Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
447 PM EST Fri Jan 10 2020

Valid 00Z Sat Jan 11 2020 - 00Z Tue Jan 14 2020

Days 1-3...

...Pacific Northwest to the Northern Rockies...
An active period is forecast to continue with very heavy snow
accumulations expected for portions of the mountains of the
Northwest and the northern Rockies.

A system currently centered west of Vancouver Island will continue
to dig southeast across the Northwest Friday night into Saturday,
producing heavy snows from the Olympics and Cascades to the
northern Rockies.  Although this system is expected to continue to
move progressively southeast of the region, persistent onshore
flow in its wake, with embedded mid level energy, will support
snow showers through Saturday before the next system arrives
Sunday morning.  Models show another well-defined shortwave
dropping southeast into the region on Sunday, bringing additional
heavy snows to the mountains.  This system will also bring colder
air into the region as well, supporting an increasing threat for
lowland snow by late in the weekend and continuing into Monday as
yet another shortwave approaches the region. 

For the three day period ending 00Z Tue, WPC PWPF is greater than
90 percent for snow totals of 2-feet or more across a large
portions of the Cascades, as well as northeastern Oregon and far
southeastern Washington mountains and the northern Rockies from
northern Idaho and western Montana to western Wyoming.


...Southern Plains and Midwest...
There remains a strong signal for significant ice accumulations
from the central Plains northeastward to the Great Lakes, with
damaging ice accumulations likely over portions of Lower Michigan.
 Heavy snow is more likely further to the northwest from southern
Wisconsin to northern Michigan.

Increasing southerly flow ahead of an amplifying southern stream
trough will continue to move moisture north of a slow moving
frontal boundary extending from the Great Lakes back into the
southern Plains.  Shallow low level cold air north of boundary
with support an icy mix, with accumulating ice becoming likely
from Oklahoma northeastward to Lower Michigan Friday night.  As
low pressure organizing over the southern Plains begins to lift
out to the northeast, this axis of freezing rain is expected to
shift farther farther southeast, with accumulating snow developing
farther to the northwest in deeper cold air.  As the low tracks
from the mid Mississippi valley, directing copious moisture toward
the Great Lakes, cold air will remain in place across the Great
Lakes with high pressure sliding northeast and remaining across
Ontario through Sunday.  This will set the stage for potentially
damaging ice accumulations across portions of southern Lower
Michigan on Saturday.  WPC PWPF is greater than 50 percent for ice
accumulations of 0.50 inch or more across this region.  Meanwhile,
as the mid-to-upper level shortwave assumes a negative tilt,
low-to-mid level frontogenesis northwest of the low along with
strong upper forcing will support a swath of moderate to heavy
snows from southern Wisconsin to northern Lower Michigan Saturday
into Saturday night.

...Northeast...
Significant ice accumulations are also expected east of the Great
Lakes into far northern New York and New England, with heavy snows
possible across a small portion of northern Maine.

A surface boundary over eastern Canada will sag southeast through
the St Lawrence Valley, with high pressure over Ontario extending
east into Quebec.  Moisture spreading north ahead of the low in
the Great Lakes and into the shallow low level cold air north of
the boundary will support freezing rain, with accumulating ice
becoming likely across far northern New York into northern New
Hampshire, Vermont and Maine Saturday night.  As high pressure
remains anchored over Ontario and Quebec, the low over the Great
Lakes will slide east across the Northeast, with freezing rain
continuing across portions of the region on Sunday.  Meanwhile
deeper within the cold air, a swath of heavier snows may develop
across far northern Maine.

Pereira