Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 PM EST Tue Jan 03 2023

Valid 00Z Wed Jan 04 2023 - 00Z Sat Jan 07 2023

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

After a brief lull today and overnight tonight, the next powerful
Pacific storm system is set to deliver yet another atmospheric
river into California on Wednesday and continuing into Thursday.
The storm system is approaching climatological records in terms of
anomalously low heights with NAEFS from 18Z Wed - 12Z Thurs
showing climatological percentiles, (1000-500mb and all mandatory
heights in between), all well into the climatological minimum.
This setup, historically, is quite significant when accounting for
both the IVT (>750 kg/m/s) and 850-700mb winds (>99.5%
climatological percentile). 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 Wednesday afternoon and 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
60-80% 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 Wednesday 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. The first draft of Key Messages are listed at the bottom of
the discussion.

...Central Plains and Upper Midwest...
Days 1-2...

The occluded wave of low pressure moving into the heart of the
Midwest will become stationary over southern Minnesota tonight and
through much of Wednesday. It will be just north of the surface
warm front, where the best 850-700mb frontogenesis resides,
heavier snow bands will look to traverse northern Wisconsin and
into the western U.P. of Michigan this evening. This is depicted
well by the HREF snowband probability tracker, which does feature
some members that suggest snowfall rates could be 1-2"/hr. While
the dynamics associated with this associated occluded front
gradually diminish, there will remain a steady cold conveyor belt
of low-level moisture embedded within 25-30 knots of easterly flow
over the U.P. and across southern Lake Superior. Meanwhile,
diffluent 500mb flow will support an atmosphere that features
rising motion aloft. Combined with the already sufficiently cold
air-mass, persistent areas of snow are expected on Wednesday from
Minnesota on east. The area with the best chances for heavy
snowfall will be in northwest Wisconsin where the best combination
of supportive upper level flow and lake-enhanced snow banding may
result in 1-2"/hr snowfall rates. Latest WPC PWPF does depict high
chances (70-90%) for >6" of snowfall from 00Z Wednesday - 00Z
Thursday in northwest Wisconsin. There is also an opportunity for
overrunning precip tonight and early Wednesday morning as the same
easterly flow regime channels over sub-freezing temperatures in
northern Wisconsin and the southern U.P. of Michigan.

...New England...
Days 2-3...

A developing wave of low pressure along the occluded front in the
eastern Great Lakes will coincide with a wedge of Canadian high
pressure over Quebec. A cold front passing through northern New
England Wednesday afternoon will provide a fresh injection of
sub-freezing temperatures into the region. At the same time, moist
W-SW 850-700mb flow aloft will overrun the developing cold air
damming signature, resulting in a classic setup for freezing rain.
Freezing rain is likely to occur as early as Wednesday afternoon
from northern New York to northern Vermont and New Hampshire. As
warm, moist air rises over the sub-freezing layer, precipitation
in these areas will fall primarily as freezing rain, with some
sleet possible closer to the U.S.-Canada border. New York's North
Country features the best odds for significant ice accumulations
with moderate chances (40-60%) for ice accumulations >0.25". The
threat for disruptive ice accumulations (>0.1" of ice
accumulation) is larger, consisting of not just New York's North
Country but also on east to the Green Mountains of Vermont (40-60%
chances) to the White Mountains of New Hampshire (also 40-60%
chances). Travel may be treacherous in these areas, with some
potential for scattered downed tree limbs and power lines where
ice accumulations surpass 0.25". The PWSSI does contain some
30-50% odds of "Moderate" impacts in New York's North Country on
Wednesday.

...Key Messages for January 1-4 Winter Storm...

--Swaths of heavy snow and light freezing rain will continue
across the Upper Midwest through Wednesday. Moderate to locally
heavy freezing rain is forecast through Wednesday night for the
North Country of New York.

--Snow rates of 1"/hr can be expected tonight south of Lake
Superior. Additional snowfall of 6-12" can be expected in portions
of the Upper Midwest.

--Wind gusts around 30 mph will continue to produce areas of
blowing and drifting snow over areas that received heavy snow over
the past day. This will result in snow-covered roads and reduced
visibility.

--Ice accumulations of 0.25" or more are likely for portions of
the North Country of New York. This may lead to dangerous travel,
scattered power outages, and tree damage.

...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.

Mullinax