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

Valid 12Z Wed Apr 19 2023 - 12Z Sat Apr 22 2023

...Pacific Northwest, Northern Rockies, and Northern High Plains...
Days 1-3...

Broad upper troughing remains over the Pacific Northwest into the
Northern Plains early this morning with several embedded pieces of
shortwave energy noted in the latest water vapor imagery. One,
currently over southern Montana will be the main driver for heavy
snow over the next 12-24 hours across central/eastern Montana into
far northwest North Dakota. That shortwave trough will continue
lifting northeast this morning and a surface low, currently over
southeast Montana, will deepen as it straddles the MT/ND border
into this evening. The combination of the broad upper level
diffluence, modest moisture wrapping into the system, and favored
northeasterly flow will support a prolonged period of moderate to
locally heavy snowfall across accumulating snow in mountain ranges
that include the Bighorns of Wyoming, the Absaroka of southwest
Montana, the Big Belt/Little Belt/Big Snowy Mountains of central
MT, and into the far northern Plains of northeast Montana. These
areas are where the latest WPC snow probabilities are greatest for
8 inches with isolated signals (10-20%) for amounts in excess of
12 inches. The highest peaks of the Big Belt/Little Belt/Big Snowy
Mountains could top 2 feet of snow.

Elsewhere in the region from the OR/WA Cascades through the rest
of the Northern Rockies southward into the Central Rockies,
another passing weak shortwave within the longwave troughing will
keep weather unsettled with light to locally moderate amounts of
snowfall over the next couple of days. Additional amounts will be
on the lighter side generally with the greatest amounts tied to
the highest peaks of the OR Cascades where additional amounts of
6-12" will be possible.


...Upper Midwest...
Days 2-3...

Beginning this evening, a strong shortwave trough will round the
base of the deeper closed low that is centered over the Northern
Rockies, pushing out into the Plains by early Thursday morning.
This shortwave energy will lead to cyclogenesis over the Midwest
with the surface low track expected through southeast Minnesota to
northwest Wisconsin by late Thursday. A coupled jet streak, one
over the central U.S. and another over parts of western Ontario
will provide large scale forcing for ascent over the region,
helping the low to deepen through Thursday night. Meanwhile,
increasing moisture from the south will wrap around the system. As
the mid/upper level low moves eastward, this will lead to a colder
air mass moving in while the frontogenetical band of precipitation
tightens, resulting in a changeover from rain to wet, heavy snow
Thursday afternoon with the greatest axis northwest of the 700 mb
center.
The heaviest precipitation is expected from Thursday afternoon
through Thursday night, but the occluding system will linger over
the region through Friday, continuing light to occasionally
moderate snowfall as individual embedded lobes of vorticity pivot.
By late Friday night into Saturday, the system gets kicked
eastward as upstream ridging builds, allowing the mean longwave
trough to settle over the eastern Great Lakes.

The latest probabilistic guidance including the WPC snow band tool
suggests snow rates approaching 1-1.5"/hr will be possible across
northern Minnesota and extreme eastern North Dakota at the peak
Thursday afternoon/evening. The latest experimental probabilistic
WSSI (pWSSI) shows a range of values for Moderate Impacts, from
slight (10-20%) around Fargo, ND and Duluth, MN to moderate and
high values (60-70%) closer to International Falls, MN. The latest
WPC snow probabilities have increased some over the last couple of
forecast cycles, now showing from near 40 to 80 percent
probabilities for at least 6 inches. Localized totals near the
Canadian border approaching 8-10 inches will be possible. As the
greatest snow rates subside late Thursday night, the primary
driver for hazardous winter weather Friday will be the gusty winds
that could make for reduced visibilities. 

Taylor