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

Valid 12Z Fri Nov 24 2023 - 12Z Mon Nov 27 2023

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

An amplified closed mid-level low with height anomalies
approaching -2 sigma will spin down across the Great Basin today
before slowly filling and ejecting eastward across the Four
Corners Saturday, and then opening into a weakening positively
tilted trough over the Central Plains on Sunday. This feature will
be the primary mechanism for ascent through the period across this
region, resulting in an impressive winter weather event from the
Central Rockies through the Southern Plains.

As this mid-level trough evolves, a southern stream jet streak
upstream of the primary trough axis will begin to pivot around the
base and then intensify as it elongated to the ESE during the
weekend. This evolution will place favorable LFQ diffluence for
ascent from the Southern Rockies into the Central Plains,
especially Saturday, with more transient ascent continuing
eastward from there on Sunday. The best overlap of lift through
this diffluence and mid-level divergence/height falls will be
across WY/CO/UT today, and then shifting to KS on Saturday. At the
same time, a southward sinking surface front will drive additional
lift through low-level convergence, intensifying frontogenesis,
and at least modest upslope ascent into the terrain. PWs on D1
will be near normal for the Rockies and central High Plains, but
pronounced moist isentropic ascent within the cooling column
supporting what appears to be, subjectively, an exceptionally deep
DGZ will drive heavy snowfall in the High Plains of CO/WY and into
NE/KS, with additional heavy snow likely focused in the terrain of
western CO and the San Juans due to more orthogonal moist flow.
WPC probabilities on D1 for more than 6 inches of snow are above
50% in most of the terrain from the Wasatch of UT through the CO
Rockies and into the Sangre de Cristos, with the highest
probabilities focused in the San Juans where 1-2 feet of snow is
likely. A secondary maxima of snow is possible in the High Plains
of KS/NE where WPC probabilities for 6 inches or more are as high
as 5%.

During D2, the evolution shifts to focus the heaviest snow from
the Sangre de Cristos eastward, with the heaviest snow signal now
appearing to be across much of Kansas. In this region, the best
PVA will overlap the most robust LFQ diffluence, within a region
of impressive 700-600mb deformation and low-level fgen. Low-level
theta-e advection from the S/SE isentropically ascending the cold
airmass behind the front will surge PWs to as much as +2 sigma
according to NAEFS, which will support an expansion of heavy
snowfall. The setup still appears to support translating bands of
snow from SW to NE, and while not all areas will see intense snow
rates, it is likely that some areas will see 1+"/hr snowfall as
cross-sections indicate a risk for CSI. The signal has again
increased tonight, and current WPC probabilities for more than 6
inches are 40-60% across much of central KS, with locally higher
amounts likely in any bands. This will likely result in
considerable impacts, and WSSI probabilities for moderate impacts
have increased to 60-70%.

Additionally, there is a small area of ice probabilities from WPC
exceeding 10% D2 near the KS/OK border. These probabilities have
come down a bit, and examination of regional soundings suggests
any mixed p-type should be more sleet/IP than freezing rain due to
a weak warm nose and potential for dynamic cooling from aloft.
However, as the front swings eastward the DGZ may begin to dry out
late Saturday while low level saturation persists. This could set
up a very light ZR/ZL event which could accumulate to a few
hundredths of an inch of ice on top of any morning freezing rain
along the thermal gradient.


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

A shortwave dropping southward out of Saskatchewan Sunday will
sharpen over the Northern Plains and then begin to take on a
negative tilt across the Great Lakes in response to a potent
vorticity maxima rotating through its base. This will result in
surface low pressure development moving across southern Canada,
with the trailing arctic front digging southward in its wake,
reaching the eastern Great Lakes by the end of the forecast period
(Monday morning.) Although there is still some spread among the
models as to the timing of this front, there is good agreement
that 850mb temps behind it will crash 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. Before Monday, this will be generally
confined to areas south of Lake Superior, east of Lake Michigan,
and maybe beginning just before the end of the forecast period
southeast of Lake Erie. D3 could be the first of a multi-day
impressive LES event in some areas, but at this time, WPC
probabilities for more than 4 inches are generally 20-40% in the
western U.P. of MI and along the eastern side of Lake Michigan in
the L.P.



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

--Periods of heavy snow continue through tonight
Heavy snow will persist across the Central Rockies and Central
High Plains today before ending Saturday morning. Additional
snowfall has a medium chance (40-60%) of exceeding 4 inches in
western NE and northwest KS. The heaviest snow is expected in the
higher terrain of UT, CO, and NM, with more than a foot likely
(>80%) in the San Juan mountains.

--Heavy snow bands in Kansas Saturday
Moderate to locally heavy snow will spread across much of Kansas
Saturday. Embedded bands with heavy snow rates exceeding 1"/hr
should lead to widespread snowfall above 4 inches, with a low
chance (10-30%) of maximum amounts exceeding 8 inches.

--Hazardous travel due to snow, wind, and icy roads
Snowfall rates of 1"/hr combined with gusty winds will cause
hazardous travel due to snow covered/slippery roads and reduced
visibility.

--Much colder temperatures for the Rockies and Plains
Much colder temperatures will overspread the Rockies and Plains
behind an arctic front. Highs and lows are expected to be 10-20
degrees below normal through Sunday, with wind chills falling into
the single digits or below 0 at times.


Weiss