Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
353 AM EDT Wed Mar 27 2024

Valid 12Z Wed Mar 27 2024 - 12Z Sat Mar 30 2024

...Northern Minnesota & Upper Great Lakes...
Days 1-2...

Cyclonic flow on the southwest flank of the winter storm now in
Ontario this morning will sustain periods of snow over the
Minnesota Arrowhead and along the showers of northern Wisconsin and
the U.P. of Michigan today. Strong CAA over Lake Superior will
result in potent LES streamers along the southern shores of Lake
Superior today with snowfall rates of 1-2"/hr expected at times. As
daytime heating wains, snow showers will dissipate in coverage
across the Minnesota Arrowhead wile low-level westerly flow
continues to support LES bands along the northern Michigan U.P.
coast into Thursday morning. LES snow bands will finally diminish
sometime Thursday afternoon as winds weaken thanks to high pressure
building in from the south and the storm in Ontario tracking
farther north and east away from the Upper Great Lakes. WPC PWPF
shows the Keweenaw Peninsula as sporting moderate-to-high chances
(50-70%) of receiving >6" of snowfall through Thursday morning.
Elsewhere, parts of the Minnesota Arrowhead, northern Wisconsin
coast, and as far east as the Porcupine Mountains of the Michigan
U.P. can expect any where from 2-4" of additional snowfall
(locally up to 8" possible in the Porcupines) through Thursday
morning.

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

This morning, a Pacific storm system is set to deliver the next
slug of Pacific moisture to into the Pacific Northwest and
Northern California today, then direct the same moisture source
into the Northern and Central Rockies tonight and through Thursday.
The IVT associated with the upper trough is quite potent (topping
out around 400 kg/m/s, or above the 97.5 climatological percentile
according to NAEFS), but it is a progressive IVT. Not only does the
IVT supply a healthy supply of moisture, but 700mb winds of 40-50
knots (or above the 90th climatological percentile per NAEFS) will
also provide plenty of upslope ascent into the Pacific Mountain
ranges. Most notably, the Sierra Nevada are in the best position
for maximizing heavy snowfall rates Wednesday afternoon and into
Wednesday night, while other Northern California mountain ranges
such as the Salmon Siskiyou, and Trinity ranges are also primed to
observe heavy snow. Even as far north as the Cascades and Olympics,
rounds of heavy snow are expected with snow levels falling as low
as 3,000ft in the WA Cascades and Olympics. Snow levels as far
south as the Sierra Nevada will fall to as low as 4,000ft by
Thursday morning. As previously mentioned, some of the same
moisture source will reach as far inland as the Blue, Boise, and
Sawtooth mountains Wednesday night. WPC PWPF shows high chances
(>70%) for snowfall totals >8" in all these mountain ranges
through early Thursday morning, while the northern Sierra Nevada
and the Salmon/Shasta mountains sport moderate-to-high chances
(50-70%) for totals >12".

By Thursday, a cold front associated with a surface low tracking
through the Northern Rockies will escort Pacific moisture and
colder temperatures aloft into ranges such as the Absaroka,
Tetons, Wasatch, Wind River, and Colorado Rockies. Through 12Z
Friday, WPC PWPC sports moderate-to-high chances (50-70%) for
snowfall totals >8" in portions of these ranges, particularly at
elevations >8,000ft (>7,000ft in the Blue and Boise Mountains).
Meanwhile, the deep upper low off the Pacific Northwest coast will
maintain a healthy fetch of onshore flow that keeps some periodic
mountains snow ongoing from the Cascades and Olympics on south to
the Sierra Nevada. Snow will pick back up in intensity on Friday as
a compact and vigorous upper low (<2.5 climatological percentile
between 850-200mb off the California coast midday Friday) directs a
rejuvenated 300-400 kg/m/s IVT at Central and Southern California.
Snow will fall at its heaviest Friday afternoon into Friday night
with some heavy snow possible in the tallest peaks of the
Transverse Ranges Friday night into Saturday morning. WPC PWPF
depicts moderate-to-high chances (50-70%) for additional snowfall
accumulations >8" in the central and southern Sierra Nevada
between 12Z Friday and 12Z Sunday. Similar moderate-to-high chances
exist for >6" of snowfall in the northern Sierra Nevada and
neighboring Northern California ranges. Over the course of the next
three days, much of the central and northern Sierra Nevada can
expect Moderate to even in some areas Major Impacts according to
the WSSI.

...New England...
Day 3...

A tricky and uncertain forecast for parts of NEw England on Friday
as most guidance is at least depicting a rapidly intensifying
storm system south of Nova Scotia Friday afternoon and tracking
over Nova Scotia by Friday night. The sub-980mb low by Friday night
will contain a robust deformation axis that could reach as far
west as Maine, but there remain intricate details that need to be
ironed out. 500mb EOF analysis shows guidance is most split on the
strength of the ridge that builds out ahead of the storm, largely
driven by differences in the GFS ensembles (more progressive) and
the EC/CMC which are slower and not as amplified. This will
continue to be closely monitored as the deformation axis would
support heavy snowfall rates as well as whipping wind gusts thanks
to the exceptional pressure gradient over the region. At the
moment, WPC PWPF shows low-to-moderate chances (20-40%) for
snowfall totals >4" in the White Mountains and along the western
Maine/Quebec border between Friday morning and Saturday morning.

Mullinax