Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
500 PM EDT Thu Mar 23 2023

Valid 00Z Fri Mar 24 2023 - 00Z Mon Mar 27 2023

...The West...
Days 1-3...

A steady stream of shortwaves spilling southeast from the Gulf of
Alaska will maintain an active pattern for much of the West,
especially from the Cascades eastward to the central Rockies and
northward to the Canadian border. With the source region from the
northwest rather than southwest, the moisture levels will be near
to below average with the QPF driven by incoming height falls/PVA
and orographic enhancement/upslope. An incoming ~150kt jet will
move southeastward into CA and weaken as a surface cold front
moves through the Intermountain West tonight.  Focus will be
squarely over the WA/OR Cascades and Coastal Range/Olympics as
multiple vort maxes slide through the area, through Friday. Snow
levels will continue to fall behind the cold front to around
1000ft and below 500ft in some places, bringing snow to some
lowland areas. All mountains passes will likely see appreciable or
significant snow accumulations across the Cascades with some
snowfall rates of 1-2"/hr likely per the WPC Snowband Tool. For
the next three days, WPC probabilities of at least 24 inches
remain highest (>50%) over the OR Cascades, and indicate that
accumulations of 1-2 ft are likely for much of the WA Cascades,
Olympics, and portions of the Oregon coastal ranges. WPC
probabilities for at least 4 inches of snow over the next 3 days
are >30% as low as 500-1000ft.

East of the Cascades, broad NW to SE flow aloft will carry the
vorticity train across the Great Basin into the central Rockies as
a closed mid-level low likely develops over southern ID early
Saturday and continues eastward into WY. With high pressure moving
through northeastern MT into the Dakotas, some easterly upslope
enhancement is forecast over southern MT into the WY ranges. WPC
probabilities of at least 8 inches of snow over the next 3 days
are highest (>60%) over these areas and into central ID. Lastly,
on the southern side of the height falls, the Wasatch into the CO
Rockies will see several rounds of snow, as the front moves in. 
Strong mid-to-upper level forcing is expected to help contribute
to some heavier snow totals over the Wasatch on Friday. 
Otherwise, snowfall amounts will be more modest here, around 6-12
inches with higher totals in the mountain peaks.

...Upper Midwest/Great Lakes into the Northeast...
Days 2-3...

Trough axis over CO/NM early Friday will lift northeastward and
strengthen beneath an increasingly S-shaped upper jet, favoring
cyclogenesis out of the Ozarks/Mid-MS Valley late Friday and
continuing across the Midwest/eastern Lower MI early Saturday.
Thermal profile will be marginal across the Corn Belt across WI,
but with precipitation commencing after dark and continued
deepening of the surface low should allow for column cooling amid
a deformation axis over southern WI northeastward across northern
Lower MI (cooler overall but later arrival time into Saturday
daytime). Though there remains uncertainty in the track of the
low, QPF amounts, and temperature profile (not to mention SLRs,
ground temperatures, and snowfall rates), ingredients, which
include favorable upper jet forcing and low-to-mid level
frontogenesis, support a stripe of heavier snowfall developing
Friday night in this general region and is supported by the higher
NBM probabilities. The 12Z HREF guidance shows snowfall rates of
1-2"/hr developing near the IA/IL border into far northern IL and
southern WI overnight before extending northeast into northern
Lower MI Saturday morning.  WPC probabilities for accumulations of
4 inches or more have increased and are generally above 30 percent
along this axis, which probabilities as high as 70 percent over
northern Lower MI, where northeast flow behind the departing low
may contribute to some additional accumulations late Saturday. 
The probabilities also reflect the growing signal for locally
heavier amounts developing along this axis.  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 that will likely linger 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 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.  While probabilities for snow accumulations of 4
inches or more remain high across northern Maine, they have come
done for areas farther to the south and west.  Some of the more
recent guidance shows moisture being directed farther to the east
along with weaker forcing contributing to a drier solution and a
diminished threat for heavy snow for portions of northern NY and
interior northern New England.

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

Pereira/Fracasso