Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 AM EST Wed Jan 04 2023

Valid 12Z Wed Jan 04 2023 - 12Z Sat Jan 07 2023

...Western U.S...
Days 1-3...
A powerful and significant storm system is set to impact
California beginning today, resulting in multiple widespread
hazards. The storm system approaching the state is highly unusual,
even for California atmospheric rivers with near record low
heights as depicted by the latest NAEFS. The moisture plume
rounding the base of the upper level feature will feature very
high IVT values (greater than 750 kg/m/s) and PWs well above
normal (+3 sigma). Finally, the system will have near record
mid-level winds (850-700mb winds >99.5% climatological
percentile). All told, the combination of intense low-mid level
winds and exceptional atmospheric moisture content will provide
the perfect recipe for prolific snowfall rates from the Shasta on
south along the Sierra Nevada and as far south as the Transverse
Range. Snow levels will initially rise as high as 7,000ft through
this evening as the warm front pushes through California, but snow
levels will then fall as low as 5,000ft by Thursday when the cold
front swings through. By Thursday, the axis of the atmospheric
river becomes oriented over Southern California and the lower
Great Basin, but prolonged onshore flow will keep period of heavy
snow in the forecast for many of the Golden State's mountain
ranges, as well as portions of the central Great Basin and even
into portions of the Intermountain West.

In terms of snowfall amounts and impacts, latest WPC PWPF depicts
greater than 80% chances for >18" of snowfall in parts of the
Shasta and Sierra Nevada. Snowfall will be measured in feet in
these ranges, with totals between 2-4 feet and locally up to 5
feet of snow. Farther south, the peaks of the San Gabriel and San
Bernadino mountains could also pick up 1-2 feet of snowfall late
Wednesday night through Thursday. The WSSI is highlighting both
"Snow Amount" and "Snow Load" as the key factors in producing
"Extreme" impacts in these ranges today into Thursday. At the
highest elevations and passes, winds will also be strong and even
potentially destructive. NAEFS between 12Z Wed - 06Z Thurs
features anywhere from 50-80 knot winds within the 850-700mb layer
along the Coastal Range and across northern and central
California. Especially for those mountainous areas that are closer
to 850-700mb, these winds would support whiteout conditions, along
with the added threat for downed tree limbs and power lines. The
winds are intense enough to also trigger "Extreme" levels for
"Blowing Snow" in the highest terrain of the Shasta, Sierra
Nevada, and San Gabriel mountains. Travel will be dangerous to
impossible in these mountain ranges with substantial impacts to
infrastructure (downed power lines, stress on building from heavy
snow load) as well.

This is yet another Pacific storm system that will be among
several more storms that look to bring additional rounds of heavy
snow, high winds, and copious amounts of rain into the medium
range. WPC has begun Key Messages for the active pattern set to
unfold not only in the next few days, but into the medium range as
well, which are listed in detail below.

...Central Plains and Upper Midwest...
Days 1-2...
A nearly stacked low pressure system will remain stationary over
the Upper Midwest today into Thursday before slowly pulling away
toward the Great Lakes Thursday night. Snowfall rates of 1"/hr to
locally 2"/hr will be possible across the heaviest snow bands,
which are expected to set up along the north/northwest side of the
mid-level low, mainly from central Minnesota through northern
Wisconsin and the U.P. of Michigan. Moist easterly flow off of
Lake Superior will help to enhance some of the snow totals where
the latest WPC probabilities show moderate to high probabilities
of at least 6" (mainly northwest WI but also around the Duluth MN
area). A secondary area of 6"+ amounts is favored across the U.P.
of Michigan thanks to some lake effect/enhancement.

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

A wave of low pressure developing along the occluded front across
the eastern Great Lakes will coincide with a wedge of cold air in
place thanks to Canadian high pressure over Quebec. As the low
passes through, moist southwesterly flow aloft will overrun the
cold sub-freezing airmass in place producing a favorable setup for
freezing rain, beginning late this afternoon across the northern
areas of New York and northern New England. The most favorable
location for significant (>0.25" ice accumulation) is across New
York's North Country, as shown by the latest WPC ice probabilities
featuring 40-60% chances. This ice accumulation may lead to
treacherous travel, with some potential for scattered downed tree
limbs and power lines where ice accumulations surpass 0.25".


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

--Strong Atmospheric River Arrives Wednesday: The next atmospheric
river will arrive in California early Wednesday and 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 40 MPH and saturated soils may make trees more
susceptible to blowing down, with power outages also possible.

--Additional Atmospheric Rivers into Next Week: A series of
atmospheric rivers are likely to continue to affect California
with heavy mountain snows and low elevation heavy rain. The next
one will arrive Saturday.


Taylor/Mullinax