Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
310 AM EST Fri Jan 06 2023

Valid 12Z Fri Jan 06 2023 - 12Z Mon Jan 09 2023


...New England...
Day 1...

A closed mid-level low ejecting from the Great Lakes will open and
pivot across New England this aftn. Despite gradual amplitude loss
of this feature, sharp height falls and PVA will move across the
area, providing locally enhanced ascent, aided by modest LFQ
diffluence as an upper jet streak extends off to the east. This
will result in a modest wave of low pressure at the surface
skirting along the Southern New England (SNE) coast, with
downstream WAA driving moist isentropic upglide at 285-290K
expanding precipitation to the north. The column looks marginally
supportive for frozen precip according to regional soundings, but
a brief period of enhanced ascent through decreasing mid-level
theta-e lapse rates and sloped 700-600mb fgen should drive some
intense UVVs beneath and into the DGZ. This evolution suggests
that where precip is heaviest, the column will dynamically cool to
support snow, with snowfall rates potentially reaching 0.5-1"/hr
at times despite working against the ever-increasing sun angle.
Although lower elevations will likely struggle to accumulate due
to a rain/snow mix (snow only during heavier precip rates), the
terrain from the Berkshires, through the Southern Greens, and
maybe most impressively the Monadnock region of NH and Worcester
Hills of MA should see at least a few inches of accumulating
snowfall. Confidence is not extremely high, but WSE plumes, and
both NBMv4.0 and v4.1 probabilities for 2-4 inches of snowfall has
increased, and WPC probabilities are 30-50% for 2", and 10-30% for
4 inches, highest across southern VT and NH. The I-95 corridor may
see a mix of rain and snow, but no accumulation is expected.


...Central Rockies...
Day 1...

A modest shortwave will amplify as it exits the Great Basin and
shifts across the Four Corners this morning before moving into the
Central Plains tonight. Broad divergence downstream of this trough
will provide ascent, enhanced by PVA and increasing diffluence
within the LFQ of a zonally oriented subtropical jet streak.
Together this will help to drive lee cyclogenesis across eastern
CO, with WAA to the east rotating cyclonically around this feature
and isentropically ascending into the Central Rockies. This low is
progged to move quickly to the east, so the overlap of pronounced
ascent and moisture should be relatively transient and confined
primarily to the first half of D1. However, this should still
result in moderate to heavy snowfall thanks to steep lapse rates
and impressive ascent collocated with the DGZ. The heaviest snow
is likely across the CO Rockies, including the Park Range, Flat
Tops, and Elk Mountains, where WPC probabilities for more than 6
inches are above 50% with locally over 12 inches likely in the
highest terrain. Additional heavy snowfall on D1 is likely across
parts of the eastern San Juans and northern Wasatch in Utah.


...Western U.S...
Days 1-3...

A continued active Pacific jet will drive repeated rounds of
precipitation across the West as multiple atmospheric rivers (AR)
spill onshore through the weekend and into next week. Both the
ECENS and GEFS probabilities indicate multiple rounds of IVT
exceeding 250 kg/m/s surging onshore each day through the weekend,
with the most impressive overlap of moisture and ascent lifting
onto the Pacific coast D3. Embedded within these ARs, multiple
mid-level impulses and periodic increases in upper diffluence will
drive locally more significant ascent, resulting in periods of
moderate to heavy snowfall above snow levels which will be
generally consistent around 4000-5000 ft today and Saturday.
However, by Sunday, a more intense region of warm advection within
the next AR and downstream of a more impressive closed low
approaching the CA/OR coast will drive snow levels towards 8000
ft, highest across CA.

For D1, WPC probabilities for 6 or more inches are highest in the
Shasta/Trinity region of northern CA where they exceed 80%, and
locally more than 2 feet of snow is expected. Despite the higher
snow levels, some impactful snowfall is possible D1 at the passes
through this terrain, including Black Butte and Snowman's Summit.
Additional heavy snow exceeding 6 inches is possible across the
Olympics and WA Cascades. During D2 and D3, the heavy snow will
only gradually spill to the east as the deepening trough over the
Pacific results in more northeast advection of features to drive
ascent, limiting the longitudinal gain of sufficient
moisture/forcing overlap to produce heavy snow. This results in D2
WPC probabilities for more than 6 inches that are generally
spatially aligned with D1, except with some moderate snow spread
into the Okanogan Highlands of WA, as well as dropping across the
length of the Sierra as well. However, the heaviest snow is likely
to surge across CA late D2 into D3 when WPC probabilities rise to
above 70% for the Sierra and Shasta/Trinity/Siskiyou regions,
resulting in storm total snowfall that will be in excess of 4 feet
in the highest terrain, especially around Mt. Shasta. As snow
levels climb D3, the characteristic of the snow will likely become
wetter and heavier, with SLRs falling below the 25th percentile of
the Baxter climatology which will likely result in enhanced
impacts as reflected by increased pWSSI for snow load.


...Key Messages for Early January Atmospheric Rivers...

--Strong Atmospheric River to Subside Tonight: A powerful
atmospheric river over California will subside tonight. Lingering
heavy rainfall could still produce flooding over coastal
California and favored upslope areas.

--Next Atmospheric River to Arrive Late Friday: The next
atmospheric river will arrive late Friday and Friday Night over
northern and central California.

--Threat of Heavy Rain, Flooding Returns Saturday: The cumulative
effect of many rounds of heavy rainfall will lead to additional
considerable flood impacts this weekend, including rapid water
rises and mudslides, over northern and central California. Flash
flooding and debris flows are possible over burn scar areas.

--Dangerous Mountain Travel Resumes Saturday: 1-2 feet of
additional snowfall this weekend will lead to dangerous travel
above 5000 feet in the mountains of northern and central
California.

--Stronger Atmospheric River to Arrive Monday: A more intense
atmospheric river will arrive over California Monday, with
additional heavy precipitation and gusty winds expected.


Weiss