Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Mon Nov 04 2019

Valid 00Z Tue Nov 05 2019 - 00Z Fri Nov 08 2019

...Northern Rockies...

Models continue to show a shortwave trough dropping southeast from
western Canada into the northern Rockies and High Plains - driving
a cold front south across Montana.  Post-frontal upslope flow in
combination with low-to-mid level frontogenesis is expected to
support a period of organized precipitation across the
northwestern to central Montana ranges Tuesday night into
Wednesday.   Overall, expect snowfall accumulations to be light,
with WPC probabilities showing little threat for amounts exceeding
4-inches away from the higher peaks.

...Northern Plains/Upper Midwest/Great Lakes/Northeast...
As the previously noted shortwave continues to drop southeast,
moving into the base of the longwave trough, models show a weak
surface wave moving east from Nebraska into Iowa on Wednesday. 
Low-to-mid level frontogenesis and modest upper forcing north of
the low is expected to support a stripe of mainly light amounts
centered across southern Minnesota and northern Iowa into southern
Wisconsin and central Michigan on Wednesday.  The progressive
nature of the system should limit accumulations, with WPC
probabilities showing little threat for widespread accumulations
exceeding 4-inches.

Light snow is forecast to move east with the initial wave into
western Pennsylvania and New York on Thursday.  Meanwhile expect
snow to redevelop farther to the west as upstream energy amplifies
the trough over the upper Great Lakes.  Model solutions begin to
diverge at this point, with the GFS becoming faster with the upper
trough and more developed with the surface wave - spreading
heavier snowfall amounts from the lower Great Lakes eastward.  In
contrast, the NAM moved to the slower side of the guidance on
Thursday.  For Day 3 across the Northeast, the WPC preference was
for a QPF/snowfall accumulation solution more similar to the
deterministic ECMWF and the European Ensemble Mean.

Meanwhile, prolonged northwest flow within the persistent longwave
trough will drive periods of lake effect snow (LES), mostly
downwind of Superior and northern Lake Michigan, where fetch is
sufficient to develop LES banding.
Snow showers off of the lakes into the U.P. and northern portions
of the L.P. of Michigan will occur periodically through forecast
period.

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

Pereira