Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
412 PM EDT Thu Mar 30 2023

Valid 00Z Fri Mar 31 2023 - 00Z Mon Apr 03 2023

...Intermountain West...
Day 1...

The latest water vapor imagery and 500 mb heights shows an
impressively deep upper trough centered over the Intermountain
West. The latest NAEFS shows the 500 mb height anomalies to be
between 2 and 3 standard deviations below normal. This is
providing sufficient vertical ascent aloft, enough anomalous
moisture, and cold air to support continued periods of heavy snow
from the northern Rockies through the Wasatch and central Rockies
to the Mogollon Rim. The heaviest snowfall through tomorrow
afternoon is favored over the Wasatch and central Rockies where
the latest WPC snow probabilities for at least 6 inches are high
(above 70%). The best chances for totals exceeding 12 inches are
across the Wasatch east of Salt Lake City (50-70% in the latest
PWPF)

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

After a brief period of mid/upper level ridging over the region
tonight through Friday, another shortwave trough will approach the
region beginning late Friday. A rather lengthy jet streak (from
the Aleutians to the western Oregon coast) will position itself
for the favored left exit region lift. As the longwave troughing
settles over the region through the weekend, a series of embedded
shortwaves will move onshore, bringing with it a steady barrage of
mid level moisture into the Pacific Northwest and northern
Rockies. The first round of heavy mountain snow arrives Friday
evening over in the Olympics and Cascades. Snow will pile up
quickly in these ranges thanks to the mean flow oriented
orthogonally to these mountain ranges. As much as 1-4 feet of snow
will accumulate in these ranges through Sunday evening. The Day 3
WSSI is depicting Moderate to Major impacts in portions of these
ranges.

Farther east, the nose of the aforementioned jet streak will be
aimed at the northern Rockies Saturday. Favorable forcing for
ascent will be provided through the left exit region of the jet
streak, the PVA associated with the incoming 500 mb vort maxes,
and a long duration of upslope flow into some mountains. 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. The latest WPC
snow probabilities are high (70-90%) for >8" of snowfall in these
aforementioned ranges this weekend. The Boise and Tetons feature
the best odds for >12" of snowfall Saturday into Sunday.

...Northern Plains and Upper Midwest...
Days 1-3...

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

The stage is set for a prolonged, disruptive winter storm across
portions of the Northern Plains and Upper Midwest beginning later
this evening through Saturday. This winter storm comes in 2 waves,
the first being a large area of precipitation north of an
advancing warm front tonight through Friday morning and then the
main cyclone and its associated heavy snow threat Friday through
Saturday. Robust 850 mb moisture transport will continue to nose
over a slowly advancing warm front positioned over the Midwest
today into tonight. An initial thump of heavy snow on the northern
edge as well as a zone of mixed precipitation is expected.
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, where the
latest WPC probabilities for ice accumulation greater than 0.1" is
between 50-70% (locally near 80% across northeast Wisconsin).

The other part of this system begins to ramp up considerably early
Friday morning through Saturday as the main upper trough pushes
out into the Plains. An area of low pressure deepens as it moves
northeast from CO to Wisconsin early Saturday. A warm conveyor
belt of 850-700mb moisture wraps around the 700mb low Friday
afternoon and it is beneath this developing "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 pivot 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, there are several factors to take into
account that can be broken down by utilizing the experimental
PWSSI.

1.) Snow Rate. With the intense banding and vertical velocities
supported across just about all guidance, snow will be able to
accumulate quickly in areas where hourly snowfall rates are above
1"/hr, even during the daytime hours (although night time will be
the best chance for rapid accumulations). There are currently
70-90% probabilities for Moderate impacts via Snow Rate from
central SD and northern NE to northern Michigan and is the primary
driver in the PWSSI. Even a slight signal (10-30%) for Major
impacts in Snow Rate are found across portions of central SD.

2.) Snow Load. With such an impressive fetch of moisture from the
Gulf of Mexico and off the Great Lakes themselves, parts of
central/northern WI and northern MI feature up to 70% chances for
Moderate impacts from the weight of the snow on trees and power
lines. There are also 30-40% chances for Major impacts as a result
of Snow Load in these areas Friday night into Saturday.

3.) Blowing Snow. The wind gusts mentioned earlier will
undoubtedly lead to significantly reduced visibilites on roads
with blizzard conditions causing drifting snow and power outages.
The pWSSI for Moderate to Major impacts due to Blowing Snow reach
20-30% across portions of central South Dakota. In areas where
snow load is a problem, the wind will only compound the risk for
tree damage and power outages in the upper Great Lakes. There is
still lingering uncertainty in track and where the TROWAL sets up,
but confidence is growing in a major winter storm that produces
numerous travel delays, closures, and cancellations from the
northern Plains to the upper Great Lakes Friday and into Saturday.
Key Messages for this storm system are below.

Mullinax/Taylor

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

-A powerful winter storm is expected to track across the central
High Plains and Great Lakes through Saturday.

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

-In the Great Lakes, similar 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.