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

Valid 12Z Sat Dec 10 2022 - 12Z Tue Dec 13 2022

...Northeast...
Day 2...

A compact shortwave moving overhead the Great Lakes Saturday will
amplify into a closed low as it drops southeast through Upstate NY
and off the Southern New England coast by Monday morning. A wave
of low pressure beneath this shortwave will drop southward as
well, while a secondary low develops south of Long Island, with an
inverted trough potentially developing in between the two. Ascent
across the region will increase rapidly, but also be transient as
systems move quickly to the south and east. Isentropic ascent at
285K will strengthen from PA northeastward, leading to an
expansion of precipitation Sunday and Sunday night. At the same
time, the most impressive synoptic lift through height falls and
PVA, combined with at least modest deformation along the inverted
trough, should result in snowfall rates increasing to 0.5"/hr or
more within the broadening precipitation shield. This will result
in a broad area of light to moderate snowfall accumulations, with
WPC probabilities indicating a 30-50% chance for 4 inches across
parts of Upstate NY and into the Berkshires of MA. Isolated maxima
of 6 inches or more are possible, especially in the Catskills
where low-level easterly flow invof the inverted trough will yield
some upslope enhancement. Light snow accumulations exceeding 1
inch may reach as far southeast as I-95 as cooling temperatures
behind the departing system change any rain to snow before precip
winds down Sunday night.


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

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

A large closed mid-level low will drop down the Pacific coast from
the Gulf of Alaska today and then shift onshore CA while
broadening to produce anomalously low heights across the majority
of the west, with 500mb heights peaking below -2 standard
deviations over the Great Basin according to the NAEFS ensemble
tables. This closed low will remain broad and amplified within
shortening wavelength flow, to just drift slowly eastward into
early next week. Around this cyclonic gyre, spokes of vorticity
will periodically rotate through the flow, combining with upper
level jet energy to drive large scale and impressive synoptic
ascent. This lift will help to drive widespread precipitation as
column moisture becomes anomalous, up to +1 standard deviation
above the climo mean, thanks to an atmospheric river (AR) moving
onshore with IVT above 500 kg/m/s.

On D1, extreme snowfall is likely across the Sierra and northern
CA ranges as 700mb flow becomes squeezed and orthogonal into the
terrain. This will drive impressive moisture inland, with ascent
maximizing thanks to impressive upslope flow beneath already
robust synoptic lift. Snow levels will climb to 4000-6000 ft
within the best WAA, but will fall to 2000-3000 ft as the cold
front moves onshore. This will manifest as snowfall rates
eclipsing 3"/hr according to the WPC prototype snow band tool,
which through this long duration event will likely accumulate to
multiple feet of snow despite modest SLR across the CA terrain.
WPC probabilities D1 are above 90% in the Sierra and near Mt.
Shasta for 12+ inches, with local maxima in the Sierra approaching
5 feet. Additional heavy snow D1 is likely extending into the
Sawtooth range of ID and the highlands of northern WA where WPC
probabilities suggest a high risk for more than 6 inches, and with
locally more than 12 inches likely.

During D2 /12Z Sunday to 12Z Monday/ heavy snowfall will expand in
coverage and extend east into the Great Basin as far as the Uintas
and Mogollon Rim, while also expanding northward to the Northern
Rockies. Sunday will be a snowy day across nearly the entirety of
the Great Basin, northern Four Corners, and points northward.
Again the heaviest snowfall is likely in the Sierra where WPC
probabilities for an additional 12 inches are 50-80%, but snowfall
of more than 12 inches is also possible D2 across the terrain of
NV and into the southern Wasatch, as well as the highest peaks of
the San Gabriels/San Bernadinos. WPC probabilities indicate at
least a low risk for 6 inches of snowfall or more across all but
the lowest terrain of the Great Basin D2. On D3 the best forcing
and moisture continues to shift eastward, with heavy snow
enveloping the eastern Mogollon Rim into the White Mountains of
AZ, the San Juans and CO Rockies, and northward into the Uintas
and as far north as the Absarokas. WPC probabilities D3 are
moderate to high for more than 6 inches in this area, generally
above 2000 ft.


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

An elongated and amplified mid-level trough will work in tandem
with phased upper level jet streaks to produce lee cyclogenesis
near the CO/WY border Monday morning. This low is likely to
rapidly deepen as it shifts into the Central Plains later Monday,
and guidance is in very good agreement for D3 as to the evolution
and placement of this system. Downstream, potent WAA will begin to
blossom drawing anomalous moisture and instability northward,
which will manifest as increasing coverage of rain, freezing rain,
sleet, and some snow across the High Plains, especially by late in
the forecast period. While initially the DGZ will be relatively
dry, the impressive moist advection should saturated the entire
column by 12Z Tuesday, resulting in increasing coverage of
snowfall from eastern WY through the Dakotas. While this will
likely be a very impressive snow event for some areas into D4, for
D3 WPC probabilities for more than 4 inches of snow are currently
highest in western SD where they reach 20-40%.


Key Messages for December 9-15 winter storm from the Pacific Coast
through the Upper Midwest:
--A significant winter storm with high winds, heavy precipitation,
and mountain snow, will continue across California today and then
spread into the Great Basin and Four Corners through Monday.

--Prolific snowfall rates of 3"/hr are expected in the Sierra
Nevada accumulating to more than 5 feet through Sunday in some
areas. 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 West Coast and Intermountain West
leading to difficult travel.

--As the system moves east, a strong surface low is expected to
develop in eastern Colorado Monday and then track northeast into
the Upper Midwest through Wednesday.

--Multiple days of significant 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 possible for parts
of South Dakota, and travel may become impossible.


Weiss