Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Sun Apr 03 2022

Valid 00Z Mon Apr 04 2022 - 00Z Thu Apr 07 2022

...Upper Midwest/Great Lakes...
Days 1-3...

A vigorous shortwave trough swinging through the Midwest this
afternoon will direct a wave of low pressure towards the Great
Lakes tonight. Upper level diffluence ahead of the trough and a
strengthening 850-700mb FGEN band support a swath of precipitation
across Wisconsin tonight and into Michigan by Monday morning. Area
averaged model soundings indicate wet bulb temperatures at low-mid
levels would be subfreezing and dynamic cooling as a result of the
strengthening vertical ascent will allow for periods of snow to
ensue. The big question remains boundary layers temps. In areas
where forcing is strongest, snowfall rates up to 1"/hr can help
surface temps fall close to freezing with light accumulations
possible. Latest WPC probabilities of >4" are highest in
north-central Wisconsin, but even those probabilities top out at
20%, suggesting perhaps some localized spots could surpass 4", but
the majority of snowfall totals should fall short of 4". Meanwhile
in southern Wisconsin, northern Illinois, and southern Michigan,
boundary layer temps in the lowest 500-1,000 feet appear to be
above freezing. In addition, a 700-300mb layer dry slot will
quickly work its way east with the progressive shortwave, making
any window for accumulating snow on the matter of only a few
hours. Expecting less accumulations south of I-96 as a result,
although some reduced visibilities from a brief burst of snow
could still occur.

On Day 3, an amplifying upper low diving southeast from southern
Canada will spawn a new wave of low pressure in the northern High
Plains Tuesday morning. Strong WAA and PVA out ahead of the trough
will give way to periods of snow across North Dakota and northern
Minnesota. The area with the best odds for snowfall totals
surpassing 4" is the Minnesota Arrowhead where the additional
snowfall enhancement, thanks to onshore flow off Lake Superior,
where WPC probabilities are up to 40%. How much snow accumulates
in parts of North Dakota and northern Minnesota remains unclear
due to lingering model spread and uncertainty in how cold the
boundary layer is in these areas. Farther west, as colder air
spills down into Montana, northern Wyoming, and the Black Hills,
upslope flow may cause periods of heavy snow to breakout on
Tuesday. The Black Hills have a 50-70% chance for snowfall totals
>4" while the Big Horns of Wyoming and the Big Snowy and Little
Belt mountains of Montana are currently pegged at 20-40% for
similar snowfall totals.


...Northwest to the northern Rockies...
Days 1-3...

Tonight, the nose of a roaring 250mb jet (~170 kts, ~3-4+ sigma)
is helping to usher in an atmospheric river across the Northwest
tonight. Substantial amounts of Pacific moisture (~2+ sigma) will
move ashore with strong westerlies at mid-levels also causing
enhanced precipitation rates in the Olympics, Cascades, and
eventually the northern Rockies Monday morning. The brisk onshore
and uplsloping flow pattern over these regions will persist
through Tuesday morning, making for a long duration heavy snow
event. Snowfall will be measured in feet in the Olympics,
Cascades, Sawtooths, and Bitterroots while farther south and east,
the Tetons and Wind River Ranges of Wyoming could also pick up
over a foot of snow. Impacts to travel, particularly highways in
the higher elevations, could be subjected to very poor
visibilities and snow covered roads, making travel dangerous in
some cases. In fact, widespread High Wind Watches and Warnings
have been posted throughout much of the Pacific Northwest and
Northern Rockies through late Tuesday, so any snowfall could lead
to near whiteout conditions at times. Snowfall rates gradually
taper off late Tuesday throughout the Pacific Northwest and
northern Rockies as drier air filters on the backside of an
emerging upper trough in southern Canada and high pressure builds
in throughout the Northwest on Wednesday.


...Colorado/New Mexico Rockies...
Days 1-2...

A weak upper trough and upslope low level flow will foster a
favorable environment for snow in the southern Colorado Rockies
and far northern New Mexico Rockies tonight and into Monday
morning. Some of the highest elevations of southern Colorado could
receive over 8" (WPC probabilities of as high as 30% in spots). As
that upper trough leaves, the intensifying system in the northern
High Plains early Tuesday morning will direct a cold front south
through the Rockies. This, along with strong upsloping flow from
the intense jet streak over the Northwest, allows for another
round of snow to envelope the Colorado Rockies. This time around
through, it is the central and northern Colorado Rockies that have
the better odds of receiving heavy snow (50-70% chance for >4" of
snow in the higher elevations). Strong winds in these impacted
areas may cause reduced visibilities and treacherous travel
conditions on Tuesday.


For Days 1-3, the probability of significant icing of at least
0.25" is less than 10 percent.


Mullinax