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FOUS11 KWBC 040819
QPFHSD

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

Valid 12Z Sun Jan 04 2026 - 12Z Wed Jan 07 2026


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

Some modest lake effect snow (LES) will continue through Sunday
aftn, but primarily within a single band south/southeast of Lake
Ontario with upstream connection to Lakes Huron and Superior.
Additional snowfall across this region should be light as reflected
by WPC probabilities for an additional 2+" of just 10-30%,
although brief 1"/hr rates are possible (10-30% chance).

Thereafter, the mid-level pattern flattens a bit as shortwave
ridging expands from the west, leading to pinched and more
progressive flow across the region. Within this flow, two back-to-
back shortwaves will traverse the region bringing rounds of wintry
weather.

The first of these will emerge from the High Plains Sunday morning
and push due east across the Great Lakes in a de-amplifying manner
as it becomes absorbed into the pinched westerlies. Despite the
weakening amplitude, height falls, PVA, and the left-exit region of
a strengthening jet streak will produce deep layer ascent, and
the intensification of this jet streak will likely result in
enhanced omega despite the weakening amplitude of the shortwave. At
the same time, increasing and impressive 925-850mb WAA will spread
northeast ahead of this impulse, aligning with the best left-exit
dynamics and the associated response to produce impressive fgen but
with limited temporal duration. Since this feature will be of
Pacific origin, the accompanying moisture will be impressive as
reflected by NAEFS PWs that are above the 97th percentile, aligned
with the strongest ascent. This suggests that as precipitation
expands from MN through the U.P., and into the northern L.P., a
narrow band of heavy snow will develop which has a 60-80% chance
of producing 1"/hr snowfall rates. The duration of these will,
again, be limited to the progressive nature of the wave, but WPC
probabilities for 4+ inches are above 50% from the Arrowhead of MN
eastward through the northern L.P. of MI, with locally 6-8"
possible (30% chance) in portions of the eastern U.P.

Additionally, south of this axis of heavy snow, a corridor of
modest mixed precipitation, including freezing rain, is likely as
the WAA surges a warm nose to around +3C at 850-800mb. The
refreezing layer is quite deep, so this may manifest more as sleet
than freezing rain, but hazardous travel is Sunday evening from MN
through central WI where WPC probabilities for 0.01" of ice are
50-70%.

This lead wave will continue to track into New England before
exiting to the Atlantic Monday night, but additional snowfall is
expected to be light, generally 1-3", although locally higher in
the Tug Hill Plateau through lake enhancement, as ascent weakens.

The next shortwave follows quickly in the wake of the first, and
emerges from CA Sunday evening before racing to reach the Corn Belt
by Tuesday morning. This impulse will track along the southern
subtropical jet, which is progged to intensify over the Central
Plains, reaching towards 140 kts. The favorable overlap of the LFQ
and height falls downstream of the shortwave will likely lead to
surface low pressure development Monday night near Iowa, with this
low then tracking steadily northeast into Ontario by Tuesday
evening. While there is still some spatial spread in the placement
of this low according to the different ensemble clusters, the
increasing ascent will lead to another round of wintry
precipitation from Minnesota through the Great Lakes (where a mix
of freezing rain and snow is likely), with additional mixed
precipitation spreading into New England by the end of the forecast
period. Total ascent with this secondary wave should be less
intense than the lead wave, and the thermal evolution is quite
complex due to potential secondary low development in the Gulf of
Maine, but at this time WPC probabilities for 2+ inches of snow are
just 10-30% in the higher elevations of Upstate NY and northern New
England.

For the freezing rain, WPC probabilities indicate a high risk
(>70% chance) of at least 0.01" of ice from central MN near the
Twin Cities through much of WI and MI, and into central and
northern New England. The greatest risk for at least 0.1" of ice is
likely in parts of northern WI, northern MI, and the higher
elevations of the Catskills, Adirondacks, and southern Greens.


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

The active west continues into early next week as an anomalous
trough persists just off the Pacific coast through Tuesday morning
before amplifying into a closed low which then undercuts towards
Baja by the end of the forecast period.

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 additionally enhanced
ascent. The combination of persistent SW mid-level flow with the
wavering jet stream will surge periods of elevated IVT onshore,
resulting in PW plumes that occasionally exceed the 90th
climatological percentile. While there may be periods of enhanced
ascent locally across different regions, in general the next 48
hours appear to be synoptically forced with broad large-scale
ascent, locally enhanced through PVA where the vorticity lobes
stream overhead, and through upslope enhancement. The strongest
upslope flow continues to be appear focused over the Sierra where
snowfall will be prolific, but heavy snow is likely through Monday
night across much of the region.

With broad SW flow encompassing the region, the accompanying 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
(12Z Sunday through 12Z Tuesday) 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, 2-4 feet is possible in the higher elevations.


...Cascades...
Day 3..

Mid-level flow across the Northern Pacific will become pinched and
zonal Tuesday, while a weak impulses approaches the coast and may
move onshore overnight into Wednesday morning. This zonal mid-level
flow will be topped by a strengthening Pacific jet streak which
will drop gradually southward such that the overlap of PVA/height
falls will match the greatest LFQ diffluence to provide ample
ascent. This will be aided by upslope enhancement into the terrain
thanks to the westerly flow, producing brief but impressive deep
layer lift into IVT that has a >80% chance of exceeding 250 kg/m/s.
While westerly IVT does not usually result in as impressive of
precipitation as that with a S or SW component, a fully saturated
column with steepening lapse rates will result leading to a period
of heavy snow in the Olympics and Cascades of OR and WA. Snow
levels will be generally 2000-3000 ft during this time, so pass
level impacts leading to hazardous travel are expected. WPC
probabilities for more than 8 inches of snow reach 70+%, with more
than 1 foot possible at the passes (Snoqualmie and Stevens
included).



Weiss


$$

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