Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
224 PM EDT Sun Mar 30 2025

Valid 00Z Mon Mar 31 2025 - 00Z Thu Apr 03 2025


...Northern Great Lakes & Northeast...
Day 1...

The winter storm is in its final hours of producing hazardous
impacts across the northern Great Lakes and northern New England.
On the backside of the storm, periods of snow are ongoing from
northern Iowa and southern Minnesota to northwest Wisconsin and
the western Michigan U.P.. Freezing rain is ongoing in parts of
north-central Wisconsin, the eastern Michigan U.P., and from the
White Mountains of New Hampshire on north into Maine. As the storm
heads northeast towards Lake Huron tonight, freezing rain/sleet in
the eastern Michigan U.P. and north-central Wisconsin will
changeover to snow as the deformation zone moves in overhead. The
CAD signature over New England will linger over the White Mountains
and much of Maine (sans the coastal areas) to keep an icy wintry
mix into the early morning hours on Monday. By 12Z Monday, snow
will finally be coming to an end across the northern Great Lakes
while it may take until midday for far northern Maine to finally
see the wintry mix transition to a plain/cold rain.

WPC probabilities show moderate-to-high chances (40-70%) for an
additional >4" of snowfall for areas along the shore of Lake
Superior in the MI U.P, the Huron Mountains of the Michigan U.P.,
and into far northern Wisconsin. In terms of additional freezing
rain, WPC probabilities show low-to-moderate chances (20-50%) for
additional ice accumulations >0.25" in the eastern Michigan U.P.,
the White Mountains, and far western Maine bordering Quebec.
Hazardous
travel conditions will linger through Monday morning with the
potential for additional power outages in the areas that could see
an additional 0.25" of ice.


...The West...
Days 1-3...

An active stretch of wintry weather has begun from the West Coast
to the Front Range of the Rockies through Tuesday due to the
prolonged influence of a deep longwave trough over the western
U.S..

This afternoon, focus shifts to an exceptional closed upper low in
the northeast Pacific that slowly makes its way east towards the
Pacific Northwest by Monday night. Heavy high elevation mountain
snow (above 6,000ft in the northern CA ranges and above 5,000ft in
both the Cascade and Blue Mountains) will be underway, but as the
upper low inches closer to the Northwest U.S., falling heights and
colder temperatures aloft will force snow levels to lower into
Monday. By 12Z Monday, NAEFS shows just about all mandatory height
levels listed are bottoming out below the 2.5 climatological
percentile while IVT values >500 kg/m/s are aimed at southern
California. The northern extent of the IVT will sport values >90th
climatological percentile through the Sierra Nevada and into the
Great Basin by Monday afternoon. From northern California to the
northern Rockies, the divergent left-exit region of a 200mb jet
streak whose wind speeds (>130 kts) are above the 99th
climatological percentile will be placed directly overhead.

The best moisture advection and upslope enhancement will be seen
from California's Siskiyou and Salmon mountains on south and west
along the spine of the Sierra Nevada. On Monday afternoon and into
Monday night, the flood of Pacific moisture aloft will advance well
inland through much of the Intermountain West. Just about every
notable mountain range in the central and northern Rockies is
likely to see measurable snow Monday night into Tuesday. Meanwhile,
cyclonic flow around the upper low will still support ongoing
upslope snowfall in the central and northern California ranges
through Tuesday night. Then by early Tuesday the parent upper low
is expected to dive southeast across southwest OR and northern
California, proving a final surge of Pacific moisture within an
area of cold air aloft. The bulk of the Pacific moisture advection
will be finished Tuesday night and into Wednesday, but the core of
the longwave trough will be over the Rockies with NAEFS showing
500mb temps that are below the 10th climatological percentile from
the Sierra Nevada to the Front Range of the Colorado Rockies. With
the help of daytime heating, expect numerous snow showers to
envelope most of the western U.S. mountain ranges given the steep
lapse rates.

Through 00Z Thursday, WPC probabilities show the northern ranges
of California (Siskiyou, Salmon, Shasta) and the length of the
central and northern Sierra Nevada have high chances (>70%) for
snowfall totals >24". Most of these ranges above 6,000ft are
currently forecast to see localized amounts between 3-5 feet
through 00Z Thursday. Heavy snow also reaches into the Oregon
Cascades with high probabilities for at least 12". Farther inland,
many Intermountain West ranges (Sawtooth, Blue, Uinta, Bear River,
Wind River, Absaroka, Big Horn, and CO/WY central Rockies) are all
likely to see snowfall totals of 6-12" with locally higher amounts
in these ranges as well. Expect hazardous travel conditions for
many road ways that remain open in these mountain ranges.


...Northern Plains and Upper Midwest...
Day 3...

By Tuesday morning, the upper trough responsible for the barrage of
mountain snow in the West will position the 250mb jet streak's
divergent left-exit region over the Great Plains. Modest 500mb PVA
and strengthening WAA at low levels will give rise to lee
cyclogenesis east of the Rockies. Throughout the day Tuesday a
strengthening LLJ, (NAEFS shows 850mb winds reaching the 97.5
climatological percentile 18Z Tuesday from north Texas to eastern
Kansas), will deliver both increased moisture and increasing WAA
aloft. The upper trough to the west will also direct an IVT of
300-500 kg/m/s into the Upper Midwest, which is topping the 90th
climatological percentile Tuesday afternoon. There appears to be
sufficiently cold enough air present that precipitation will
transition over from rain to snow from the Black Hills on east
across South Dakota and as far east as central Minnesota Tuesday
afternoon that is largely due to strong 300K isentropic ascent and
850-700mb WAA resulting in FGEN at those mandatory height levels.

There are ongoing differences in the position and strength of this
winter storm. The GFS/CMC camps sports a deeper and more northerly
track storm system. The ECMWF and its AI counterpart (EC-AIFS) are
farther south and take a little longer to ramp up the storm system.
The GEFS is on the slower side of the upper trough's progression,
allowing it to be deeper and phase more effectively with nearby
shortwave troughs. The GEPS is more in between the ECENS, with the
latter being a little flatter and taking longer for the trough to
deepen, thus forcing the storm's more southerly track initially.

The ECMWF EFI continues to depict an increasing signal for a
potentially disruptive winter storm from the Dakotas on east across
northern Minnesota. This also aligns fairly well where WSO values
>50% exist on Days 3-4. Both snow amounts, along with snow load
and blowing snow, are likely to be hazards that the Northern Plains
and Upper Midwest contend with through Wednesday, with freezing
rain most likely on the eastern edge of the WAA across northern
Wisconsin and northern Michigan. Current WPC snowfall probabilities
(through 00Z Thursday) for >6" are moderate- to-high chance
(50-80%) across the eastern Dakotas and northern Minnesota. There
are also low-to-moderate chance probabilities (20-40%) for snowfall
totals >12" along the Minnesota Arrowhead. Residents and those
traveling to/from these regions should keep close eyes on the
forecast as additional changes in the types of impacts, the
severity of those impacts, and where/when these impacts occur are
likely to fluctuate for another day or so.


Mullinax