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FOUS11 KWBC 041921
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
221 PM EST Sun Jan 4 2026

Valid 00Z Mon Jan 05 2026 - 00Z Thu Jan 08 2026


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

Over the next few days, two back-to-back progressive shortwave
troughs will bring a combination of snow and ice to the Upper
Midwest, Great Lakes, and interior Northeast.

The first disturbance is making its way over the Upper Midwest and
this afternoon and will be positioned over the Great Lakes tonight.
500mb height falls, sufficient PVA, and the left-exit region of a
strengthening jet streak will produce deep layer ascent. At the
same time, increasing and impressive 925-700mb WAA will spread
northeast ahead of the shortwave trough, creating a narrow FGEN
band over northern WI and the MI U.P. this evening. Moisture
associated with this feature comes from the highly active Pacific
jet stream pattern, reflected by NAEFS PWs that are above the 97th
percentile. With these synoptic and mesoscale ingredients at play,
a narrow band of heavy snow will develop which, according to latest
12Z HREF guidance, show high chances (>70%) of producing at least
1"/hr snowfall rates from far northern WI to the southern periphery
of MI's U.P. and the Tip of MI's Mitten this evening. Duration
will be limited given the upper-level disturbance's progressive
nature, but WPC probabilities for 4+ inches are above 50% from the
central MI U.P. to the tip of MI's Mitten, with locally 6-8"
possible (30-50% chance) in portions of the eastern U.P..

Farther south, an icy wintry mix is likely as WAA produces a warm
nose to around +3C at 850-750mb. The refreezing layer is quite deep
below 850mb, so some sleet may help to cut into the freezing rain
accumulations. Nevertheless, hazardous travel is expected this
evening from northern MN through central WI and central MI where
WPC probabilities for 0.01" of ice are 50-70%.

This first shortwave trough will race into the Northeast before
exiting to the Atlantic Monday night, but snowfall is expected to
be light. Snowfall totals over northern NY and as far east as the
Green Mountains are likely to receive 1-3" of snowfall, although
locally higher in the Tug Hill Plateau due to lake enhancement is
possible.

As snow concludes across the Northeast Monday afternoon, the next
Pacific shortwave reaches the Corn Belt Monday night. This
disturbance will have sufficient synoptic-scale forcing thanks to
a strengthening subtropical jet streak that strengthens to the tune
of 140 kts. Left-exit region dynamics and height falls downstream
of the shortwave will spawn low pressure over eastern NE Monday
evening, with the low then tracking east across the Midwest
Tuesday morning and over southern Ontario by Tuesday evening. There
remains some uncertainty on the placement of the heaviest QPF, but
increasing ascent will foster another round of wintry
precipitation from Minnesota through the Great Lakes Monday night
into Tuesday. As the low reaches southern Ontario Tuesday evening,
additional mixed precipitation spreads across New England Tuesday
night and into Wednesday. Northern New England has the better odds
for light-to- moderate snowfall on Wednesday as a coastal low may
support a deformation zone of snowfall that produces 1-4" of
snowfall.

While the first system had a fair amount of snowfall over the Upper
Great Lakes, the second system from the Midwest and Great Lakes to
the interior Northeast will largely be an ice producer. WPC
probabilities indicate a high risk (>70% chance) of at least 0.01"
of ice from central MN near the Twin Cities through the heart of
the Great Lakes and into the higher terrain of the interior
Northeast. The greatest risk for at least 0.1" of ice is likely in
parts of central MN, northern WI, and the higher elevations of the
Catskills, Adirondacks, and southern Greens.


...Western U.S. Mountain Ranges...
Days 1-2...

The active west continues into the first half of the week as an
anomalous trough persists just off the Pacific coast through
Tuesday morning before amplifying into a closed low that heads
towards Baja by Wednesday. This should conclude the snow in the
Sierra Nevada by Tuesday, but lingering snow in the Northern
Rockies will continue into Wednesday.

Downstream of this trough, nearly continuous S/SW flow will shed
spokes of vorticity onshore, with the accompanying height falls/PVA
leading to rounds of precipitation across much of the western
CONUS. At the same time, a subtropical jet will amplify and waver
from CA through the Central Plains, providing additional
synoptically-induced ascent. The combination of persistent SW mid-
level flow will foster elevated IVT today and into Monday,
resulting in PW plumes that occasionally exceed the 90th
climatological percentile. While periods of enhanced ascent
will differ across different regions, the next 36-48 hours appear
to be synoptically forced with broad large-scale ascent and
through upslope enhancement. The strongest upslope flow continues
to be appear focused over the Sierra Nevada through tonight where
snowfall will be prolific at times, but heavy snow is likely to
wind down late Monday as the upper low pinches off from the polar
jet and dives south of CA Monday night.

With broad SW flow engulfing the region, ongoing WAA will surge
snow levels to as high as 6000-8000 ft in the Great Basin and
Rockies D1 (but generally 3000-5000 ft elsewhere), before steadily
falling through D2 as a cold front pushes eastward and the trough
axis finally swings farther east to the coast. By 12Z Tuesday, snow
levels are expected to be generally 1500-2500 ft in the north, to
5000-6000 ft in the south. 2-day WPC probabilities (00Z Monday
through 00Z Wednesday) feature a high risk (>70% chance) for 12+
inches in the Sierra, northern CA ranges, Uintas, Tetons, Wind
Rivers, and portions of the Salmon River/Sawtooth/Bitterroots, with
lesser snowfall expected at the other regional terrain. In the
Sierra, an additional 1-2 feet is expected in the higher
elevations.


...Cascades & Olympics...
Days 2-3...

A powerful storm system traversing southern AK and an unusually
strong subtropical ridge north of Hawaii will create a strong
thermal gradient that that directs a strong >140kt 250mb jet
streak at the Pacific NW. The Olympics and Cascades will reside
beneath the left-exit region of this jet streak at the same time as
500mb height falls unfold on Tuesday. Between falling heights and
upslope enhancement driven by a westerly IVT topping 300 kg/m/s,
snow levels will fall to as low as 2,000ft Tuesday night in the WA
Cascades and Olympics and 3,000ft in the OR Cascades. By Wednesday,
a secondary disturbance ejecting out of the Gulf of AK trough will
bring another round of mountain snow to the Cascades on Wednesday.
WPC probabilities show high chances (>70%) for snowfall >12" above
3,000ft in the Cascades and Olympics with 2-4 feet of snow likely
in the northern WA Cascades above 4,000ft. Expect impacts at pass-
level in WA as the WSSI currently shows Moderate Impacts (hazardous
driving conditions and potential closures) for Snoqualmie and
Stevens Passes where as much as 1-2 feet of snowfall is forecast.
Note that the WSSI-P shows low-to-moderate chances (20-40%) for
locally Major Impacts, which imply increased odds for closures and
delays at these passes.


Weiss/Mullinax





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