Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
332 PM EST Fri Nov 24 2023

Valid 00Z Sat Nov 25 2023 - 00Z Tue Nov 28 2023

...Southern Rockies through the South/Central Plains...
Days 1-2...

A closed low over Utah will open and begin to eject east as a
weakening positively tilted trough tonight, crossing the Central
Plains through Saturday. As this mid-level trough evolves, a Wly
southern stream jet streak will intensify over the southern
Plains. This evolution will place favorable left exit diffluence
for ascent from the Southern Rockies into the south-central Plains
into Saturday before intensifying/shearing systems over the
Midwest Sunday night. The best overlap of lift through this
diffluence and mid-level divergence/height falls will be across KS
on Saturday. At the same time, the front that has pushed down the
southern Plains will lift north into Oklahoma Saturday and
intensify frontogenesis along the KS/OK border. Low-level theta-e
advection from the S/SE isentropically ascending the cold airmass
beyond the front will surge PWs to as much as +2 sigma according
to NAEFS, which will support an expansion of heavy snowfall north
from near the OK/KS border. The setup will support translating
bands of snow from SW to NE with embedded rates of 1"/hr as
cross-sections continue to indicate a risk for CSI. The focus for
the greatest snow has come into better focus on the cold side of
the transition zone over south-central KS where there are moderate
Day 1.5 probs for >6" with a sharp cut off to the southeast which
is in or near the Wichita metro area. These bands should weaken
Saturday evening as they reach northeast KS/the KC metro where
2-4" is more likely. The risk for 2" continues over northern MO
into northern IL with Day 2 snow probs for >2" generally 10-20%.

Along the baroclinic zone just north from the warm front lifting
into OK Saturday is an area of overrunning with a wintry
transition zone expected between the rain and snow with a swath of
freezing rain likely to develop. Day 1.5 ice probabilities for
>0.1" are lower moderate (30-50%) over northwest OK. Furthermore,
as low pressure shifts east along the surface front Saturday, the
DGZ may begin to dry out while low level saturation persists which
could set up very light ZR/ZL over west-central OK late Saturday.


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

A shortwave dropping out of Saskatchewan Saturday evening will
slide in behind the weakening trough axis over the south-central
Plains before phasing Sunday over the Upper Midwest. This allows
the resultant trough to take on a negative tilt across the Great
Lakes Sunday night with an associated arctic front crossing the
Great Lakes Sunday night through Monday with 850mb temps behind it
generally to -5C to -10C, and possibly as low as -15C over western
Lake Superior. These cold 850mb temps will move atop lake surface
temperatures that are generally +5C to +12C according to GLERL,
which will result in steepening lapse rates, 500-1000 J/kg of
SBCAPE, and rising inversion heights coincident with a saturated
but lowering DGZ, all supporting increasing coverage and intensity
of lake effect snow in the favored W/NW snow belts. The lake
effect snow machine begins Sunday night for areas south of Lake
Superior and east of Lake Michigan, expanding to the eastern Great
Lakes Monday. This looks to be the beginning of a multi-day,
impressive LES event in some areas, with Day 3 probabilities for
>6" generally moderate in favored U.P. and L.P. belts (Porcupine
and eastern U.P. coast and near Grand Traverse Bay.

The negatively tilting trough shifting east over the Great Lakes
allows and a surface low to develop along the New England coast
Monday. While this system is rather mild thermally, sufficient
cold air for snow reaches northern New England Monday with Day 3
probabilities for >6" currently low to locally moderate for the
White Mtns and well interior northern Maine.



***Key Messages for Thanksgiving Weekend Winter Storm***

--Southern Rockies heavy snow through tonight
Heavy snow through tonight in the higher terrain of southwest
Colorado and northern New Mexico, with more than a foot in the San
Juan mountains.

--Heavy snow bands in Kansas Saturday
Moderate snow with embedded heavy bands with 1"/hr rates will
spread across much of Kansas Saturday with moderate chances
(40-60%) for snowfall above 6 inches in south-central Kansas.

--Hazardous travel due to snow, wind, and icy roads
Locally heavy snow combined with gusty winds will cause hazardous
travel due to snow covered/slippery roads and reduced visibility.

--Cold Conditions for the Rockies and Plains
Highs and low temperatures are expected to be 10-20 degrees below
normal through Sunday, with wind chills falling into the single
digits or below 0 at times.


Jackson