Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Fri Mar 31 2023

Valid 12Z Fri Mar 31 2023 - 12Z Mon Apr 03 2023

...Pacific Northwest to Northern Rockies...
Days 1-3...

An extended jet streak stretching from the Aleutians to the
Pacific Northwest will be responsible for a series of upper level
disturbances that produce copious amounts of snowfall along the
Cascade Range starting Friday and lasting into the upcoming
weekend. The Pacific Northwest will lie beneath the diffluent
left-exit region of a 130 knot jet streak off the coast while a
steady deluge of 850-700mb moisture flux is aimed at the Olympics
and Cascades. In addition to the favorable synoptic-scale lift
aloft, 850-300mb mean winds are out of the west, meaning they are
favorably oriented for upslope flow into these mountain ranges.
The upslope flow component will be key in generating heavy
snowfall rates for prolonged stretches Friday night and into the
day on Saturday. As the first pair of disturbances that lead to
heavy snow through Saturday exit east, an amplifying upper trough
off the coast of British Columbia will dig south Sunday morning
and be placed off the WA coast. With the trough more intense than
the first pair of disturbances Friday into Saturday, temperatures
aloft will be colder and the fetch of Pacific moisture will also
be positioned farther south into OR and northern CA. Even by
Sunday night as the upper low advances inland into the interior
Northwest, there will still be ongoing periods of snow along he
Cascades and into the Trinity/Salmon Mountains, as well as the
northern Sierra Nevada. Three-day snowfall totals along the
Cascade Range will be measured in feet with totals ranging between
1-4 feet (localized amounts up to 6 feet possible in the tallest
peaks). Some areas along northern CA's coastal range and the
Salmon/Trinity mountains could pick up over a foot of snow on
Sunday. The Days 1-3 WSSI show an expansive swath of Major to
Extreme impacts along the Cascade Range with the highest
concentration of Extreme impacts focuses along the OR Cascades.
Even the Olympics and northern CA coastal range could see Moderate
to Major impacts this weekend.

Farther east, the nose of the aforementioned jet streak will be
directed at the northern Rockies Saturday. Favorable forcing for
ascent will be provided through the left exit region of this jet
streak, the PVA associated with the incoming 500 mb vort maxes,
and a prolonged period of upslope flow into some mountains ranges.
Heavy snow is expected from the Bitterroots, Sawtooth, Salmon
River, and Boise mountains of Idaho, the northern Lewis Range of
Montana, and the Tetons of western Wyoming and eastern Idaho. By
Sunday, heavy snow will unfold as far south as the Wasatch. The
latest WPC snow probabilities are high (70-90%) for >8" of
snowfall in these aforementioned ranges through Saturday. The
Boise and Tetons in particular feature the best odds for >12" of
snowfall late Friday through Sunday morning. The Days 1-3 WSSI
show Moderate impacts throughout many of these ranges, as well as
the Blue Mountains of northeast OR. The tallest peaks of the
northern Rockies mountain ranges can anticipate Major impacts this
upcoming weekend.

...Northern Plains and Upper Midwest...
Days 1-2...

...A powerful multi-day winter storm is forecast to unfold from
the Dakotas and northern Nebraska to the Upper Great Lakes...

The first in a two-part winter storm spanning from the Northern
Plains to the Upper Great Lakes has begun tonight. The first part
of the winter storm is unfolding this morning as periods of snow
and a wintry mix have developed north of an advancing warm front
this morning. Strong 850 mb moisture transport will intersect the
slowly advancing warm front positioned over the Midwest this
morning. An initial thump of heavy snow on the northern side of
the warm front Snowfall on the order of several inches are
expected from far northern Wisconsin and the U.P. of Michigan.
South of that, a zone of wintry mix including freezing rain is
likely to develop from parts of central Minnesota through central
Wisconsin. Latest WPC probabilities for ice accumulation greater
than 0.1" is between 20-40% in east-central SD and north-central
WI.

Meanwhile, the primary area of low pressure that will be the
catalyst for the impending winter storm will continue to organize
in the heart of the Midwest this morning.  A warm conveyor belt of
850-700mb moisture revolves around the 700mb low Friday afternoon,
prompting the development of a "Trough of Warm Air Aloft" (TROWAL)
across portions of the Dakotas where the most intense snowfall
banding and rates are expected. The TROWAL will pivot across
central SD where both 1-2"/hr snowfall rates and wind gusts of
30-40 mph will track across the state and advance northeast into
eastern SD and central MN by Friday afternoon. As colder northeast
winds strengthen Friday night via both the intensifying low over
the Great Lakes and the dome of high pressure to the north, any
precipitation that was originally a rain/wintry mix will
transition to snow by Friday night and continue into Saturday. The
U.P. of MI and northern WI will not only have the TROWAL to
contend with, but also some lake enhancement that could drive
snowfall rates above 2"/hr at times. Wind gusts from the Upper
Mississippi Valley to the Upper Great Lakes could range between
35-45 mph with peak gusts above 50 mp through Saturday morning.
The storm races into southeast Canada by midday Saturday, which
aside from lingering lake effect snow showers, will mean the storm
effectively comes to an end in the Great Lakes by Saturday
afternoon.

In terms of impacts, the WSSI depicts the myriad of expected
winter weather hazards well. When it comes to Snow Amount, it
identifies much of central SD and into southeast ND and western MN
with Moderate impacts. It also shows some Major impacts in the
heart of SD, where considerable disruptions to daily life are
anticipated. This includes dangerous to even impossible driving
conditions. Farther east into northern WI and the U.P. of MI, the
WSSI shows Moderate to Major impacts from both Snow Amount and
Snow Load. This is due to the added weight of the snow from the
combination of rich Gulf of Mexico moisture, as well as lake
enhancement that lead to a heavy/wet snow. As snow load weighs
down tree branches and power lines, this combined with wind gusts
of 40-50 mph can lead to power outages in the Upper Great Lakes.
Speaking of those strong wind gusts, they will be strongest in the
SD, southeast ND, and western MN where Moderate to Major impacts
are likely. Lastly, there is also a lingering ice component to
this event over central MN and north-central WI, but impacts will
top out on the Minor side in these areas Friday afternoon and into
Friday night. The latest Key Messages for this winter storm are
below.


...Key Messages for Mar 30 - Apr 1 Winter Storm...

-A powerful winter storm will track across the central High Plains
and Great Lakes today and through Saturday.

-A combination of heavy snow rates (1-2â€/hr) and strong wind
gusts as high as 50 mph are expected to create blizzard conditions
from portions of the Dakotas to western Minnesota.

-In the Great Lakes, similarly strong wind gusts along with a
heavy and wet snow may cause extensive tree damage and power
outages.

-There is also the potential for treacherous ice accumulations
from eastern South Dakota to central Minnesota and northern
Wisconsin.

-Hazardous travel conditions are expected in impacted areas that
could include snow covered roads, near impossible travel from zero
visibility and whiteout conditions.

Mullinax