Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
433 AM EST Sat Nov 24 2018

Valid 12Z Sat Nov 24 2018 - 12Z Tue Nov 27 2018

...Western U.S....
Day 1...

A well-defined mid-upper level shortwave trough will continue to
drop southeast across the northern to central Rockies on Saturday,
with model guidance continuing to show low-mid level
frontogenesis, along with strong upper forcing, supporting locally
heavy mountain snows from the ranges of southwestern Montana to
northern Utah and Colorado.  Snow are expected to diminish from
west to east late Saturday into early Sunday as the shortwave
moves out into the central Plains.

...Central Plains to the Great Lakes...
Days 1-3...

As the previously noted shortwave digs southeast into the central
Rockies, expect precipitation to spread east into South Dakota and
the central Plains on Saturday.  Snow is forecast  across the High
Plains, with locally heavy accumulations possible in the Black
Hills and Nebraska Panhandle regions on Saturday.  Rain changing
to snow is expected further east as the shortwave moves out into
the Plains, with colder air surging down the backside of the
associated surface low as it tracks across the central Plains
Saturday evening. This system will continue to track further east
toward and into the lower Missouri valley Sunday morning, with low
to mid level frontogenesis and left-exit region upper jet dynamics
beginning to support a stripe of moderate to heavy snows on the
northwest side of the low.  These ingredients will likely continue
to support moderate to heavy snows as the low tracks further to
the northeast into the upper Great Lakes region late Sunday into
early Monday.  Unfortunately, the overnight guidance continued to
show significant differences with respect to the track of the low,
especially by late Sunday as the low begins to lift northeast from
the lower Missouri valley toward the Great Lakes.  The GFS and its
ensemble members are overall faster than the ECMWF and its
ensembles.  The GFS is also further to the north than the ECMWF
with the track of the low and its stripe of heavier snow amounts. 
While several guidance members have trended faster than their
previous runs, the 00Z ECMWF presented an opposing trend and was
slower than its daytime run, placing it on the western edge of the
guidance.  With forecast confidence limited, the WPC preference
was for a compromise solution, similar to ones offered by the
UKMET, Canadian and the ensemble means.

...Northeast...
Day 3...

Models show the shortwave over the Midwest lifting across the
upper Great Lakes into eastern Ontario and southern Quebec, with
trailing energy moving toward the Northeast coast Monday night.  A
surface low developing along the associated frontal boundary will
become the dominate surface reflection as it lifts along the
Northeast coast Monday night into Tuesday morning.  As the
trailing energy begins to lift along the Northeast coast, strong
divergence aloft, along with strong low level moisture advection
and ample low level cold air are expected to support the potential
for locally heavy snow accumulations across portions of the
Adirondacks and northern New England mountains.

...Pacific Northwest...
Day 3...

Overnight guidance offered a fairly good signal for appreciable
ice amounts from the northwestern Columbia Basin region into the
mountain valleys of the northern Cascades on Monday.  Models show
the low levels warming ahead of an approaching shortwave trough,
while easterly winds help to maintain subfreezing boundary layer
temps across portions of the region.  WPC probabilities show a
Moderate Risk for ice accumulations of 0.25 or more across
portions of this region.

Pereira