Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
447 AM EST Mon Nov 23 2020

Valid 12Z Mon Nov 23 2020 - 12Z Thu Nov 26 2020


...Pacific Northwest/Northern Rockies...
Ongoing precipitation associated with an upper level shortwave and
weakening frontal system is expected to continue Tuesday morning,
with additional light snow accumulations expected across the
Olympics, Cascades and northern Rockies.  Dry conditions are
expected to briefly follow as a shortwave ridge moves across the
region late Tuesday into early Wednesday.  Wet conditions return
Tuesday afternoon with a frontal boundary and approaching
shortwave.  This is expected to bring a period of heavier
precipitation, including mountain snows to the Olympics and
northern Cascades beginning Tuesday afternoon and continuing into
Wednesday as the upper trough moves across the region.  As the
trough moves inland, snows will extend south into the central
Cascades and east into the northeastern Oregon and southeastern
Washington mountains, as well as portions of the northern Rockies.
 Areas impacted by heavy accumulations will likely include the
Washington and Oregon Cascades, where WPC PWPF indicates
widespread potential for storm totals of a foot or more.

...Central Rockies/Plains...
As an amplifying upper trough moves across the Great Basin and
Southwest, areas of heavy snow are expected to develop and
continue through Monday across portions of the western and central
Colorado ranges.  Heavy amounts are especially likely across the
San Juan Mountains, where WPC PWPF shows high probabilities for
accumulations of a foot or more across portions of the region. 
Snows are likely to continue into Tuesday before diminishing late
in the day as the upper trough moves east into the Plains. 

Snows are expected to develop along and east of the I-25 corridor
as the upper trough moves east and an upper low begins to develop
over the High Plains on Tuesday.  While accumulating snows are
likely over the mountains and foothills, marginal temperatures are
expected to largely limit amounts over the Plains.  However,
dynamical cooling within a developing deformation band may be
sufficient enough to produce a swath of wet accumulating snow
across portions Nebraska and Kansas Tuesday and Tuesday night.

...Upper Mississippi Valley/Great Lakes...
An upper trough moving across the West on Monday is expected to
split as it moves across the Rockies into the Plains on Tuesday. 
As the previously noted low develops within the southern branch
over the central Plains, models show the northern stream portion
moving from the northern Plains toward the upper Great Lakes. 
Models continue to show a low level warm front, along with a
favorable upper jet couplet, supporting snow moving northeast from
eastern Iowa and northern Illinois early Tuesday to northern
Wisconsin and Michigan Tuesday night.  While widespread heavy
accumulations are not expected, some areas could see a few inches,
with WPC PWPF showing some higher probabilities for amounts of
4-inches or more across central Wisconsin.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira