and
into Saturday morning over interior New England. WPC probabilities
show moderate-to-high chances (50-70%) for snowfall totals >4"
from northern Vermont (excluding the Champlain Valley) on east
through northern New Hampshire and west-central Maine. The peaks
of the White Mountains could see snowfall totals top 8". WSSI-P
does show some moderate chances (40-60%) for Minor Impacts in these
affected areas of Interior New England.

This active pattern reloads over the Central Plains on Saturday as
the longwave trough in the West ejects a shortwave disturbance
over the Rockies. This feature will spawn a new wave of low
pressure along the 925-850mb front that remains stationary from
Nebraska and southern Minnesota on east through Michigan. There
remains some discrepancies in guidance on the evolution/strength of
this storm system. The ECMWF SATs MSLP guidance shows the low
organizing over eastern Kansas that heads into eastern Iowa by
Sunday morning. Ultimately, the 290K isentropic ascent and strong
WAA mechanisms that supported the first round of wintry weather
will still be present Saturday night and into Sunday from the
Midwest to New England. The ECMWF-AIFS does show the potential for
a deformation zone on the backside of the storm over eastern South
Dakota and into central Minnesota, although thermodynamic profiles
are not sold yet on which areas can dynamically cool enough to stay
all snow. There is higher confidence in yet more freezing
rain/sleet from northern and central Wisconsin on east through
northern Michigan and as far east as northern New England. WPC
probabilities show moderate chances (40-60%) for ice accumulations
>0.1" in these areas with low chances (10-30%) for >0.25",
especially in the more elevated terrain. In terms of snowfall, WPC
probabilities show low-to-moderate chances (30-50%) for snowfall
totals >4" in far northern Wisconsin and the western Michigan U.P..

Pereira/Mullinax


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