Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 PM EST Fri Jan 01 2021

Valid 00Z Sat Jan 02 2021 - 00Z Tue Jan 05 2021


Day 1...
...Pacific Northwest to the Northern Rockies...
An upper level shortwave and its associated frontal band are
expected to support a round of organized precipitation, including
mountain snow, as they move across the Northwest into the northern
Rockies late today into the overnight.  Even though this system is
expected to quickly weaken and move east, unsettled weather is
likely to continue as strong westerly flow ahead of the next
system begins to channel deeper moisture into the region. 
Guidance shows organized heavier precipitation returning to the
Northwest as this next system's associated warm front moves into
the region late Saturday.  One day snow totals are likely to
exceed a foot across the higher elevations of the Olympics and
Washington Cascades.

...Northeast...
The low currently centered over the mid Mississippi valley this
afternoon will continue to move northeast toward the Great Lakes
overnight.  Meanwhile, warm advection precipitation ahead of the
system will continue to interact with a cold high moving off of
the Northeast coast.  This will support a wintry mix with a period
of snow, changing to sleet and freezing rain across much of
Upstate New York and western New England, with mainly snow farther
east across most of Maine.  While some areas, especially portions
of the Catskills to the southern Adirondacks, and the Berkshires
to the southern Green Mountains, could see some significant ice
accumulations, the overall trend in the models has been toward a
colder solution, with a longer period of snow especially over the
Adirondack region into northern Vermont overnight.  As the parent
low moves east of Lake Ontario, energy is expected to quickly
transfer to a coastal low that is forecast to quickly develop and
move from southern New England into the Gulf of Maine Saturday
morning.  Low-to-mid level frontogenesis is likely to support
moderate to heavy snowfall within the deformation band setting up
north of the low.  The the 12Z HREF guidance shows several members
suggesting snowfall rates of 0.5-1 in/hr across an area stretching
the North Country region of New York to western Maine Saturday
morning, with rates increasing as the band shifts farther east
across Maine during the afternoon.  The latest WPC PWPF, shows
high probabilities for snow accumulations of 4-inches or more
covering a large portion of the North Country, northern Vermont,
northern New Hampshire and most of interior Maine.  Probabilities
for 8-inches or more have increased across portions of Maine, with
probabilities of 50 percent or greater covering a large portion of
central Maine. 

Days 2-3...
...Pacific Northwest/California to the Northern Rockies...
Heavy precipitation is likely to continue, with more widespread
potential for heavy snow as the previously noted system moves
across the region Saturday night and Sunday.  Additional heavy
snows are likely for the Washington Cascades, with snow levels
dropping below pass level Saturday night.  Heavy accumulations are
also likely from the Wallowa Mountains in northeastern Oregon to
the central Idaho ranges, as well the Idaho Panhandle to the
northwestern Montana ranges. 

As a front lingering near the Oregon-California border lifts back
to the north, this is expected to bring additional organized
precipitation, including additional heavy snows back into the
Olympics and northern Cascades early Monday.  Then by late Monday,
an amplifying shortwave approaching the coast is expected to
support heavier precipitation farther south -- increasing the
potential for heavy snows across the southern Cascades into the
northern Sierra.

...Ohio Valley/Great Lakes to the Northeast...
An upper low moving into the southern Plains on Saturday is
forecast to follow a track similar to its predecessor -- lifting
across the mid Mississippi valley into the lower Ohio valley and
then the southern Great Lakes region Sunday morning.  This is
likely to produce some light precipitation, including light snows
across the Ohio valley into the Great Lakes Sunday morning.  At
this time, heavy amounts do not appear likely.  Meanwhile, as the
system moves through the Ohio valley, a secondary low is expected
to develop over the Southeast and track to the Mid Atlantic coast
by midday Sunday.  Precipitation developing between the two
centers will spread north through the northern Mid Atlantic into
the Northeast on Sunday, likely beginning as a period of snow with
light accumulations from Pennsylvania into New York and southern
New England.  While the coastal low is likely to continue to
deepen as moves away from the Mid Atlantic coast, its track and
potential impacts on the Northeast are far from certain.  While
the 12Z GFS takes the low closer to the coast -- showing some
potential for significant snows developing across New England on
Monday -- the general consensus of the remaining models is farther
east, with lighter snow accumulations through late Monday.

Pereira