Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
408 AM EST Tue Nov 05 2019

Valid 12Z Tue Nov 05 2019 - 12Z Fri Nov 08 2019

...Northern Rockies/High Plains...

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 higher amounts in windward terrain
and lighter amounts in valley locations/downslope areas.

...Northern Plains/Upper Midwest/Great Lakes...
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 most areas received 2-4 inches in the band of
snow with the wave.

Areas of cross lake flow and leeshore convergence will drive
periods of lake effect snow (LES), mostly downwind of Superior and
northern Lake Michigan, where fetch is sufficient to develop LES
banding.  The higher amounts day 1 are expected to be 4-6 inches
on the eastern shore of the UP of MI, with lower amounts in the
western UP and also in northwest lower MI. On day 2, the low level
frontogenesis combines with lake enhancement to continue light
snow in western lower MI. 2-4 inches of snow is possible in west
central lower MI.  Lee shore convergence weakens as does ascent
and snow shower intensity off Lake Superior into the UP of MI and
off Lake MI into northwest lower MI, so amounts trend down a
little n day 2. On day 3, northwest flow reestablishes confluent
flow and leeshore convergence in the eastern up of MI, so snow
should pick up again in coverage/amounts, with snow showers
extending down into northwest lower MI as well.

...Northeast...
Light snow is forecast to move east as the 700 mb wave crosses
northwestern Pennsylvania and western New York on Thursday.
A mixture of precip types and initial wet snow are likely to keep
accumulations limited in PA/western NY.  Expect snow to increase
in coverage/intensity as the trough amplifies crossing New York
into New England.  Mid level deformation and frontispieces should
drive an axis of moderate to heavy snow across the NY Adirondacks
to the VT Green Mountains and NH White Mountains, finally into
central Maine. An area of 6-12 inches of snow is possible, but
confidence is not high yet due to changing forecasts from model to
model and run to run.

Model solutions begin to diverge at this point, with the 00z ECMWF
trending significantly faster/further north towards the GFS
solution of the low off the Maine coast by Fri morning.  In
contrast, the NAM remained south of the GFS/ECMWF, while the
Canadian and Ukmet showed a weaker sfc reflection that moved off
the coast faster. For Day 3 across the Northeast, the WPC
preference was for a QPF/snowfall accumulation solution  to the
deterministic ECMWF and the European Ensemble Mean.

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

Petersen