Probabilistic Heavy Snow and Icing Discussion...amended
NWS Weather Prediction Center College Park MD
724 PM EST Thu Feb 17 2022

Valid 12Z Fri Feb 18 2022 - 12Z Mon Feb 21 2022

...Western Great Lakes to northern New England...

A shortwave currently over Missouri will become absorbed into the
westerlies and shear into confluent mid-level flow towards the
Great Lakes through tonight.  This feature will interact with a
northern stream shortwave emerging from the Upper Midwest to drive
an amplified but positively tilted longwave trough across the
area.  A wave of low pressure is progged to lift rapidly northeast
from the Ohio Valley and then across New England by Friday morning
while quickly intensifying.  This low will move along a
southeastward advancing cold front, and it is this front which
will cause precip to change from rain, to a mix, to snow, in a
swath from the Great Lakes to northern New England.  In terms of
snowfall, the heaviest accumulations should be across southeast
Ontario and southern Quebec, but there will likely be a swath of
moderate snow from northern Indiana to southeast Michigan, and
also reaching far northwestern and northern New York and
northwestern Maine, with some 6+ inch totals just north of the
Adirondacks and far northwest Maine.

A corridor of mainly light freezing rain just south of the main
snowfall axis is expected as a warm nose aloft overruns
subfreezing temperatures near the surface.  While widespread heavy
ice accumulations are not expected, WPC probabilities do indicate
the potential for localized accumulations of 0.10 inch or greater
across northern portions of Upstate New York and central/northern
Maine. Where freezing rain occurs, there is some concern that a
rapid deepening of the cold nose could promote more sleet than
freezing rain, but even where freezing rain does occur it may not
accrete efficiently due to heavier rates, so freezing rain
exceeding 0.25" is not likely.


...Northern Plains through the Great Lakes...

A potent shortwave will dive quickly track southeastward out of
Alberta on Friday morning, and this will be accompanied by a
clipper type low moving towards the Great Lakes, and accompanied
by an intense cold front sweeping southeast across the area.  This
system will be accompanied by modest warm air advection ahead of
it, followed by a strong surge of arctic air in its wake.  A
diffluent jet streak will aid in synoptic ascent, and the
combination of height falls and the warm advection will produce a
swath of light to moderate snow across the Great Lakes, including
much of Michigan, and then downwind of Lake Erie and Lake Ontario.
 While some of this snowfall will be driven by the leading warm
advection, the majority of the snow is likely to occur from lake
effect snow owing to intense cold air advection behind the front,
and there is about a 30% probability of greater than 4 inches
across the eastern U.P. and northwestern L.P. of Michigan on
Saturday, and then reaching the Tug Hill Plateau with probs of the
same accumulations up to 70 percent.  There is also the potential
for some snow squalls across portions of the Northern Plains and
Upper Midwest Friday along the actual cold front, and these could
severely limit visibility and result in hazardous road conditions,
albeit for brief periods of time with most locations affected
getting under 2 inches.  This will be something to monitor in
future forecasts.

...Pacific Northwest...

A shortwave embedded within confluent mid-level flow will reach
Washington State by Saturday night, accompanied by modest upper
level diffluence and rising PWs.  This will result in
precipitation across the mountainous terrain of the Pacific
Northwest to the Montana Rockies.  Snow levels late Day 2 are
expected to be in the 2000-3000 foot range, but should rise as
warm advection onshore drives precipitation to the east ahead of a
surface cold front dropping southward beneath the shortwave.  As
the front drops southward Saturday morning, snow levels will
likely drop once again to as low as 1000-2000 feet in Washington,
and 2000-3000 feet elsewhere.  Therefore, much of the
precipitation should be in the form of snow in the Olympics,
Cascades, Blue Mountains, and Northern Rockies, where WPC
probabilities for 8 inches are above 80% for the northern
Washington Cascades, and and generally 40 to 70 percent for the
higher terrain of northern Idaho and near Glacier National Park.
Locally more than 12 inches is likely in the Washington Cascades,
with significant accumulations for the main mountain passes as
well.


Hamrick/Pereira/Weiss


Key Messages for the Feb 17-18 Winter Storm:

-A strong winter storm will impact the Great Lakes to northern New
England into Friday, producing wintry hazards of moderate to heavy
snow, sleet, and freezing rain.

-A swath of moderate snow is expected across northern Indiana, and
southern Michigan through this evening and tonight, extending into
far northern New York and northern Maine by Friday morning, with
the highest snowfall totals likely just north of the Canadian
border.

-The combination of wet snow and strong, gusty winds will lead to
reduced visibility and  hazardous travel conditions, particularly
for untreated roads.

-A corridor of light freezing rain and sleet are likely just to
the south of the heavy snow axis, with more than 0.1 inches of
freezing rain likely for portions of northern New York. Some
impacts to travel and power are possible in these areas.

-Temperatures are expected to drop quickly overnight.  Any
untreated wet surfaces could freeze quickly.