Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
321 AM EST Fri Dec 29 2023

Valid 12Z Fri Dec 29 2023 - 12Z Mon Jan 01 2024

...Northern New England...
Days 1-2...

Periods of freezing rain across central Maine and snow in northern
Maine continue this morning as a surface low south of Nova Scotia
funnels Atlantic moisture into Maine via 850mb easterly flow.
Sufficiently cold air remains in place for an icy wintry mix
despite an increasing southerly component causing a classic
"overrunning" setup, highlighted by sub-freezing temps at the
surface but >0C temps within the 900-800mb layer. As the low south
of Nova Scotia tracks father east, precipitation will transition
over to primarily snow Friday afternoon. By Friday night, a ribbon
of positive vorticity advection will support the development of a
secondary low along the coast of Maine come Saturday morning. The
reinvigorated area of low pressure will aid in the development of
moderate-to-heavy snow in central and northern Maine throughout
the day on Saturday. Snow should wind down by Saturday evening as
the strongest forcing quickly moves east over the Atlantic
Canadian Maritime.

WPC PWPF is only topping out at a low 5-10% for snowfall totals
>4" around Caribou, while additional ice accumulations >0.1" are
low chance as well (20-30%). Still, the combination of both snow
and freezing rain will make for dicey travel conditions across the
northern half of "The Pine Tree State." The latest WSSI shows
Minor impacts for most areas north of Bangor and into the North
Woods. While snow and ice accumulations are not expected to be
overly heavy, the blend of both snow and ice can make for
hazardous travel conditions, particularly on any untreated
surfaces through Saturday.

...Central Appalachians...
Days 1-2...

Periods of snow return Friday evening in the Central Appalachians
as an upper low slowly tracks overhead. There remains a lack of
sufficiently cold air, so a change over to snow in the central
Appalachians will initially 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 (20-40%) for snowfall totals >4" in the
Yew Mountains of eastern West Virginia, which includes portions of
the Potomac Highlands. Latest WSSI does depict Minor Impacts in
the higher terrain of eastern West Virginia. This suggests a few
inconveniences to daily life, while any light snowfall will also
being a welcomed sight to ski resorts in the region.

...California...
Days 1-2...

A deep upper level trough off the West Coast will direct an
atmospheric river at the West Coast today that results in periods
of rain along the California/Pacific Northwest coast. The
atmospheric river, as evident by the NAEFS suggesting IVT values
(>500 kg/m/s), are above the 97.5 climatological percentile Friday
midday with an 850-925mb moisture tap that extends into the
subtropical East Pacific. As the upper trough axis moves ashore
tonight, 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 and even low chances
(10-30%) for snowfall accumulations >12" late Friday into
Saturday. The WSSI depicts Minor to Moderate Impacts in parts of
the central and southern Sierra Nevada, particularly for
elevations >7,000ft. Treacherous travel and delays in parts of the
Sierra Nevada are anticipated through Saturday.


Mullinax