FOUS11 KWBC 280748
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 AM EDT Fri Mar 28 2025

Valid 12Z Fri Mar 28 2025 - 12Z Mon Mar 31 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 day or so.
However, even as the system lifts out, snow levels across the
Northwest will continue to drop 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 today, 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
and a break in precipitation on Day 2. 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. Day 1 WPC probabilities for
over 8 inches of snow are 60-80% for parts of the Olympics,
Cascades, Klamath Mountains, and the northern Sierra Nevada. Then,
the next system approaches the West Coast by Day 3 as the closed
upper low off the Pacific Northwest reloads from energy dropping
southward across the eastern Aleutians. This leads to additional
moderate onshore flow amid initial snow levels around 4000-5000ft
(up to 6000ft in the central/southern Sierra). WPC probabilities
for at least 8 inches of additional snow on Day 3 across the Sierra
and northern CA ranges are 40-70%, but are expected to increase as
the bulk of the precipitation moves onshore after 12z/Mon.

Meanwhile, moisture spreading inland across the northern Great
Basin and Rockies 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 and ending with a weak area of convergence on Sunday
between western MT and northern CO. 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. WPC Days 1-2
probabilities for at least 8 inches of snow are generally 50-70%
from the Blue Mts of northwest OR through the central ID ranges
into southwest MT, as well as the Absarokas and Wind River Range.
These probs also extend south to the Park Range and Medicine Bow
Mountains of CO/WY.

...Northern Minnesota through the Northern Great Lakes... Day 1...

A longwave upper trough in the West working in tandem with the
right-entrance region of a 250mb jet streak this 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. This WAA 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 (primarily
north of the U.S.-Canadian border) and ice starting as early as
this morning across far northern MN, but becoming more widespread
across northern MN/WI/MI by this evening. The strong WAA over sub-
freezing boundary layer temps is a favorable setup for freezing
rain/sleet that will make for hazardous travel conditions even
where only light amounts occur. WPC probabilities for at least
0.25" of ice accretion on Day 1 is medium (50-70%) across the
eastern U.P. and northern L.P. of MI, with 20-40% chances extending
westward across the U.P. and into the MN Arrowhead.

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

This same system is expected to spread a narrow swath of heavy snow
and area of mixed precipitation into the Interior Northeast and New
England by late D1, but with wintry precipitation lingering for
several days. This long-duration event made possible by a trailing
shortwave and developing central U.S. trough which will continue to
reinforce a favorable jet pattern for upper divergence as well as
sufficient mid-upper level moisture content. However, the snowfall
threat peaks early Saturday morning as a jet max passes to the
north of New England and 925-850mb WAA is at it's strongest. 00z
HREF depicts 30-40% chances for 1-2"/hr snowfall rates around
13z/Sat across far northern NY and northern VT/NH. WPC snowfall
probabilities for at least 6 inches of snow on days 1-2 are medium
(40-60%) from northern NH, VT, and into central ME.

South of this heavy snow threat between the northern Adirondacks
and much of central/southern VT and NH, 850-750mb warm nose allows
for a transition area of sleet/freezing rain. After the brief
burst of heavier precip early Sat, forcing weakens and there is
left a weak stream of moisture overrunning a CAD signature due to a
strong 1030mb High situated over the James Bay. This potentially
significant icing threat is expected to linger through Sun morning
until the surface low across the Midwest begins to strengthen and
push warmer air northward across New England by Day 3. However, the
CAD signature in place will continue to support freezing rain
across the typically colder valley locations and areas banked along
the Green/White mountains. WPC probabilities for the entire
forecast period (ending 12z/Mon) are high (70-80%) for at least
0.25" of ice accretion from the NY border with eastern Ontario
through the northern Adirondacks and into central VT/NH.
Probabilities for at least 0.5" also impressive and 40-60% (highest
across western Jefferson county of NY).

...Northern High Plains through the Upper Midwest/Great Lakes... Days 2-3...

A shortwave trough moving into the Southwest today 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
and potent 700mb closed low 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
southeastern Montana and northeastern Wyoming into parts of
western to central SD and NE. The highest amounts are forecast to
fall across the Black Hills, but locally heavy totals are also
possible eastward along southern SD and north- central NE. WPC
probabilities for at least 4 inches are 40-70% across the Black
Hills and 10-30% into south-central SD and north- central NE.

As this system develops further Saturday night through Day 3,
increasing WAA and a blossoming precipitation shield is expected to
spread into the Upper Midwest and Great Lakes. Surface low pressure
is expected to track along along the 925-850mb front that remains
stationary from NE and southern MN on east through Michigan. There
still remains some discrepancies in guidance on the evolution/strength
of this storm system, but there is a growing consensus with the
00z guidance on the synoptic scale. The GFS finally has a more
amplified solution (in line with the CMC and ECMWF), which paints
an axis of snow fall extending east-northeast from SD through
central MN into northern WI and the MI U.P.. There is also higher
confidence in yet more freezing rain/sleet from northern and
central WI on east through northern Michigan as high pressure
remains locked in place over James Bay and cold northeast flow
continues to filter into the Upper Midwest/Great Lakes. This is a
very favorable setup for potentially significant freezing rain from
northern WI through northern MI, especially for late-March. WPC
probabilities show moderate chances (30-60%) for ice accumulations
>0.25" in these areas with low chances (10-30%) for >0.5" across
parts of northern WI. In terms of snowfall, WPC probabilities show
moderate chances (30-60%) for snowfall totals >6" in far northern
WI and the western MI U.P..

Snell
$$
--- SBBSecho 3.20-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)