Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Sat Nov 25 2023

Valid 00Z Sun Nov 26 2023 - 00Z Wed Nov 29 2023

...South/Central Plains into the Midwest...
Day 1...

The now open mid-level trough over eastern CO this afternoon will
cross KS tonight. Meanwhile left exit upper diffluence increases
as the WSWly jet over the southern plains intensifies. Downstream
theta-e advection to the north will continue to overrun a leading
warm front producing moist isentropic upglide and direct banded
snow northeast across the rest of KS, northern MO, and eastern
IA/northern IL tonight with persistent, but slowly weakening
banded structures. Day 1 snow probs for an additional >4" is
moderate over eastern KS with moderate probs for >2" then northern
MO  and along the WI/IL border.

Farther northeast over western Michigan, the approaching northern
stream wave allows some redevelopment on Sunday, which combined
with lake enhancement brings renewed risk for >4" snow which is
moderate along the western MI shore and before LES begins
(discussed in the section below).

Some additional minor ice accretion is possible this evening as
the DGZ may begins to dry out while low level saturation persists
which could set up very light ZR/ZL over west-central OK.


...Great Lakes and Northern New England...
Days 2-3...

The onset of a significant lake effect snow (LES) event sweeps
across the Great Lakes Sunday afternoon through night and lasts
through Tuesday as much colder air drops across the still warm
lakes. A sharp shortwave trough will deepen as it tracks across
Manitoba tonight, extending a trough axis and resultant height
falls into the Upper Midwest Sunday. This feature will take on a
negative tilt Sunday night as a potent vorticity maxima rotates
through its base and reaches the Northeast Monday, while secondary
vorticity energy shifts into the trough from upstream. This will
produce significant cyclonic flow across the eastern third of the
country, with the coldest air beneath this trough advecting over
the Great Lakes on Monday characterized by 850mb temps of -10C to
-15C. This will occur across lake surface temperatures that are
generally +5 to +12C as reflected by GLERL, producing steep low
level lapse rates and SBCAPE possibly exceeding 1000 J/kg in some
areas. Lake effect snow is likely to begin first in the U.P of MI
Sunday afternoon behind the accompanying surface cold front, and
then spread through the L.P. Sunday night, eventually developing
east of Lakes Erie and Ontario by Monday morning. Once LES begins,
it looks persist until period of WAA on brief shortwave ridging
moves across the region on Tuesday.

With generally westerly flow veering northwesterly through the
event, the heaviest LES is expected Sunday night/Monday across the
U.P. and western portions of the L.P. of MI where Day 2 WPC
probabilities for >6" are moderate in the Porcupine Mtns and parts
of the eastern U.P. as well as east of Grand Traverse Bay on the
L.P. with the caveat that localized totals twice this amount are
likely should bands only slowly pivot. More significant LES is
progged to develop over the eastern Great Lakes late Monday
through Tuesday when the DGZ crashes concurrently with their depth
increasing, while forcing remains quite robust over the lakes.
Additionally, flow on Monday could support some effective fetch
from an upstream connection providing additional moisture
downstream of Lakes Erie and Ontario where snowfall could be quite
heavy. Snowfall rates will likely exceed 1"/hr, and Day 2.5 WPC
probabilities for >6" are highest along the Chautauqua Ridge and
Tug Hill Plateau, where they exceed 80%. Much heavier snowfall
totals are possible in these areas given the prolonged nature of
this event. There remains some uncertainty into the amount of wind
shear and exact wind direction, but in some places this setup
appears to be favorable for significant and impactful snow
accumulations, especially late Monday through Tuesday.

This negatively tilting trough and accompanying vorticity maxima
shifting east over the Great Lakes and into New England will help
drive surface cyclogenesis the New England coast Monday. This
near-shore track results in a marginal thermal structure for any
wintry precipitation outside of the White Mtns and far western
Maine where Day 2 snow probs are moderate in places for >6".


Jackson