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FOUS11 KWBC 290829
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Sat Nov 29 2025

Valid 12Z Sat Nov 29 2025 - 12Z Tue Dec 02 2025


...Northern High Plains through the Midwest into the Northeast...
Days 1-2...

...Significant winter storm ramps up across the Midwest today...

A positively tilted shortwave trough diving out of Montana will
sharpen today across the Central Plains, and eventually take on a
negative tilt by this evening in response to secondary vorticity
energy rotating through its base. This evolution will result in
intensifying deep layer ascent in response to more significant
height falls overlapping mid-level divergence, and this will
additionally be aided by increasingly coupled upper jet streaks.
The surface low developing within this impressive ascent will then
deepen as it lifts northeast from Kansas /12Z Saturday/ to Michigan
/12Z Sunday/. This system will become increasingly progressive
through D2, reaching the Canadian Maritimes by 12Z Monday.

This evolution will yield an impressive swath of snow from the
Northern Plains through much of the Midwest and then into the Great
Lakes. As the low develops, increasing warm/moist advection will
surge PWs out of the Gulf, and PW anomalies above the 90th
percentile are progged by NAEFS to reach as far north as MO/IA.
This will occur during a period of intensifying 290K isentropic
ascent, which when occurring beneath the larger scale synoptic lift
will cause snowfall to expand quickly, and become heavy, especially
where 700-600mb fgen intensifies (both due to the WAA and the
ageostrophic response of the upper jet position). At the same time,
cross-sections continue to indicate a threat for CSI due to folding
theta-e surfaces, which additionally will support periods of heavy
snowfall rates as laterally translating bands shift northeast
through the region. Snowfall rates of 1-2"/hr are likely (>80%
chance according to the HREF), which, although won`t be constant,
will help rapidly accumulate snowfall even with SLRs that will be
around to slightly below climo most of the event.

This intense snowfall will result in widespread significant
impacts, and the WSSI-P indicates a 60-80% chance of major impacts
for parts of IA, IL, and WI, including the Des Moines, Chicago, and
Milwaukee metro areas. This will snarl travel during the busy
post-Thanksgiving holiday weekend. Where snow is the most intense
and longest lasting, record or near-record November snowfall is
possible, especially across eastern IA, southern WI, and northern
IL. This is also where the ECMWF EFI is above 0.9 with an SoT of
1-5, further indicating the potentially record-breaking November
snowfall.

WPC probabilities D1 are high (>70%) for at least 6" of snow from
central IA though southern MI, with locally more than 12" possible
(10-30% chance) in eastern IA, southern WI, and northern IL.
Isolated higher totals are possible as well where any banding can
persist due to local convergent features such as indicated by many
of the high-res models near Lake Michigan.

The most significant snowfall should wane quickly from west to east
by 12Z Sunday, but post-system NW flow will create some modest lake
effect snow (LES) across the U.P. and L.P. of MI, as well as east
of Lakes Erie and Ontario Sunday night. Additionally, some modest
warm-advection snowfall is likely across the higher terrain of the
interior Northeast before snowfall comes to an end by 12Z Monday.
Additionally snowfall of more than 4 inches is likely (50-90%
chance) according to WPC probabilities from the Chautauqua Ridge
northeast through the higher terrain of the Adirondacks, and into
VT/NH/ME.

Key Messages remain in effect for this system (Key Message 2
below).


...Central Rockies through the Ohio Valley...
Days 2-3...

The next system follows quickly behind the powerful storm across
the Midwest as progressive flow dominates the CONUS.

This system begins as a positively tilted shortwave emerging from
the Pacific Northwest and then digging rapidly into the Four
Corners states by 12Z Monday. As this impulse dives southeast, the
associated vorticity lobe will swing rapidly east, forcing the
evolution into a neutral tilt over the Central/Southern Plains D3,
and potentially becoming negatively tilted late in the forecast
period across the Mid-Mississippi Valley. During this evolution, a
downstream upper jet streak will amplify and pivot poleward over
the Central Plains to enhance deep layer lift. This should cause an
area of heavy snowfall D2 across the Central Rockies, including
the Wasatch, Uintas, and CO Rockies where snow levels of around
4000-5000 ft are expected. Although the system is progressive, PW
anomalies above the 90th percentile will support periods of heavy
snow, aided by upslope into the terrain, and WPC probabilities for
more than 4 inches of snow have increased to 70-90% across the Ruby
Mountains in NV, as well as the expanse of the Wasatch, Uintas, CO
Rockies, and San Juans. Locally as much as 12" of snow is possible
in the highest terrain of Colorado.

The forecast becomes more challenging into D3 as this system
pushes east. Although the surface wave is likely to wane across the
Southern/Central Plains, the downstream impressive jet streak and
height falls invof the upper trough will persist. Late Monday, some
of the guidance indicates that the downstream jet and a secondary
sub-tropical jet streak emerging across Mexico will phase, leading
to secondary low pressure development across the Mississippi
Valley. As this occurs, precipitation will expand from the Gulf
Coast through the Mid-Mississippi Valley in response to
intensifying moist isentropic ascent along the 290K-300K surfaces.
Mixing ratios within this plume are progged at 4-8 g/kg, around the
75th-90th percentile for the date, indicative of the impressive
moisture availability in this system.

While there are still important timing, location, and thermal
differences among the various global members and their associated
ensembles, the trends are for a wetter and colder system which
could produce significant snow and mixed precipitation before 12Z
Tuesday. The AIFS and ECMWF EFI are both well southeast of the
highest NBM probabilities during D3, but the D3/D4 clusters
indicate high variability due to timing differences of the trough.
Despite this uncertainty, current WPC probabilities for at least 2"
of snow are moderate (30-70% chance) from central Kansas through
the western Ohio River Valley, with the potent jet streak aloft
helping to produce potential banded snow in this region.

Additionally, with warm moist air flooding northward from the Gulf
and a retreating cold high pressure in place, a zone of mixed
precipitation, including freezing rain, is likely from the Ozarks
through the southern Appalachians. There continues to be much
uncertainty in the axis of heaviest ice accretions, but
probabilities for at least some modest ice have come up tonight,
leading to increasing confidence in areas of impactful freezing
rain. At this time, WPC probabilities for 0.1" on D3 are as high as
30% in the Ozarks, and 30-50% in the southern Appalachians.

This system will likely continue beyond this forecast period while
strengthening, and may create heavy snow across the interior Mid-
Atlantic and Northeast on Tuesday. For this reason, key messages
have been initiated (Key Message 3 below).


Weiss



...Winter Storm Key Messages are in effect. Please see current
 Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_3.png



$$