Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
318 PM EDT Thu Apr 20 2023

Valid 00Z Fri Apr 21 2023 - 00Z Mon Apr 24 2023

...Pacific Northwest and Northern/Central Rockies...
Days 1-2...

An upper low tracking into British Columbia this evening will
become an open trough as it tracks through southern British
Columbia Thursday night. This trough will position its best PVA
over the northern Rockies Thursday night into Friday morning, then
into the central Rockies midday Friday and into the evening hours.
These portions of the Rockies will also reside beneath the
difluent left-exit region of a 130 knot 250mb jet streak during
this same time, fostering favorable vertical ascent atop the
atmosphere. There will also be some added 850-700mb Pacific
moisture into these regions, but the fast progression of this
disturbance will keep snowfall duration relatively short. With
also a lack of sufficiently cold air aloft, periods of heavy snow
will be confined to the tallest peaks of the Olympics, Cascade
Range, and into the higher terrain of the northern and central
Rockies. WPC probabilities contain 40-60% probabilities or >6" of
snowfall in ranges such as the Blue Mountains of Oregon, the Bear
River Range, the Tetons, Wind River, and northern Colorado
Rockies. Elevations with the best odds of seeing >6" reside above
5,000ft in the Pacific Northwest and above 7,000ft in the Rockies.
The WSSI generally caps impacts in these mountain ranges at Minor,
but some localized Moderate impacts are possible.

...Upper Midwest & western Great Lakes...
Days 1-2...

Periods of heavy snow will continue this afternoon and into the
evening hours as the cyclone over western WI continues to deepen
while the remnant upper low over western North Dakota gradually
opens up into an elongated upper trough on the western flank of
the storm. Moisture will be supplied around the northwest flank of
the storm via easterly 850mb moisture transport while a dome of
Canadian high pressure provides adequately cold enough air for the
primary precipitation type to be snow. This warm conveyor belt
will prompt the formation of snow bands beneath the TROWAL on the
western flank of the low this evening, and with the sun setting,
will provide a more opportune window for accumulating snowfall. In
addition, wind gusts of 30-40 mph will lead to blowing snow and
near whiteout conditions tonight. As the low occludes and weakens
Friday morning, so will snowfall rates. Widespread snow showers
are expected from the Dakotas to northern Minnesota but with
daytime heating ongoing, the lighter snowfall rates will make it
increasingly difficult for snowfall to accumulate on most
surfaces. Latest WPC probabilities show an additional 6-12" of
snowfall is likely from northeast North Dakota to northern
Minnesota tonight and into Friday morning. The latest WSSI does
depict Moderate to even in some Major impacts in these areas,
largely due to the Snow Amount component, but Snow Load and
Blowing Snow are also contributing factors.

Scattered snow showers will continue across the eastern Dakotas
and Upper Mississippi Valley Friday evening while a surface trough
revolves around the storm as it tracks east into Ontario. This
trough will keep snow showers, and in some cases organized bands
of snow, in the forecast through Friday night and into Saturday
morning from Minnesota to the northern Great Lakes. Most
accumulations will be light due to the lighter snowfall rates
during daytime hours, but the lingering wind gusts above 30 mph
could still lead to poor visibility for motorists. There is a good
amount of uncertainty regarding the snowfall potential out to Day
3 as another low pressure system forms farther east. Most
probabilities >4" were <10%, but this still bears watching as lake
effect snow bands could result in localized totals >4" along the
U.P. of Michigan Saturday night into early Sunday.


The probability for icing over 0.1" is less than 5 percent for
Days 1-3.


Mullinax