Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 PM EDT Tue Apr 18 2023

Valid 00Z Wed Apr 19 2023 - 00Z Sat Apr 22 2023

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

From the Pacific Northwest to the northern High Plains, a broad
upper level troughing pattern containing several disturbances is
responsible for rounds of heavy snow, primarily in the mountain
ranges of the northwestern U.S.. Starting in the Pacific
Northwest, an upper level low of the British Columbia coast is
directing a plume of 850mb moisture at the Cascade Range. At the
same time, temperatures are anomalously cold as NAEFS shows
temperatures in the 850-500mb layer are below the 10th, and even
at times below the 2.5th, climatological percentile. This is a
recipe for more mid-spring snowfall from the Olympics and Cascade
Range to as far south as the Salmon Mountains of far northern
California tonight and into Wednesday. WPC probabilities for >8"
of snowfall in these ranges are between 50-80% and the WSSI shows
Moderate to even Major impacts through Wednesday. The Olympics,
most notably, feature the largest concentration of Major impacts
and are largely driven by a combination of Snow Amount and Snow
Load.

Farther east, a shortwave trough taking on a negative tilt over
the northern Rockies this evening will promote strong vertical
ascent aloft from northern Wyoming to much of Montana and western
North Dakota. The strong upper level divergence aloft will aid in
the development of a deepening 850mb low north of the Black Hills
tonight and direct a conveyor belt of southeasterly 850mb moisture
flux with origins as far south as the western Gulf of Mexico. By
Wednesday morning, a 700mb low will develop over western North
Dakota and 700mb moisture. As moisture wraps around the mean
500-700mb low, it will place sufficient moisture aloft within an
air-mass sufficiently cold enough to support 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. WPC probabilities show high chances (70-90%) for snowfall
>6" in these aforementioned areas from Tuesday night into
Wednesday evening. The WSSI shows Moderate to Major impacts in
portions of these areas, with the highest concentration of Major
impacts in the mountains of central Montana and northwest North
Dakota.

...Upper Midwest...
Days 1-3...

Day 1...
Out ahead of the developing upper low in the northern High Plains,
a weaker vorticity maximum breaking off from the longwave trough
over the northwestern U.S. will contain both divergent flow aloft
and WAA at low levels. This WAA at low levels will race north of
the warm front lifting north through the Midwest Wednesday
morning, where surface wet bulb temperatures will be cold enough
for arriving precipitation to fall in the form of a wintry mix.
WPC probabilities do suggest up to 30% odds for ice accumulations
>0.1" through Wednesday afternoon. Accumulations should generally
be less than 0.1" given the bulk of the wintry mix will occur
during the daytime hours, allowing for the added solar radiation
to warm surfaces enough to limit accumulating ice to tree branches
and grassy surfaces. Still, cannot rule out some slick spots in
the MN Arrowhead of northern WI where recent snowfall may be able
to keep surface temperatures below freezing longer than modeled
and thus could lead to some hazardous travel conditions.

Days 2-3...

By Wednesday evening, a 500mb shortwave trough rounding the base
of a 250-500mb upper low over the northern High Plains will help
spawn a storm system in the Central Plains that tracks along a
frontal boundary that extends north and east into the Midwest.
There are coupling jet streaks to note: one in the southern High
Plains and another in Ontario, that will place their diffluent
quadrants over the Midwest. This example of jet coupling, along
with strong warm air advection through the Mississippi River
Valley, lays the ground work for the aforementioned area of low
pressure to organize and strengthen as it tracks into the Upper
Midwest Thursday afternoon. As moisture wraps around the
developing 700mb low, it will also wrap around towards the remnant
700mb low over North Dakota, creating a double-barrel storm
structure over the North-Central U.S. By Thursday night and early
Friday morning, the 700mb low over North Dakota looks to open up
into a trough on the backside of the newer 700mb low north of the
Minnesota Arrowhead. The end result is a longer period of heavy
snow from northern MN to North Dakota Thursday afternoon and
lingering into Friday morning.

The latest experimental PWSSI shows 40% probabilities for Moderate
impacts over northeast Minnesota, while the footprint or 50%
probabilities of Minor impacts is more expansive; stretching west
from northern Minnesota to northwest North Dakota. This is due to
the combination of Snow Amount and Snow Load, while winds may be
gusty enough on the backside of the storm Friday morning for lower
probabilities of Blowing Snow. Much of the snow does begin at
night, which helps snow accumulation potential given the time of
year. But by Friday morning, with snowfall rates gradually
diminishing, the primary impacts would come from gusty winds of
25-35 mph that could cause reduced visibility. In terms of
snowfall totals, WPC probabilities do show 60-80% chances for
snowfall totals >6" between Thursday evening and Friday morning
over northern Minnesota. Some snow accumulations are possible in
northern Wisconsin as well, but the timing of when the storm forms
and its track make the range of potential snowfall amounts wider
and less confident.

Mullinax