Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sun Apr 02 2023

Valid 12Z Sun Apr 02 2023 - 12Z Wed Apr 05 2023

...Pacific Northwest to the northern High Plains...
Days 1-3...

An amplifying upper trough over the Pacific Northwest providing
strong vertical lift atop the atmosphere will also supply Pacific
moisture into the Pacific Northwest, the northern Sierra Nevada,
and northern Great Basin. The Cascade Range in OR features the
highest probabilities for >8" of snowfall (70-90%) Sunday and into
Sunday night. As the trough continues to deepen over the West
throughout the day on Sunday, the surge of 850-700mb moisture will
be directed at the Wasatch, Uinta, and Teton ranges. They will
reside just north of a deepening area of low pressure in eastern
UT and western CO with heavy snow breaking out north of the low's
strengthening surface front. By Monday morning, a cold front will
push south through the Wasatch with even strong vertical ascent
transpiring over the Uinta and on east to the Laramie and Medicine
Bow ranges. WPC probabilities show high chances (70-90%) for >8"
of snow in these ranges through Monday afternoon.

By Monday afternoon, lee cyclogenesis east of the CO Rockies and a
dome of high pressure over southern Canada sets up a strengthening
easterly wind component into southeast WY, western NE, and
southwest SD. By 00-06Z Tuesday, NAEFS shows easterly U-wind
anomalies that are forecast to fall below the CFSR climatology for
this time of year. These unusually strong easterlies will act as
upslope flow into the central High Plains and mountains ranges of
WY. The emerging 700mb low in southern WY will also contain an
impressive southeasterly integrated vapor transport (IVT) with
values above the 97.5 percentile in northeast CO and western NE.
With sufficient sub-freezing temps in place, ample moisture,
strong orographic ascent, and diffluent flow aloft in the
left-exit region of a strengthening jet streak over the southern
High Plains, the stage is set for a classic early April major
winter storm to track from the central Rockies Monday night and
into the Midwest by Tuesday. As the storm continues to strengthen
Tuesday afternoon, the deformation axis of heavy snow will be
positioned from western NE and the Dakotas to northern MN. Along
the warm front lifting north through the Upper Midwest,
precipitation may fall as a wintry mix. This would help cut down
snowfall totals, but would result in icy conditions near the MN
Arrowhead, northern WI, and the Michigan U.P. Latest WPC
probabilities do contain 30-50% chances of >0.1" of ice from
northern WI to the U.P. of MI.

In terms of impacts, The Days 1-3 WSSI shows an expansive area of
Major impacts from the Wasatch and mountain ranges of southern WY,
to the high plains of eastern WY and central SD. There are some
Extreme impacts also being denoted in the Laramie Range and The
Badlands. The primary impacts driving the WSSI are Snow Amount and
Blowing Snow. Latest WPC probabilities show high chances (70-90%)
for >12" of snowfall from southern WY to southeast ND. There are
also moderate probabilities (40-60%) for >18" in the same area
from southeast WY and central SD to southeast ND. In addition to
the significant snowfall totals, the wind gusts will be strong to
potentially destructive. The 12Z Euro EFI between 00Z Wed - 00Z
Thurs shows >0.9 values over parts of the Dakotas, southern MN,
and in the MN Arrowhead. The NBM also contains 60-70%
probabilities for wind gusts >50 mph across the eastern half of
the Dakotas and into northern NE. These winds will likely result
in blizzard conditions that lead to high snow drifts and whiteout
visibilities. These wind gusts are also capable of causing
extensive tree damage and power outages. Motorists are encouraged
to make sure they have essential supplies with them in the event
they are stranded. Below are the latest Key Messages.

...Key Messages for April 3-5 Winter Storm...

--Heavy Snow Totaling Over One Foot Likely
A major winter storm is likely to produce over a foot of snow from
the Central Rockies to the Northern Plains, which could challenge
some April snow records in the Dakotas and northwest Minnesota.
Avalanches and significant impacts from snow load are possible in
the Intermountain West, particularly in portions of Utah.

--Blizzard Conditions Likely in the Northern Plains
Strong winds and heavy snow will create whiteout conditions at
times Tuesday and Wednesday, causing dangerous to impossible
driving conditions and considerable disruption to daily life.

--Life Threatening Cold for Anyone Stranded
Wind chills will fall to near or below zero during the blizzard in
the Northern Plains, which could be life-threatening to anyone
stranded.

--Damaging Winds From the Southwest to the Midwest
Widespread gusts in excess of 50 MPH will spread from the
Southwest and Four Corners Monday, to the Plains Tuesday, and
Upper Midwest Wednesday. This may lead to power outages, wind
damage, and blowing dust, even outside of thunderstorm activity.

Mullinax