Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 PM EST Sun Dec 27 2020

Valid 00Z Mon Dec 28 2020 - 00Z Thu Dec 31 2020


...Great Lakes..
Days 1-2...
A wave of low pressure moving across WI and MI tonight will spread
moderate snow across the area. While some moderate snow is likely
across WI this evening, any significant accumulations from this
event are likely from LES developing as the low pulls away. CAA
behind the departing system moving across the lakes on NW flow
will lead to rounds of moderate to heavy snow in the eastern U.P.
and NW L.P. of MI on D1. WPC probabilities are moderate for 4".
Into D2, the associated cold front moving across the area will
lead to redeveloping LES downwind and to the east of Lake Erie and
Ontario, with an effective fetch from Huron transporting
additional moisture. WPC probabilities D1.5 into D2 are high for 4
inches, with amount up to 8" possible in the Tug Hill Plateau.

Additionally, as the arctic front crosses the Northeast brief snow
squalls are possible which are not likely to accumulate much, but
may lead to short duration reduced visibility and hazardous travel.


...Central Plains/Midwest...
Days 2-3...
Longwave trough moving eastward from the Great Basin will send an
intensifying jet streak around its base placing favorable upper
diffluence for surface pressure falls into the lee of the Rockies.
As this occurs, coupling of the new jet streak and the residual
zonal streak to the north will drive deep layer ascent for low
development and strengthening, with a surface low expected to move
northeast into WI and the Great Lakes by Wednesday aftn. Guidance
today has overall weakened due to less northern and southern
stream interaction, leading to better split flow and a more
progressive solution, with phasing only occurring late in the
forecast period. This is agreed upon by the GFS and ECMWF along
with their ensembles which were used heavily for this forecast.

As the low begins to lift northeastward, it will be accompanied by
a band of low-level fgen and intense WAA from western KS into
southern SD. Within this band, intense ascent into the DGZ and an
increasing isothermal layer within model soundings suggest intense
snow rates of >1"/hr for a few hours. However, the WAA will
eventually overwhelm the cold air to cause a p-type change to
freezing rain and eventually rain across KS and parts of NE/IA.
Further north, a band of freezing rain is likely to linger from NE
Kansas into central IA and southern WI. North of there, the column
should stay cold enough for primarily snow. As the two streams
interact Wednesday morning, some enhanced deformation may occur NW
of the surface low causing a local uptick in snowfall across NE IA
into central WI. This is in addition to the leading WAA snowfall.

With a more progressive solution and less energy phasing, WPC
probabilities for heavy snow have trended down just slightly.
However, there is still a high risk for 4 inches from central NE
into southeast SD on D2, expanding into northern IA, southern MN,
and much of WI D3. The heaviest snow, which may exceed 8" is most
likely across NE IA into WI.

South of there, from central KS through northern MO and into
northern IL/southern WI, there is likely to be a longer duration
of freezing rain. Eventually in these areas p-type may also change
to rain before ending, but WPC probabilities indicate a moderate
risk for 0.1" of accretion on D2-3, with isolated pockets to .25"
possible.


...CA/Great Basin/Rockies...
Days 1-2...
An upper trough moving onshore CA early Monday will advect
eastward into the Great Basin Monday and into the central and
southern Rockies Tuesday. This feature will be accompanied by
persistent and robust jet energy crossing central to southern CA
inland across central to southern NV/UT, then CO, providing deep
layer ascent and spreading moisture eastward.

Upper level divergence combines with topographically forced ascent
in windward terrain to produce snows across the central to
southern CA Sierra Nevada mountains, followed by the Transverse
Ranges of southern CA.  The jet axis for ascent will then cross
into UT and into western CO, leading to several inches of snow
across the mountains of western CO. The heaviest snow day 1 is
likely in the Sierra and transverse ranges of CA which are more
orthogonal to the flow and WPC probabilities are high for 8
inches. Heavy snow is also likely in the San Juans D1 due to WAA
snows spreading northward and interacting with the terrain. By D2,
the heaviest snow should shift is expected in the San Juans and CO
Rockies due to stronger low level jet that leads to stronger
ascent in windward areas.
WPC probabilities on D2 are high for 8 inches in the San Juans
with maximum amounts over 12" likely.


Weiss