FOUS11 KWBC 300721
QPFHSD

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

Valid 12Z Sun Mar 30 2025 - 12Z Wed Apr 02 2025

...Upper Midwest, Northern Great Lakes, and Northeast... Day 1...

Ongoing prolonged late-season ice storm continues across portions
of northern WI into the northern MI L.P. and central/eastern U.P.,
as well as New England today. A negatively tilted 250-500mb trough
axis working in tandem with the strongly diffluent right- entrance
region of a 250mb jet streak has organized an area of low pressure
tracking across the Great Lakes today. Sufficiently cold
temperatures remain locked in place to the north of the robust
925-700mb FGEN warm front that is oriented W-E from northern WI to
as far east as New England. This is all thanks to a 1030mb+ high
pressure system over southeastern Canada that is supporting the
ongoing icy setup. As strong SWrly winds overrun the sub-freezing
air-mass, a >0C warm nose protruding into the 850-700mb will cause
precipitation to fall in the form of freezing rain and sleet
throughout the Upper Midwest, Upper Great Lakes, the northern
Appalachians, and New England into the end of Day 1. By Monday
morning the low pressure system is forecast to push into southeast
Canada and surge above freezing temperatures throughout the entire
East Coast, with precipitation also gradually ending across the
Great Lakes. While ice will be the more commonly observed winter
weather hazard, there will be light to moderate snow beneath the
deformation zone on the northern and western flanks of the 850mb low.

Periods of snow will be ongoing across central MN into northern WI
and the MI U.P. this morning, as well as wrapping around the
western side of the low as far south as northwest IA. South of the
snow axis, freezing rain and sleet will be common in northern
Wisconsin, the eastern Michigan U.P., across the tip of Michigan's
Mitten, and into interior New England. The 0C surface line will
gradually lift northward throughout the day east of the low
pressure, but will be stubborn to lift across northern MI and
northern New England. The deformation axis will continue its swath
of snow from southeast Minnesota to central Wisconsin Sunday
afternoon, but it will be progressive and snow falling during the
daytime hours will be tougher to accumulate. Snow within the
deformation axis may continue over northern Michigan Monday morning
before finally exiting to the northeast over Ontario by Monday afternoon.

WPC probabilities show moderate-to-high chances (60-80%) for >6"
of snowfall for areas along the shore of Lake Superior in the MI
U.P.. In terms of freezing rain, WPC probabilities show moderate
chances (40-60%) for additional ice accumulations >0.25" in
northern Wisconsin, northern Michigan, and in the New England
mountain ranges such as the Greens and especially the Whites.
There is also an area of low chances for an additional >0.5" of
ice accumulation as far eastern MI U.P.. All of these areas can
anticipate hazardous travel conditions with the potential for
additional power outages in those areas sporting better chances
for >0.50" of total ice accumulation.

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

An active stretch of wintry weather is expected from the West
Coast to the Front Range of the Rockies through Monday due to
persistent troughing. A pair of shortwaves traversing the Rockies
early this morning is responsible for ongoing mountain snow from
as far north and west as the Absaroka and Bitterroots through the
Wyoming ranges and on south to the Colorado Rockies. As high
pressure builds in over the Canadian Prairies today, easterly
upslope flow and residual moisture aloft will foster additional
mountains snow in the Black Hills and Big Horns. Some lower
elevations snowfall is expected during the day on Sunday as well,
but accumulations will be minor due to the late season sun angle
limiting accumulations. WPC probabilities do depict moderate
chances (40-70%) for snowfall totals >6" in the highest elevations
of the Absaroka, Wind River, and northern Colorado Rockies.

By 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 cause 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. Then by early Tuesday the parent upper low is
expected to dive southeast across southwest OR and northern CA,
proving a final surge of Pacific moisture within an area of cold
air aloft. From northern California to the interior Northwest, the
region will also be located beneath the divergent left-exit region
of a 200mb jet streak whose wind speeds (>130 kts) are above the
99th climatological percentile. With the IVT located farther south,
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.

A very wintry few days are coming to the mountain ranges of the
western U.S.. Through 12Z Wednesday, 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 3-4
feet through 12Z Wednesday. Heavy snow also reaches into the OR
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 above the 90th
climatological percentile 18Z Tuesday from north Texas to the Red
River of the North), will deliver both increased moisture and
increasing WAA aloft. There also 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 night.

There are ongoing differences in the position of this developing
deformation axis. All three deterministic guidance members
(EC/GFS/Canadian) along with the EC- AIFS all show this storm
growing in strength and size heading into Day 4, but they do this
in different ways both synoptically and on the mesoscale as well.
The ECMWF EFI does show a growing signal for a potentially
disruptive winter storm from the Dakotas on east across Minnesota
and into the northern Great Lakes, which is in line with where WSO
values of 30-80% 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 WI and MI. Current WPC snowfall probabilities (through 12Z
Wednesday) for >4" are 40-80% across the eastern Dakotas and
central MN, with a majority of the snow expected into Day 4.
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 the rest of the weekend.

Snell/Mullinax
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