Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 PM EST Sat Dec 10 2022

Valid 00Z Sun Dec 11 2022 - 00Z Wed Dec 14 2022

...Northeast...
Days 1-2...

A shortwave trough traversing the Great Lakes late Saturday will
work in tandem with a ~120 knot jet streak over the Ohio Valley to
produce strong vertical ascent atop the atmosphere early Sunday
morning. At the surface, a cold air damming signature wedged into
the Northeast will reside ahead of an approaching weak surface low
emerging from the eastern Great Lakes Sunday morning. A steady
southerly feed of 850mb moisture will be directed into a
sub-freezing air-mass across the interior Northeast early Sunday
morning as 850mb warm air advection (WAA) aids in the production
of wintry precipitation. The heaviest snowfall will occur during
the morning hours on Sunday and into the mid afternoon hours when
the best combination of synoptic-scale lift, moisture, and perhaps
some upslope enhancement in the higher terrain of the Catskills,
Poconos, Adirondacks, and Berkshires occur. Latest WPC PWPF for
snowfall totals >4" are as high as 40-60% in these mountain
ranges, with slightly lower percentages in the Lower Hudson Valley
and back towards NY's Finger Lakes. Note lower accumulations (>2"
snowfall totals) are as high as 70% in northern NJ and western CT.
Latest WSSI does depict "Minor" impacts for some of these areas,
so those traveling on Sunday will want to take their time and use
caution on the roads.

...Pacific Northwest and California through the Great Basin &
Rockies...
Days 1-3...

...Widespread heavy snow likely across the West through early next
week...

The upper low responsible for the prolonged stretch of heavy
snowfall in the West will continue to deliver ample amounts of
Pacific moisture into the West Coast and the Intermountain west
tonight and into to start of next week. The approaching upper
trough contains temperatures in the 700mb layer that is 2-3
standard deviations below normal, prompting snow levels to
continue lowering and SLRs to rise. The Day 1 (00Z Sun - 00Z Mon)
WPC PWPF shows >18" snowfall probabilities >95% throughout much of
the Sierra Nevada, speaking to just how impressive these snowfall
totals are. This is captured will in the Day 1 WSSI which
maintains "Extreme" impacts up and down the Sierra Nevada. The
barrage of Pacific moisture into Southern California and falling
heights aloft makes the Transverse Range prone to heavy snow as
well. The WSSI does depict "Major" to "Extreme" impacts in the
higher elevations of the San Gabriel and San Bernadino Ranges
where WPC PWPF also shows >50% chances for >8" of snowfall.

As the persistent fetch of 850-700mb moisture moves into the Great
Basin and Intermountain West, so will the threat for heavy snow
along many mountain ranges. The most notable of them being the
Mogollon Rim of AZ on north through the San Juan of Colorado,
across southwest Utah and up the Wasatch and Uinta Ranges, then
finally up to the Absaroka, Bitterroot, and Sawtooth Ranges of the
northern Rockies. WPC PWPF probabilities range generally between
50-80% in the highest terrain of these ranges. By late Monday into
Tuesday, more of the Colorado Rockies on north into southern
Wyoming will receive heavy snow, with WPC PWPF putting 40-70% odds
of snowfall >6" in these areas.

...Northern/Central High Plains...
Day 3...

The upper low traversing the Intermountain West will find itself
in a favorable position to strengthen early next week, resulting
in a deepening surface low in lee of the central Rockies Monday
morning. The synoptic-scale setup is textbook for a significant
winter storm in the central and northern Plains. A 150 knot 250mb
jet streak in southern Arizona will have its divergent left exit
region position over the central High Plains Monday afternoon. In
terms of moisture, a deep moisture feed emanating out of the
western Gulf of Mexico will be transported north via a
strengthening low level jet throughout the day on Monday.
Southerly 850mb winds are progged to be 40-45 knots over the
southern and central Plains late Monday, which is above the 90th
climatological percentile. Meanwhile, a pair of high pressure
systems; one over southeast Canada and another in the Canadian
Prairies, are acting to supply sub-freezing temps.

As the 700mb low emerges off the central Rockies, it will deepen
quickly as it tracks through western Kansas Tuesday morning. WAA
at 850-700mb will intersect a cold front over the Northern Plains,
resulting in a pivoting axis of heavy snow over the Dakotas and
into parts of western Nebraska and eastern WY/MT. As the 700mb low
consolidates, a warm conveyor belt containing 700mb moisture flux
should setup somewhere in southeast WY, northeast CO, and
potentially into northwest KS as well. It is in these two areas
where the strongest vertical ascent, combined with sufficient
moisture and sub-freezing temperatures where snowfall will be
heaviest. In addition to snowfall, winds will be picking up to
very strong levels. The ECMWF EFI highlighted an area of 0.8-0.9
in portions of southeast WY, western NE, and into far northeast
CO, which is a strong signal for potentially hazardous winds. The
combination of whipping wind gusts and snow builds the case for
near whiteout, or even blizzard, conditions late Tuesday.

The challenges in the forecast are where the emerging dry slot in
the 300-700mb layer tracks, which will act to effectively cut-off
the production of quality dendritic snowflakes. There is also the
question of the warm nose around 800mb along the Missouri River
and into the Midwest, which may lead to a changeover from snow to
an icy wintry mix, particularly over southern MN and northern IA.
These details should come into focus over the next couple of
forecast cycles, but the track of the storm, as well as the depth
of the sub-freezing layer aloft will help to determine how long
these areas can stay snow before making a switch over to a wintry
mix, and possibly even all rain. Latest WSO showed a 50-80% area
for potentially warning-criteria level snowfall over north-central
MN with slightly lower odds (10-30%) in central WI. WPC PWPF does
depict 30-50% probabilities for >0.1" of freezing rain
accumulation over parts of southern MN and northern MN on Day 3.


Key Messages for December 9-15 winter storm from the Pacific Coast
through the Upper Midwest:
--A major winter storm with high winds, heavy precipitation, and
extreme mountain snow will continue across California into Sunday
and then traverse the Great Basin and Desert Southwest through
Monday.

--Expect prolific snowfall rates of 3â€/hr in the Sierra Nevada,
accumulating to more than 5 feet through Sunday in the Sierra
Crest. This will result in nearly impossible travel due to near
zero visibility and snow covered roads, especially across mountain
passes.

--Elsewhere, snowfall totals of 1-3 feet are forecast for many of
the mountain ranges of the Southern California and the
Intermountain West leading to difficult travel.

--As the system moves east, a strong surface low will develop in
eastern Colorado Monday and then slowly track northeast over the
Central Plains through Wednesday.

--Multiple days of major impacts due to snow and blowing snow are
likely for much of the Central and Northern High Plains into the
Upper Midwest. Blizzard conditions are likely for parts of the
Central and Northern Plains, and travel may become impossible.


Mullinax