Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 AM EST Mon Dec 28 2020

Valid 12Z Mon Dec 28 2020 - 12Z Thu Dec 31 2020


...Great Lakes..

Days 1-3...
A wave of low pressure moving across MI is forecast to cross into
Ontario and then Quebec today. Cold advection behind the departing
system will lead to moderate to heavy snow in the lee shore areas
downstream from lake Superior in the eastern U.P. and NW L.P. of
MI on D1. The warm front south of the low crosses Lake Ontario,
with low level south to southwest winds transporting moisture and
ascent into the areas east of Lake Ontario, including the Tug Hill
and western Adirondacks.  This moves downstream until a front
approaches later tonight, with boundary layer winds veering.  This
results in the peak low level convergence occurring near the
southeast corner of Lake Ontario tonight, as the 00z GFS has a
bulls eye in ascent there by Tue morning.

On Day 2, the snow downstream from Lake Ontario winds down, with
the snow switching to the upper Lakes within the broad low-mid
level theta-e advection and convergence zone crossing WI, Lake
Michigan, and into the the UP of MI and northwest lower MI.

Light icing is possible in southern lower MI as warm advection
changes precip types quickly from snow to sleet and freezing rain.
 Light amounts of each type are expected in southern lower MI.
Several inches of snow are expected in northern lower MI to the
eastern UP of MI on Day 3/Wed due to the combination of synoptic
snow, followed by post-system cold advection  and lake effect
snow. Also, on Day 3, the warm/moisture advection zone crosses
Lake Ontario, continuing into the Tug Hill and western
Adirondacks.  The continuing warm advection also leads to the
chance of a period of light icing in northern New York.

...Central Plains/Midwest...

Days 1-3...
A longwave trough moving eastward from the Rockies crosses the
northern to central Plains. A coupling of the 2 jet streaks will
drive deep layer ascent for low development and strengthening,
with a surface low expected to move northeast across the mid MS
Valley Tue night to the upper Great lakes Wed.
As the low lifts northeastward, it will be accompanied by a band
of low-mid level frontogenesis and warm advection from KS across
NE into southern SD. Within this band, ascent near the low-mid
level front and warm advection suggest snow rates of 1"-1.5"/hr
for a few hours. Several inches of snow are expected in northern
NE and southern SD to southern MN, northern IA and southern WI. 
The higher amounts are expected in portions of northeast IA to
southern WI within the longer duration low level jet and front.

The low level warm advection causes a change from snow to to
freezing rain and eventually rain across most of KS and northern
MO into parts of southern NE/southern IA, followed by northern IL.
 The existence of multiple precip types and uncertainty regarding
the duration of each one drive the forecast uncertainty.
The possibility of ice accumulations as much as a quarter inch
exists in northwest MO to southern IA.  WPC probabilities indicate
a moderate risk for 0.1" of accretion on Day 2 Tue, with isolated
pockets to .25" possible.

With a more progressive solution and the continued northeast
movement of the low-mid level front,  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, northern IA,
southern MN, and much of southern to central WI. The heaviest
snow, which may reach 8", is most likely across NE IA into
southern WI.

...CA/Great Basin/Rockies/Southern Plains to Mid MS Valley...

Days 1-3...
An upper trough moving onshore CA early Monday will advect
eastward into the Great Basin today and into the central and
southern Rockies Tue. 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 southern CA Sierra Nevada
mountains, and Transverse Ranges of southern CA.  Heavy snow is
also likely in the San Juans of CO/adjacent northern NM today into
early Tue due to WAA snows spreading northward and interacting
with the terrain. As upper divergence maxima persist in the ranges
of western CO, locally heavy snow are expected in windward areas.
WPC probabilities on D2 are high for 8 inches in the San Juans
with maximum amounts over 12" likely.

On Day 3, the upper trough moves across the southern Plains.  An
increasing number of forecasts show a 700-500 mb low developing
over the southern Plains.
The cool near the mid-upper low may allow snow to develop on the
western edge of the precip shield over portions of west Texas
north into Missouri.
The low level cold air also leads to a band of mixed sleet and
freezing rain in the AR Ozarks across southern MO, where the
greatest threat of significant icing exists on Day 3/Wed.

Petersen