152
FOUS11 KWBC 270840
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Thu Nov 27 2025

Valid 12Z Thu Nov 27 2025 - 12Z Sun Nov 30 2025

...Great Lakes...
Days 1-2...

Long duration and impressive lake effect snow (LES) event ramps up
today and continues through Friday with prolific snowfall totals
likely in the favored snow belts SE of the Lakes.

This event is driven by a large closed 500mb low which will be over
southeast Ontario to begin the period. This low will spin nearly in
place through this evening before secondary vorticity energy
rotating into its base helps pivot it off to the east on Friday. A
final piece of energy, a strung-out vorticity lobe, will then move
across the Great Lakes Friday aftn, pulling the entire system to
the east and resulting in warm-to-neutral temperature advection by
Friday night, bringing an end to this event.

With strong cyclonic flow covering the region, NW winds will bring
strong cold advection (CAA) across the lakes to produce this
impressive LES. 850mb temps are progged to fall to -10C to -15C,
which when moving across lake temperatures that are still generally
+6C to +10C according to GLERL, will produce increasingly steep
lapse rates, SBCAPE as high as 800-1000 J/kg, and inversion depths
potentially reaching 700mb from the surface. This all will support
intense snowfall rates for which both the HREF and WPC snowband
tool suggest will reach 1-2"/hr within narrow bands, and locally
heavier snowfall rates cannot be ruled out.

While the bands may twist and pivot at times due to wind
fluctuations, in general they should be pretty persistent and
focused across the northern U.P and far NW L.P. of MI through
Friday morning, with more focused and longer-duration bands
impacting the Chautauqua Ridge and Tug Hill Plateau into Friday
night. This will result in storm total snowfall that has a high
probability (>70%) of exceeding 12 inches near Traverse City, MI,
as well as east of Lakes Ontario and Erie. Locally, more than than
2 feet of snow is possible (10-30% chance according to WPC
probabilities) across the Chautauqua Ridge and Tug Hill Plateau.

Key Messages remain in effect for this system, and are linked below
(Key Message #1).


...Northern High Plains through the Great Lakes...
Days 1-3...

...Significant winter storm likely to impact portions of the
Northern Plains, Upper Midwest, and Great Lakes through this
weekend...

A closed low off the WA coast will dive rapidly southeast while
opening, reaching the Northern Rockies by Friday morning. This wave
interacting with a stationary front draped across the region and a
strengthening upstream jet streak will help spread precipitation
into the High Plains of MT very late D1 into D2 (around 12Z
Friday). Temperatures will be plenty cold for all snow as
precipitation continues to expand into the Dakotas, and the
evolution of this shortwave will drive the stationary front
southward as a cold front, leading to even colder temperatures and
dry fluffy snow into D2. Ascent during this period will be
generally modest, but as snowfall expands, WPC probabilities
suggest a moderate risk (50-70% chance) for at least 4 inches of
snow from northeast Montana through eastern South Dakota. This
jet-streak forced band has intensified in recent model progs, so
locally heavier snow is possible, especially across South Dakota
where a stripe of 700-600mb fgen gets intensified through the jet
structure.

Thereafter, the system amplifies to become much more impressive
with expanding heavy precipitation into the Upper Midwest Friday
night through Saturday. The mid-level trough diving into the
Central Plains will sharpen, and as the vort swings into the base
of the longer wave trough it will force a negative tilt coincident
with the development of coupled jet streaks (one strengthening over
the Central Plains and another exiting the Great Lakes) to produce
robust ascent. This overlapping synoptic lift will help strengthen
a surface low developing in the lee of the CO Rockies and then
tracking east, and eventually northeast, from KS through IL and
into MI by 12Z Sunday.

Downstream of this surface low, warm and moist advection will
rapidly intensify as 850mb winds surging to above 50kts draw
elevated PWs northward from the Gulf, reflected by PW anomalies
surging above the 90th percentile according to NAEFS into KS/MO/IA.
This will be accompanied by a theta-e ridge lifting northward as
well, and although a true TROWAL is not currently progged, this
theta-e ridge will still promote additional moisture and some
elevated warm air to promote modest potential instability. This
evolution suggests intense WAA, which is reflected as well by model
output, which will help expand precipitation from the Dakotas and
Iowa Friday night, to encompass most of the Upper Midwest Saturday,
and then reach the Great Lakes Saturday night.

As far as snowfall amounts and impacts, there is still considerable
uncertainty due to model placement and timing of this system. In
general, the models are in good agreement with the large-scale
evolution, but with such strong WAA in place, even minor timing or
latitudinal differences can result in significant changes to
impacts. Evaluation of D3 and D4 clusters suggests the GEFS is
potentially a bit under-dispersed with its faster trough
progression, while the ECENS/CMCE feature more spread to support a
variety of solutions suggesting a continued ensemble approach
especially by D3. However, some condensing of the spread D2 boosts
the confidence that light to moderate snow will expand into the
Corn Belt before 12Z Saturday, before pushing east thereafter.

In general, this event appears to be of longer-duration with
moderate snowfall rates driven by 280-290K isentropic ascent
combined with modest fgen beneath the weak theta-e ridge aloft.
However, as is the case with most strong WAA events, a narrow
corridor of heavier snowfall with rates greater than 1"/hr appears
likely, especially near MO/IA where conditional instability is in
place and strong fgen drives lift into the DGZ. The DGZ appears to
be elevated and shallow in most places (although some SREF
probabilities for 100mb of depth do eclipse 30%), but locally some
banding could produce more intense rates leading to more
substantial amounts and impacts.

Currently, WPC probabilities are extremely high (>90%) for more
than 4 inches in a large swath from eastern SD through most of IA,
northern IL, southern WI, and into parts of lower Michigan.
However, the axis of heaviest snowfall is likely from southern IA
through western Michigan, where WPC probabilities are high (>70%)
for more than 8 inches of accumulation, and locally 12+ inches of
snow is possible, and there are several members within the WSE
plumes which indicate more than 15 inches in some areas. The exact
placement of the heaviest snow is still in question due to a
continued southern trend in guidance, but the long duration of this
event, combined with locally heavier snow rates, leads to high
confidence in an impactful event. This is additionally reflected by
WSSI-P probabilities which exceed 90% (50%) for moderate (major)
impacts for portions of IA, IL, and WI. Post-Thanksgiving travel is
likely to be extremely disrupted during this event.

Key Messages are in effect for this system and are linked below
(Key Message #2).


...Cascades and Northern Rockies...
Days 1-2...

Closed mid-level low west of WA will push onshore Thursday evening
while opening into a shortwave as it lifts into the Northern
Rockies. Downstream of this impulse, modest warm and moist
advection will surge IVT to above 250 kg/m/s into the Cascades,
leading to increasing coverage and intensity of precipitation. Snow
levels will climb to around 6000 ft within this IVT plume, but then
crash quickly as the cold front accompanying this system pushes
east as well. At the same time, a secondary front will dig
southward from Alberta, Canada into the Northern Rockies, and
westerly flow aloft pushing higher moisture into this region will
interact with the terrain and this front to produce ascent and
expanding precipitation. At the same time, the overlap of the
strongest height falls downstream of the filling shortwave with an
amplifying jet streak will likely cause surface low development
Friday morning, helping to additionally enhance ascent before
everything kicks off to the east by Friday night.

This evolution will likely cause moderate to at times heavy snow
(heaviest in the Northern Rockies due to fgen and upslope
enhancement), with snow levels fluctuating around 3000-4000 ft, but
remaining near the surface east of the Continental Divide and into
the High Plains. WPC probabilities are moderate (>50%) for 4+
inches D1 across the Cascades, but high (>70%) for 6+ inches near
Glacier NP and the surrounding Northern Rockies. During D2 snowfall
shuts off across the Cascades, but continues in the Northern
Rockies with an additional moderate chance (50-70%) for 6+ inches
of snow again. Travel across most of the Northern Rockies passes
will likely be impacted during this snow.


The probability of significant ice across the CONUS is less than
10 percent.

Weiss


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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

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


$$