Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 PM EDT Wed Apr 19 2023

Valid 00Z Thu Apr 20 2023 - 00Z Sun Apr 23 2023

...Pacific Northwest, Northern Rockies, and Northern High Plains...
Days 1-2...

Periods of snow will continue in portions of the northern High
Plains and as far west as western Montana as 850-700mb moisture
continues to wrap around the 700mb low positioned along the
Montana/Canada border this evening. The heaviest snowfall is
likely to occur in the Little Belt, Big Snowy, and Bearpaw
Mountains of central Montana, as far west as the Lewis Range, and
on south to the Big Horns of Wyoming where favorable upslope flow
supports enhanced snowfall rates. Farther east, the TROWAL on the
western flank of the low in northwest North Dakota is likely going
to set up over northeast Montana and northwest North Dakota
through Thursday morning. Eventually, this low will open up into a
trough connected to the new primary low in the Upper Midwest
Thursday afternoon. This will favor additional periods of snow
through the day time hours, although accumulating snowfall will be
harder to come by given the strong solar input. Periods of snow
will eventually wind down from the Montana Rockies to the High
Plains of western North Dakota by Thursday evening. WPC
probabilistic snowfall output suggests high chances (70-90%) for
additional snowfall totals >6" in the aforementioned mountain
ranges of western and central Montana through Thursday night. In
northeast Montana and northwest North Dakota, probabilities for
>6" of snowfall are most commonly in the 30-50% range, with areas
featuring the higher chances for those snowfall amounts along the
U.S./Canada border. The Day 1 WSSI shows Moderate to even in some
cases Major impacts in the areas mentioned above, suggesting
hazardous travel conditions are likely through Thursday evening.

In the Pacific Northwest, a potent upper low of the coast of
British Columbia will deliver another slug of Pacific moisture and
divergent upper level flow over the Olympics and Cascades on
Thursday. At the surface, a warm front associated with a storm
system in the northeast Pacific will approach Thursday morning and
force precipitation to track into the the Pacific Northwest.
Precipitation will fall in the form of snow in the Olympics and
Cascades, which will be heavy at times Thursday afternoon. WPC
probabilities show up 40-60% odds for >6" of snowfall in these
ranges through Thursday evening. This round of snowfall is short
lived as it quickly heads east into the Northern Rockies Friday
morning, but moisture and lift aloft will be less sufficient to
support heavy snow.

...Upper Midwest...
Days 1-2...

An occluding low pressure system over western North Dakota will
give way to a new area of low pressure developing on its
associated triple point in the heart of the Midwest this evening.
As a 500mb disturbance rounding the base of the 500mb low over the
Missouri Valley takes on a negative tilt early Thursday morning,
the storm system will continue to deepen as it tracks northeast
into northern Wisconsin by Thursday afternoon. The strengthening
of this storm system is also supported by strong divergent flow
within the left-exit region of a 120 knot 250mb jet streak. As the
slug of moisture arrives ahead of the warm front lifting north,
ice will be the primary concern in the Minnesota Arrowhead,
western U.P. of Michigan, and far northern Wisconsin. The deep
snow pack still in place will act to keep the air-mass adequately
cold enough for temperatures to remain below freezing. Meanwhile,
warm southeasterly flow at 850mb will cause a burgeoning warm nose
aloft that causes snow aloft to melt and as it approaches the
surface, either freeze into sleet or reach the ground as freezing
rain. The wild-card in this setup is much of the ice does come
during the daytime hours, and the added solar input could limit
icing potential. However, surfaces from snow pack that feature
temperatures below freezing would still have the potential to
garner better ice accumulation. Latest WPC probabilities for ice
accumulations >0.10" have increased to 40-60% through Thursday
night, with up to 10% chances just north and west of Duluth for
ice accumulations >0.25".

Farther north and west, the northern and western flanks of the
850mb low are where the conveyor belt of 850mb moisture will wrap
around the low and result in a pronounced TROWAL by Thursday
evening from northern North Dakota to northern Minnesota.
Temperatures within the boundary layer will also be cold enough to
support snow, thanks in large part to a cold dome of Canadian high
pressure to the north. Snow initially during the daytime hours may
struggle to accumulate as the TROWAL becomes more defined and
hourly rates are lighter, but by the afternoon and evening hours,
banded snow should take shape and provide a better opportunity for
accumulating snowfall, especially after sunset. The storm system
will become stationary north of the Minnesota Arrowhead through
Friday morning, resulting in periods of snow and gusty winds from
the eastern half of North Dakota to northern Minnesota. Snowfall
rates will gradually diminish throughout the day Friday, and given
the time of year, any accumulations would likely be minor. The
lingering wind gusts though may still result in poor visibility in
some cases.

The Days 1-2 WSSI show Moderate impacts along the North
Dakota/Minnesota and Canada border with Snow Amount being the
primary driver in the WSSI algorithm. Snowfall amounts from far
northern North Dakota to northern Minnesota can range between
6-12" (localized amounts >12"), while totals in the 1-4" range are
more common from central North Dakota to the Minnesota Arrowhead.
The winds on the backside of the low could lead to blowing snow
and reduced visibility. The WSSI does contain some Minor to
Moderate impacts in parts of south-central North Dakota to account
for the potential effects from blowing snow. Snow will quickly
taper off by Friday evening as high pressure builds in from the
Canadian Prairies for the upcoming weekend.

Mullinax