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

Valid 12Z Thu Mar 30 2023 - 12Z Sun Apr 02 2023

...Intermountain West...
Days 1-2...

An impressively deep upper trough traversing the western U.S.
today is responsible for providing the sufficient vertical ascent
aloft, atmospheric moisture, and sub-freezing temperatures within
the column to support periods of heavy snow from the northern
Rockies and Wasatch to the central Rockies and Mogollon Rim. Upper
levels heights at the 700-500-200mb levels are all well below the
10th climatological percentiles Thursday and into Thursday evening
according to NAEFS. The upper trough's moisture fetch is also
anomalous with a narrow conveyor belt of >90th climatological
percentile integrated vapor transport (IVT) stemming from southern
AZ to the central Rockies this morning. The heaviest snowfall will
be favored in areas where both elevations are >7,000 feet and
where the best orographic enhancement from upslope flow is
present. Latest WPC PWPF 48-hr probabilities between 12Z Thurs -
12Z Sat shows moderate chances (40-60%) for >8" of snow in the
Tetons, Wasatch east of Salt Lake City, and western CO Rockies. It
is in these areas where snowfall will be most impactful as the
WSSI does show Moderate to even some Major impacts, but the Major
impacts will likely be confined to the tallest and more remote
peaks of the Wasatch and CO Rockies. Meanwhile, WPC PWPF between
12Z Thurs - 12Z Fri does show similar probabilities for >6" of
snowfall in portions of the Absaroka and along the mountains
ranges bordering ID/MT. The Mogollon Rim of AZ can also expect
some snowfall totals to eclipse 6" at elevations >6,000 feet as
WPC PWPF places the odds for the highest elevations of having
40-50% odds of snowfall amounts >6" on Thurs. Periods of snow look
to linger in the Tetons, Wind River Range, Wasatch, and CO Rockies
through Friday as quick moving disturbance advects 700mb moisture
flux into these mountain ranges, but rates will be less heavy and
gradually diminish by Friday evening.

...Pacific Northwest to Northern Rockies...
Days 2-3...

An upper trough tucked west of the Alaska Panhandle will direct
the nose of a lengthy jet streak (starting north of the Aleutians
and ending off the Pacific Northwest coast) directly at the
Pacific Northwest Friday evening that will then move into the
northern Rockies this weekend. This jet stream pattern will see a
series of upper level disturbances round the base of the longwave
trough in the northeast Pacific and track into the northwestern
U.S., leading to a steady barrage of 850-700mb 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 as mean
850-300mb winds will be out of the west and oriented orthogonally
to these mountain ranges. As much as 1-4" of snow will accumulate
in these ranges through Saturday night alone and more snow will
continue to fall into the second half of the weekend. The Day 3
WSSI is already depicting Moderate to Major impacts in portions of
these ranges.

Farther east, the nose of this 130 knot 250mb jet streak will be
aimed at the northern Rockies on Saturday. Between the favored
location beneath the diffluent left-exit region of the jest
streak,the PVA associated with incoming 500mb vort maxes, and
prolonged upslope flow into some mountain ranges, snow will fall
Saturday and into Sunday from the Bitterroots, Sawtooth, Salmon
River, and Boise mountains of ID, the northern Lewis Range of MT,
and the Tetons of western WY and eastern ID. WPC PWPF shows high
probabilities (70-90%) for >8" of snowfall in these aforementioned
ranges this weekend. The Boise and Tetons feature the best odds
for >12" of snowfall between 12Z Sat - 12Z Sun with 50-70% chances
in these ranges.

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

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

The stage is set for a prolonged period of disruptive wintry
weather from the Northern Plains to the Upper Great Lakes starting
as early as Thursday afternoon. This particular setup can be
broken down into two particular events; one driven by mainly
warm-air advection (WAA) Thursday night and the other being a more
organized, powerful cyclone in the Midwest on Friday. Starting
with today, the upper trough responsible for the periods of heavy
mountain snow mentioned above will work in tandem with a strong
ridge of high pressure over the Southeast to cause a robust 850mb
moisture transport throughout the southern and central Plains. The
IVT areal extent from TX to the Upper Mississippi Valley Thursday
afternoon and into Friday is remarkable; NAEFS shows a large area
of >90th climatological percentile IVTs in these areas, and
eventually working into the MS and OH River Valleys Friday
afternoon and Friday night. This first initial thump of snow, as
well as ice, comes from the 290K isentropic ascent and uniform
850-700mb WAA north of the surface warm front lifting through the
MS Valley Friday afternoon and evening. As the surface low
strengthens in the Central Plains Thursday night, easterly winds
from eastern MT to the Michigan U.P. will ensue as high pressure
over south-central Canada also builds in. Temperatures will be
more marginal for this event through early Friday morning, but
particularly where there is still snow pack that is measured in
feet, surface temperatures will be stubborn to get above freezing.
Latest WPC PWPF shows the MN Arrowhead and northern coastal WI
with the highest probabilities for >4" of snowfall (60-80% Duluth
on east), but the ice >0.1" footprint is larger, encompassing
areas from northeast SD and central MN to northern WI and along
the WI/MI U.P. border. Probabilities range between 40-60% there,
while northern WI does contain up to a 30-40% chance for ice
accumulations >0.25". Given the strong warm nose and strong
vertical velocities, there is a plausible for thunder in either
snow, sleet, or freezing rain areas via some weak, but still
available, elevated instability Friday morning.

The instability only grows over the Upper Midwest and into the
Great Lakes heading into Friday afternoon as the surface cyclone
rapidly deeps in the Midwest, thanks in large part a 150 knot jet
streak placing its left-exit region overhead and intense WAA at
mid-levels. By Friday afternoon, the 500mb low is set to track
from northern NE across the Missouri River and into the Upper
Mississippi Valley Friday night. With the 850-500mb lows not yet
vertically stacked by 00Z Saturday, further intensification of the
surface low is forecast as a warm conveyor belt of 850-700mb
moisture wraps around the 700mb low Friday afternoon. It is
beneath this developing "Trough of Warm Air Aloft" (TROWAL) 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 50-60% probabilities for Moderate impacts via Snow Rate
from central SD and northern NE to northern Michigan and is the
primary driver in the PWSSI. 2.) Snow Load. With such an
impressive fetch of moisture from the Gulf of Mexico and off the
Great Lakes themselves, parts of northern WI and northern MI
feature up to 60% chances for Moderate impacts from the weight of
the snow on trees and power lines. There are also 20-30% chances
for Major impacts as a result of Snow Load in these areas Friday
night into Saturday. 3.) Blowing Snow. The wind gusts mentioned in
the previous paragraph will undoubtedly lead to significantly
reduced visibilities on roads with blizzard conditions causing
drifting snow and power outages. 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.


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

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

--A combination of heavy snow rates and strong wind gusts as high
as 50 mph may cause blizzard conditions from portions of the
Dakotas to southwest Minnesota.

--In the Great Lakes, similar wind gusts along with a heavy/wet
snow may cause 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 likely in impacted areas that
could include both snow/slush covered roads and whiteout
conditions.

Mullinax