Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 PM EST Fri Jan 17 2020

Valid 00Z Sat Jan 18 2020 - 00Z Tue Jan 21 2020


Day 1...

...Northern Plains to the Great Lakes/Northern Mid Atlantic...

A 700 mb low moving northeast across the northern Plains is
forecast to induce the area of snow now crossing eastern SD and
southern MN to move northeast in tandem across eastern ND and MN,
and downstream across WI into MI.  The heaviest totals are still
forecast in the MN arrowhead, as a prolonged period of
southeasterly flow will also contribute to locally heavy, lake
enhanced amounts north of Lake Superior.  A foot of snow is
expected in this area. Another area of enhanced snow from Lake MI
is expected to develop once the cold front passes Sat, with snow
showers picking up across western lower MI.

Strong isentropic lift and low level warm air advection ahead of
an upper level shortwave will support precipitation spreading
northeast from the mid Mississippi valley into the northern Ohio
Valley, lower Great Lakes, central Appalachians, and northern mid
Atlantic Sat.

The low-mid level warm advection results in snow changing over to
sleet across northern IL and southern WI east across IN, OH, and
PA. The precip may end as a brief period freezing rain or drizzle.
Significant ice accumulations are possible, with another one to
two tenths of an inch icing likely across northeast MO into
western Illinois this evening. Lighter amounts are expected
further northeast across the northern IL and IN into OH and PA.
Light icing is also forecast in the central Appalachians, with one
to two tenths of an inch expected in ridge areas. 

...WA Cascades...
A warm front lifting north across the region will be associated
with deep layer warm advection and moisture transport.  The
combined lift from the jet and warm advection and frontal
convergence in the terrain supports a long duration snow. The
highest rates appear to peak with ascent maxima tomorrow morning
in the WA Cascades.  The long duration of favorable moisture and
lift favors heavy amounts across portions of the Olympics and
northern Cascades.  Locally a couple of feet is possible in
favored upscale areas.

Day 2...

...Great Lakes/Northeast...
Low pressure is forecast to slide east into the St Lawrence Valley
Saturday night as a second low begins to develop along the
northern New England coast.  In the warm advection pattern ahead
of the low, models are in good agreement --indicating light to
moderate snows spreading east across New England, with a wintry
mix changing to rain in southern New England.
 
Low pressure developing along the northern New England coast
should focus locally heavy amounts on the mid level deformation
zone across portions of New Hampshire and Maine before moving
south of Nova Scotia Sunday morning, bringing the event to a
close. Probabilities are moderate for 6 inches of snow from
northern NH across most of interior Maine. 

Heavy lake effect snow totals are likely east of Lake Ontario
through the Tug Hill region into
the western Adirondacks of NY. Westerly winds will support a
prolonged period of lake enhancement that will begin Saturday
night and continue into early Sunday. There is a strong potential
for locally heavy amounts of a foot or more across this region. 

The veering winds late Sunday will reduce cross lake trajectories
across Lake Ontario into western NY and shift the bands further
south. Westerly winds and leeshore moisture convergence will also
support lake effect snows east of Lake Erie, while northwesterly
winds will produce locally heavy amounts across portions of
northern Michigan.

The probability of significant icing (0.25 inch or greater) is
less than 10 percent Day 2.

Day 3...

...Great Lakes...
Lake effect snow showers Sunday evening are forecast by the NAM
and Canadian Regional GEM to focus off Lake Michigan into
southwest lower MI and nearby northern IN.
A building ridge moving in from the upper MS Valley will reduce
both moisture and lift late Sun night into Monday, so potential
for locally heavy snow is minimal.

The probability of significant icing (0.25 inch or greater) is
less than 10 percent Day 3.

Petersen