Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 AM EST Thu Jan 05 2023

Valid 12Z Thu Jan 05 2023 - 12Z Sun Jan 08 2023

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

An active period continues across the West as repeated impulses
and associated atmospheric rivers (AR) move onshore into the
weekend. The first of several shortwave impulses embedded within a
broad Pacific coast trough will move into CA this aftn, with
height falls occurring along the length of the Pacific coast to
drive ascent through D1. A second shortwave is progged to move
into the Pacific Northwest on D2 as a large gyre spins across the
Northern Pacific Ocean south of the Gulf of Alaska, while yet a
third, and even more impressive trough spins towards the coast on
D3. Confluent flow downstream of each of these waves combined with
persistent, albeit variable, jet energy and associated diffluence
will help surge moisture onshore characterized by PW anomalies of
+0.5 to as high as +2 standard deviations above the climo mean,
coincident with IVT exceeding 500 kg/m/s today, and then again D3,
as forecast by both ECENS and GEFS probabilities from CW3E. This
will spread copious moisture onshore through the period, with
waves of heavy precipitation occurring each day. Although the
heaviest precipitation will likely remain across CA through the
forecast period, at least moderate precipitation, falling as snow
in many areas, will spread as far east as the CO Rockies. Snow
levels will vary across the West, but should generally waver
between 3000-5000 ft each day, but could rise as high as 7000 ft
with the second significant impulse D3, highest also in CA.

For D1, the heaviest snowfall will occur across the Sierra,
especially before 00Z tonight as ascent and moisture maximize,
aided by orthogonal/upslope flow into the Crest. WPC probabilities
for more than 8 inches of snow are above 90% in the Sierra, with
1-2 feet likely at some of the important passes above 5000 ft,
including Donner, Echo Summit, and Carson Passes, with more than 3
feet of snow likely in the highest terrain, especially in the
southern Sierra. With spillover moisture spreading all the way
into the Great Basin and Four Corners on D1, heavy snowfall
exceeding 6 inches is likely into the Wasatch and Kaibab Plateau,
as well as farther south into the San Gabriels. Heavy snow is also
likely in the Shasta/Trinity/Siskiyou region, and as far north as
the Olympics of WA, as the aforementioned broad height falls occur
along the breadth of the coast.

By D2 the lead shortwave moves towards the CO Rockies bringing WPC
probabilities above 40% for 6 inches into much of western CO. At
the same time, the shortwave lifting into WA state brings
additional heavy snowfall to the Olympics and WA Cascades, with
locally 6-10 inches likely, although generally above pass level.
Late D2, the next impressive AR approaches as the larger mid-level
impulse moves eastward across the Pacific. This will spread more
significant precipitation onshore once again, from the WA Cascades
into the Blue Mountains of OR, through the northern CA mountains,
into the Sierra. WPC probabilities D3 for more than 6 inches reach
above 90% in the Shasta/Trinity region and northern Sierra, with
multiple feet of snow possible above 7000 ft. Elsewhere, WPC
probabilities for more than 6 inches are modest across these areas.


...Eastern Great Lakes through New England...
Days 1-2...

Two waves of wintry precipitation will impact the Northeast into
the weekend, followed by locally heavy Lake Effect Snow (LES).

The first wave will be ongoing at the start of the forecast period
this morning as WAA overrunning Canadian high pressure will spread
precipitation across primarily northern New England. The attendant
wave of low pressure will lift northward into Ontario, leaving
pronounced WAA across central and northern New England through
tonight. At the same time, the aforementioned high will wedge more
intensely southward allowing for a southward push of cold
ageostrophic drainage to deepen the sub-freezing near-surface
layer. This will result in a mixture of p-type with snow likely
across northern Maine, but a mix of sleet and freezing rain likely
as far south as the Worcester Hills of MA. The guidance has gotten
a bit deeper with the sub-warm nose cold depth, suggesting more
sleet than freezing rain, with a more rapid drying of the column
from SW to NE resulting additionally in less ice accretion. Still,
a messy D1 is likely with WPC probabilities indicating a 10-30%
chance for 4+ inches across the northern Whites and towards
Caribou, ME. WPC probabilities for 0.1" or more of freezing rain
is as high as 20% from central NH through coastal ME, with modest
probabilities for at least 0.01" of ice as far south as the
Berkshires and Worcester Hills.

Although light snow will continue across northern Maine into D2,
there is generally a brief respite in precipitation expected until
Friday morning across southern and central New England. At that
time, the core of the closed upper low over the Great Lakes is
progged to pivot eastward while opening, providing briefly intense
ascent through height falls, PVA, and downstream divergence, with
some synoptic lift also enhanced through a weak LFQ of a departing
jet streak. The guidance has become a bit more aggressive with
this feature, and while antecedent thermals are marginal for snow,
enhanced ascent aided by instability aloft and some negative
theta-e lapse rates should result in a burst of heavier precip
which will dynamically cool the column and drive a near-isothermal
layer beneath the DGZ. This suggests the likelihood for a burst of
moderate to heavy snow, which could accumulate rapidly to a few
inches, and both NBM and WSE probabilities have increased this
morning. There are current no probabilities for 4 inches in this
area, but locally 2+ inches of snowfall is possible, especially in
the Worcester Hills and Southern Greens.

Behind this ejecting shortwave, W/NW flow will develop across the
Great Lakes providing a favorable environment for some LES,
especially Friday night into Saturday morning downstream of Lakes
Erie and Ontario. Inversion depths reaching 800mb will support
heavy snowfall rates despite modest 850-sfc deltaT, aided by
theta-e lapse rates that are 0C/km or less at times. This should
support 1"/hr snowfall, with the most favored banding likely
focused into the Tug Hill Plateau and Chautauqua Ridge where WPC
probabilities for more than 4 inches of snow peak on D2.5 above
50%, and locally 8 inches of snow is possible.


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

--Strong Atmospheric River Arrives Wednesday: A powerful
atmospheric river is arriving in California today and will
continue into Thursday.

--Dangerous Travel Conditions in the Mountains: Extremely heavy
snow rates above 3 inches per hour are nearly certain above 5000
feet, which will lead to dangerous, and at times impossible,
travel in the mountains of northern and central California.

--Heavy Rain, Flooding Likely at Lower Elevations: Considerable
flooding impacts are likely, especially Wednesday Night to
Thursday. Rain rates over 1 inch per hour may lead to rapid water
rises and mud and rock slides. Coastal areas of California and the
Sacramento Valley are most at risk.

--Downed Trees and Power Outages Possible: Widespread wind gusts
in excess of 50 MPH and saturated soils may make trees more
susceptible to blowing down, with power outages also possible.

--Additional Atmospheric Rivers into Next Week: The next
atmospheric river will arrive Saturday, and another one on Monday.
The cumulative effect of many rounds of heavy precipitation may
lead to increasing impacts.

Weiss