Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 AM EST Thu Dec 28 2023

Valid 12Z Thu Dec 28 2023 - 12Z Sun Dec 31 2023

...Central Appalachians...
Day 2...

The upper low responsible for the major winter storm that produced
blizzard conditions and significant ice accumulations to parts of
the Northern and Central Plains around Christmas will meander over
the Mississippi-Ohio Confluence today. There is a chance for a
couple additional inches of snow in parts of the Middle
Mississippi Valley today (low chances, 20-40%) in parts of western
Illinois, northeast Missouri, and southeast Iowa with pockets of
Minor Impacts on the WSSI at most in parts of these areas.

The chance for heavy snow returns as early as Friday afternoon in
the Central Appalachians as the upper low slowly tracks east
through the Ohio Valley. There remains a lack of sufficiently cold
air, so a change over to snow in the central Appalachians will
largely be due to the lower/colder heights associated with the
upper low tracking overhead Friday night. Then, as the upper low
tracks east into the Mid-Atlantic on Saturday, northwesterly flow
will result in upsloping into the higher terrain of the central
Appalachians throughout the day before winding down Saturday
evening. Latest WPC PWPF depicts low-to-moderate chances (10-30%)
for snowfall totals >4" in eastern West Virginia, which includes
portions of the Potomac Highlands. Latest WSSI does depict Minor
Impacts in some of these areas and extending as far north as the
southern Laurel Highlands. This suggests a few inconveniences to
daily life, while also being a welcomed sight to ski resorts in
the region.

...Northern New England...
Days 1-3...

Low pressure off the Northeast coast will track north become
located off the Massachusetts Cape by Thursday evening. A
stationary front extending from southern Quebec to central Maine
will act as the demarcation for wintry precip with snow the
primary precip type in northern Maine, while central Maine is more
likely to contend with freezing rain starting Thursday night. This
is due to overrunning of milder east-to-southerly flow atop a
sufficiently cold boundary layer will allow for precipitation to
be in the form of freezing rain through Friday morning. WPC PWPF
has increased the potential for freezing rain accumulations >0.1"
to a low-to-moderate risk of 30-50% and the WSSI depicts Minor
Impacts are possible due to Ice Accumulation in its algorithm.
Hazardous travel conditions are possible Thursday night and into
Friday, including but not limited to untreated roads and sidewalks.

By Friday evening, a storm system associated with an upper level
trough approaching from Ontario will introduce CAA into the low
levels and allow for precipitation to changeover to snow Friday
night into Saturday. The wildcard in snowfall accumulations will
be if this next storm system to strengthen and pivot over Downeast
Maine, which could result in several hours worth of heavy snow
over central and northern Maine. WPC PWPF continues to show low
chances (10-20%) for snowfall totals >4" in northern Maine, while
the WSSI shows Minor impacts in portions of the Central Highlands
and North Woods due to snow. Snow is forecast to taper off by
Saturday evening as the storm tracks east of Nova Scotia.

...California...
Days 2-3...

A deep upper level trough off the West Coast will direct an
atmospheric river at the West Coast on Friday that results in
periods of rain along the California/Pacific Northwest coast. The
atmospheric river is an exceptional one, as evident by the NAEFS
suggesting IVT values (>500 kg/m/s) are above the 97.5
climatological percentile Friday midday. As the upper trough axis
moves ashore Friday night, snow levels will gradually drop and
SLRs will incrementally increase as the atmospheric column cools
and upslope flow strengthens. By Saturday morning snow levels will
fall to as low 6,000ft and snow will fall heavily from the tallest
peaks of the Shasta/Salmon mountains of northern California to the
Sierra Nevada. WPC PWPF highlights the Sierra Nevada as having
high chances (>70%) for receiving >6" of snowfall late Friday into
Saturday. The Probabilistic WSSI (WSSI-P) shows high chances
(>70%) of experiencing Minor Impacts during that same time frame,
particularly for elevations >7,000ft. The Snow Amount and Snow
Rates algorithms are currently the two primary drivers in causing
treacherous travel and delays in parts of the Sierra Nevada on
Saturday.

Mullinax