Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 PM EDT Thu Oct 29 2020

Valid 00Z Fri Oct 30 2020 - 00Z Mon Nov 02 2020

...Northeast...
Days 1-2...

A mid-level wave racing northeastward to off the Mid-Atlantic
coast Friday will spawn weak surface cyclogenesis behind the
remnants of Tropical Storm Zeta. Modest synoptic ascent combined
with isentropic lift will continue to spread precipitation across
Upstate New York and Southern/Central New England. Late tonight,
strong CAA will commence from the north, coincident with some
brief but intense frontogenesis. The combination of this enhanced
ascent during a time of more large scale column cooling will cause
precip to changeover from rain to snow, first in the higher
terrain and then eventually into the lower elevations before
precip winds down Friday aftn. The snow may be briefly heavy, with
rates potentially approaching 1"/hr at times. After a long
duration rain event, accumulations will be slow to develop.
However, the higher snow rates should eventually overcome this,
especially in the terrain, and WPC probabilities for 4 inches are
moderate to high in the Catskills, Berkshires, Southern Greens,
and Monadnocks, with lower probabilities extending into the
Adirondacks and Worcester Hills. Elsewhere, light accumulations of
1-2" are possible across the lower terrain and into central CT,
RI, and eastern MA.


...Pacific Northwest...
Day 1...

A modest shortwave digging down from the Gulf of Alaska will be
accompanied by a Pacific jet streak and surface cold front to
produce snowfall in the high terrain of the WA Cascades and
Northern Rockies. Snow levels will generally be high, so
significant accumulating snow should be confined to the highest
terrain. Snow levels will fall with the passage of the shortwave
and cold front, but this will also bring an end to the
precipitation, so snow accumulations in the valleys is not
expected. WPC probabilities for 4 inches are moderate.


...Michigan...
Day 3...

A potent low pressure system will move across Ontario dragging a
strong cold front across the Lakes Saturday night into Sunday.
Behind this front, strong CAA across the still warm lake waters
will produce a favorable for LES as N/NW winds develop across Lake
Superior, Michigan, and Huron by the end of D3. Modest mid-level
ascent will aid in LES production, but the short temporal duration
of aligned forcing will limit overall accumulations. Still, a
period of moderate to heavy snow is likely across the U.P. and NW
portions of the L.P. where an effective fetch may setup as well.
WPC probabilities for 4 inches are high across much of the U.P.
and into the far NW corner of the L.P. near Traverse City.


The probability for significant ice (0.25" or greater) is less
than 10 percent all 3 days.

Weiss