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

Valid 12Z Sun Dec 27 2020 - 12Z Wed Dec 30 2020


...Great Lakes..
Days 1-3...

A wave of low pressure is likely to move from IL to WI and across
Michigan today. As this low moves northeast, a closed 850 mb low
drives a warm front across eastern WI and lower MI. 850 mb
warm/moisture advection and convergence will drive an area of
light snow across east central WI and northern lower MI.
The models forecast a thin band of 0.25 inches liquid equivalent,
so 2-4 inches of snow is likely across portions of east central WI
to northern lower MI.
WPC probabilities are as high as 30% for 4 inches in these areas.

Behind this low, another round of LES is likely for parts of the
U.P., northern L.P. Sunday night into Monday. Amounts should be
modest, as WPC probabilities indicate
low probabilities for 4" in the favored snow belt regions.  As the
downstream warm front crosses Lake Ontario into the Tug Hill on
Monday,snow showers develop for a period of several hours before
the front departs.  Additional snow showers are expected as the
cold front approaches Tue morning.  Several more inches of snow
are possible.

...CA/Great Basin/Rockies...
Days 1-2...

An upper trough moving onshore CA late tonight into early Monday
continues east across the Great Basin Monday and into the central
and southern Rockies Tuesday. This feature will be accompanied by
persistent jet energy crossing central to southern CA inland
across central to southern NV/UT, then CO. Upper level divergence
combines with topographically forced ascent in wind terrain to
produce snows across the central to southern CA Sierra Nevada
mountains, followed by the Transverse Ranges of southern CA.  The
jet axis on Day 2/Mon crosses UT and into western CO.  The jet
axis crossing the ranges lead to several inches of snow across the
mountains of western CO. The heaviest snow day 2 is expected in
the San Juans 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.

As the upper trough moves slowly east out of the Rockies and
across the Plains D3/Tue, snow coverage and intensity drops off,
with a few inches of snow expected in the southern CO mountains of
New Mexico before activity ends.

...Central Plains/Upper MS Valley...
Day 3...

As the long wave trough moves east across the Plains, low-mid
level southwest flow increases.  The increased low level jet
drives warm/moisture advection with a 850-700 mb front moving
northeast across Nebraska and Kansas through eastern South Dakota,
Iowa, southern Minnesota, Wisconsin and northern Illinois.
The combination of the front and warm advection produces ascent
and snow across northeast Nebraska and southeast South Dakota
across southern Minnesota, northern Iowa,  and into southern to
central Wisconsin.

There remains considerable spread in both the placement and timing
of the 700 mb low and resultant QPF and where the precip type
transition zone sets up across the upper MS Valley to the Upper
Great Lakes.  The GFS/GEFS differ from the ECMWF/ECMWF Mean,
UKMET, Canadian global, and NAM. A preferred blend included the
GFS/EC, as  combination despite the NAM's further north track
reduced QPF amounts and potential snow in eastern Nebraska and
eastern South Dakota.

South of this area, warm advection turns snow to sleet and
freezing rain across northern Kansas to northern Missouri and
Illinois, continuing into Iowa and southern Wisconsin. WPC
probabilities indicate a low risk for 0.25" of freezing rain
accretion in northeast KS to northern MO and southern IA.

The probability of 0.25 inches of icing is less than 10 percent
Days 1-2.

Petersen