Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Fri Mar 24 2023

Valid 12Z Fri Mar 24 2023 - 12Z Mon Mar 27 2023

...Pacific Northwest/Northern and Central Rockies...
Days 1-3...

Persistent troughing from the Gulf of Alaska into the Northwest
will maintain an unsettled pattern as a lead cold front exits the
Rockies. Snowfall totals/coverage will decrease with each day as
the upper trough and trailing vort maxes or weakly closed lows
move through. With moisture values below normal for late March,
orographics will do the heavy lifting and the
Cascades/Olympics/Coastal Range will see appreciable snow D1, in
addition to farther east over southern Montana southward to the
Wasatch and CO Rockies (including onto the High Plains) where
totals will be lower. Snow levels are currently lowering over the
Rockies as the front continues eastward, and will bottom out below
1000ft and down to 500ft in some places in western OR/WA. All
mountains passes will see appreciable or significant snow
accumulations across the Cascades with some snowfall rates of
1-2"/hr likely per the WPC Snowband Tool especially today. For
D2-3, vort maxes will dampen out as another system drops southward
well offshore. For the next three days, WPC probabilities of at
least 18 inches remain highest (>50%) over the OR Cascades as well
as across south central Montana, where accumulations of 1-2+ ft
are likely. WPC probabilities for at least 4 inches of snow over
the next 3 days are >30% as low as 500-1000ft.


...Upper Midwest/Great Lakes into the Northeast...
Days 1-3...

Mid-level trough moving onto the Plains today will lift
northeastward and strengthen beneath an increasingly S-shaped
upper jet, favoring cyclogenesis out of the Ozarks/Mid-MS Valley
this evening and continuing across the Midwest/eastern Lower MI
early Saturday. This sets the stage for a quick-hitting and
focused axis of snowfall in a deformation band on the northwest
side of the surface low north of ~41N where a marginally cold air
mass exists. Low pressure is forecast to reach peak intensity
across Lower Michigan in the mid/upper 980s mb as the mid-level
circulation closes off. LFQ of a 130kt jet streak will provide
broad lift as a moisture plume from the central Gulf streams
northward and wraps around the circulation. Precip shield will
extend into the colder air just before dawn Saturday with rain
changing to snow as intensity picks up. Lingering uncertainty in
the track of the low, QPF amounts/placement, SLRs, ground
temperatures, and snowfall rates has narrowed a bit but overall
ingredients (also including low-to-mid level frontogenesis) still
support a stripe of heavier snowfall from northern IL and across
southeastern WI into northern Lower Michigan. 90th percentile
amounts exceed 12 inches per the NBM probabilities which seems
like a reasonable upper bound given the relatively short duration.
This coincides with the higher 00Z HREF probs for snowfall rates
of 1-2"/hr developing near the IA/IL border right around 12Z
Saturday and progressing northeastward through the late morning
into the early afternoon. WPC probabilities for accumulations of 4
inches or more have increased and are generally above 50 percent
along this axis, with probabilities as high as 90 percent over
northern Lower MI, where northeast flow behind the departing low
may contribute to some additional accumulations late Saturday. 
Southeast of the heavier snow, an icy transition zone is expected
to produce some minor ice accumulations across portions of the
central Lower MI Friday night into early Saturday morning. 

Farther east, WAA-driven precip will move into the Northeast north
of a warm front across the Mid-Atlantic as the triple point starts
to develop near NJ Saturday night. Air mass ahead of the
precipitation will be near to sub-freezing over much of the
interior and a wintry mix is likely for some areas away from the
coast to start. But with the parent low track into southern
Quebec, a changeover from southwest to northeast will likely take
place through much of NY and into central New England as well as
coastal Maine with easterly flow. Coldest temperatures will lie
across northern Maine where probabilities for snow accumulations
of 4 inches or more remain high (>70%). Elsewhere, snowfall will
be light (generally <4") over the Adirondacks and into Vermont
where light icing is also likely (generally less than 0.10").

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

Fracasso