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FOUS11 KWBC 261927
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 PM EST Tue Nov 26 2024

Valid 00Z Wed Nov 27 2024 - 00Z Sat Nov 30 2024

...Ohio Valley through the Interior Northeast...
Days 1-3...

Two areas of low pressure will bring rounds of wintry
precipitation to the region during this Thanksgiving week.

The first wave of low pressure will be moving off the coast of
Maine to start the period, extended from an occluded low positioned
near James Bay. This low will remain progressive to the east as a
weakening shortwave shedding around a larger closed low north of
the Great Lakes moves into the Canadian Maritimes. A ribbon of
modest PW advection will occur downstream of this impulse and
rotate cyclonically into New England, leading to a swath of
precipitation moving across the region, which will fall as snow
only in the higher terrain of NH and ME due to a relatively warm
column. Some modest snowfall is likely as the accompanying WAA
drives periods of heavy snow rates around 1"/hr before the column
dries out from the west and precipitation ends Wednesday morning.
WPC probabilities for an additional 2+ inches of snow are modest,
and confined to the higher terrain of Maine.

A more significant area of low pressure will then begin to take
shape across the Ohio Valley/Midwest as a shortwave pivots through
the broad cyclonic flow across the east. This shortwave will
maintain a positive tilt as it shifts from the Midwest Wednesday
aftn to off the coast of Maine by Thursday night. Although it will
remain progressive, the accompanying ascent will intensify through
Thursday as a downstream jet streak intensifies to 130kts and
becomes modestly coupled as it pivots poleward across the Mid-
Atlantic. This synoptic ascent impinging upon a surface baroclinic
gradient (enhanced by downstream WAA) will drive cyclogenesis in
the TN VLY, and this low will deepen as it moves progressively into
the Mid-Atlantic and then near Cape Cod Thursday night before
exiting to the Canadian Maritimes.

There still remains considerable spread in the placement and
intensity of this low which is reflected by cluster analysis. The
GFS/GEFS continues to be on the weaker/shallow end with its 500mb
heights leading to more of a sliding and weaker low, while the
EC/ECENS and some Canadian members are stronger resulting in a
deeper and somewhat slower low. The trends have been for a slightly
more northward/deeper low, with the exception of the GFS/NAM, and
a solution near the CMCE appears reasonable based on DESI variable
IQRs. While the exact track and intensity will be critical as
moisture shifts northward on robust theta-e advection into a column
that is marginally supportive of snow outside of terrain, it is
probable that heavy snow will occur well inland from I-95 on
Thursday.

This is most likely within what could be a potent band on
the N/NW side of this low, with the evolution fitting the
conceptual model of a laterally-quasi-stationary band moving from
IN Wednesday night through northern PA and then across central New
England into central ME. Persistent WAA into the system results in
some increasingly sloped fgen, especially in the 700-500mb layer,
which will effectively intersect the DGZ, furthering support for
this band. These types of bands can produce sharp snowfall
gradients, and the intensity can dynamically cool the column
sufficiently to overcome near-freezing surface temps to cause
impactful, low SLR snowfall. At this time, confidence in a band is
increasing, although placement remains very uncertain, and the snow
footprint remains very much in question. Regardless, at this time
WPC probabilities on Thanksgiving /D2/ and into Friday are 50-70%
for more than 4 inches across the Adirondacks, Greens, Whites, and
much of northern ME. Locally as much as 8" of snow is possible as
reflected by WSE plumes. Additionally, where this band occurs,
local modest accumulations of an inch or two are possible as far SW
as IN/OH as shown by WPC probabilities for 1"+ reaching 10-30%.


..Great Lakes...
Day 3...

***Significant and long lasting Lake Effect Snow event becoming
 more likely beginning Friday***

Although periods of light to moderate lake effect snow (LES) in
the typical N/NW band areas is possible D1 and D2, the significant
event begins D3. WPC probabilities D1 (near the Tug Hill Plateau)
and D2 (western and central U.P.) are generally 10-30% for 4+
inches.

Then, on D3, a closed mid-level low positioned near James Bay late
Thursday will retrograde to the west and then pivot almost in
place while deepening Friday. This will result in amplification of
the eastern CONUS trough, leading to persistent cyclonic NW flow
across the Great Lakes. This will create enhanced CAA, with 80mb
temps falling from -3C (east) and -10C (northwest) to as low as -8C
and -15C, respectively, by the end of D3. This cold air will be
moving across very warm waters, with GLERL noting 0% ice cover and
lake- surface temperatures around +8C in Lake Superior to +15C over
Lake Erie. This will produce extreme sfc-850mb temperature
gradients with super-adiabatic lapse rates and lake-induced
instability that is already progged to approach 1000 J/kg in the
recent NAM. A shortwave moving across the Lakes D3 will help
enhance the CAA to produce the onset of what is very likely to
become a prolonged and significant LES event, with multiple
shortwaves and somewhat variable wind trajectories leading to
widespread heavy LES into the medium range. While initially, the
immediate coast of the lakes may be too warm for all snow except
during the most intense rates, eventually the entire column should
cool to support heavy snow even to the lakeshore. For D3, WPC
probabilities are high (>50%) for 6+ inches in the U.P., near
Traverse City, MI, in the Tug Hill, and most impressively along the
Chautauqua Ridge. Locally 12"+ of snow is possible on D3, with
much more to come through the weekend.

The arctic-sourced cold and subsequent LES off the Great Lakes
through the weekend is the subject of the Key Messages which are
linked below.


...Great Basin through the Southern Rockies...
Days 1-2...

A positively tilted trough dropping along the OR coast this morning
will begin to fill as it ejects into the Great Basin at the start
of the forecast period /00Z this evening/. This feature will weaken
rapidly during D1 as it becomes embedded in what is generally broad
zonal flow across the CONUS, becoming just a weak ripple in the
flow by Wednesday evening over the Southern Rockies. Despite the
loss of amplification, pinched flow to its south and downstream
will continue to advect significant moisture eastward within a
narrow atmospheric river characterized by IVT reaching above the
99.5 percentile within the CFSR database through the Four Corners
region. As the trough fills, the accompanying IVT surge also
weakens, but moisture will remain sufficient to produce heavy
precipitation from the Great Basin through the CO Rockies and
Sangre de Cristos into Thursday.

As this weakening trough pivots east, it will interact with a broad
baroclinic zone resulting in waves of low pressure moving through
the region. Additionally, the associated temperature gradients and
eastward advance of the weakening upper jet will drive elongated
axes of mid-level fgen to enhance ascent, which could produce
stripes of embedded heavier precipitation within otherwise modest
precipitation rates. The most likely areas to experience heavy snow
from this event will be in the higher terrain where westerly flow
will upslope favorably to drive pronounced lift, including across
the Wasatch, Uintas, San Juans, Sangre de Cristos, and CO Rockies
where WPC probabilities for 8+ inches of snow D1 exceed 90%,
primarily above 5000-6000 ft. Locally, more than 2 feet is possible
in the highest terrain, especially in the San Juans and portions of
the CO Rockies. However, where any bands can occur to produce
heavier snowfall rates, light accumulations into elevated valleys
are also possible.

East of the Continental Divide, there is uncertainty as to how much
moisture can wring out and reach atop the Front Range. However,
lee cyclogenesis progged to occur Wednesday should surge a cold
front south across the High Plains. This will provide some
additional upslope ascent while concurrently deepening the DGZ and
enhancing fgen east of the terrain. This will yield at least some
snow along the I-25 urban corridor south of Denver, with the most
significant accumulations likely in the higher terrain of the
Palmer Divide and Raton Mesa. This is reflected by WPC
moderate probabilities (10-50%) for 4+ inches of snow on Wednesday.



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

$$