Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
219 PM EST Mon Dec 26 2022

Valid 00Z Tue Dec 27 2022 - 00Z Fri Dec 30 2022

...Great Lakes...
Day 1...

The seemingly endless period of cyclonic flow over Lakes Erie and
Superior looks to continue for one more day as a secondary
shortwave trough rotates beneath the base of the upper low over
northern Quebec. The shortwave will provide some modest PVA over
the eastern Great Lakes while 850mb winds remain oriented out of
the WSW, keeping low level convergence and upper level divergence
positioned over the Great Lakes. This means one more day of
single-band lake effect snow bands downwind of Lakes Erie and
Ontario in areas that do not need anymore snowfall in wake of the
deadly winter storm. As a clipper system approaches the northern
Great Lakes and high pressure slides over the Mid-Atlantic Tuesday
afternoon, temperatures will become warmer at low levels and winds
will become almost purely out of the SW. This will finally cause
the lake effect snow machine to turn off. Latest WPC PWPF still
shows moderate probabilities (40-50%) for >4" of snowfall on
Monday night into Tuesday morning near the Buffalo metro area,
with high probabilities (70-90%) downwind of Lake Ontario near the
Tug Hill. In fact, there are even some 70% probabilities for >8"
near Watertown, NY.

...Interior Pacific Northwest Ice Potential
Day 1...

While the Arctic air-mass is gradually eroding away in the Pacific
Northwest, surface temperatures in the Columbia Basin will remain
generally below freezing as a powerful Pacific storm system
approaches from the west. Its associated warm front will continue
to advance deeper into the Pacific Northwest Monday night into
Tuesday morning, leading to an optimal setup for an above freezing
layer nosing its way over atop sub-freezing surface temperatures.
Latest WPC PWPF shows a small area of 10-20% probabilities for
freezing rain accumulations >0.10" near Yakima. Latest WSSI still
depicts "Minor" impacts in the Columbia Basin and even into the
Columbia River Gorge, suggesting motorists should continue to use
caution while driving tonight into Tuesday morning.

...The West...
Days 1-3...

A robust upper low tracking into the Pacific Northwest will
introduce an anomalous fetch of Pacific moisture into the West
Coast and Intermountain West tonight and into the middle of the
work week. This upper low is impressive, featuring 850-700mb
heights in the <1% climatological percentile over western
Washington and Oregon around 18Z Tuesday. At the same time, the
nose of a 150kt 250mb jet will be oriented over northern
California. While the heights over Pacific Northwest are
exceptionally low, the temperatures in the 700-500mb layer are
remarkably mild for late December over California and the
Southwest. NAEFS mean temperatures between 12Z Tues - 00Z Wed at
500mb are between the 90-97.5 climatological percentile, which is
forcing snow levels to be exceptionally high at the start. Snow
levels will start off as high as 10,000' tonight and Tuesday
morning, but will gradually decrease through the afternoon and
evening hours Tuesday as a cold front makes its way into
California. This will result in a heavy, wet snow in the Sierra
Nevada, Cascade Range, and as far inland as the Boise and Sawtooth
of central Idaho. WPC PWPF shows probabilities for >12" of
snowfall between 60-80% in portions of these ranges tonight
through Tuesday afternoon. WPC WSSI shows "Extreme" impact
potential in the Sierra Nevada weighted toward the Snow Load
component of the WSSI algorithm. Travel is likely to be dangerous,
if not impossible, in the hardest hit areas. In addition, there is
a strong wind component to this event too, highlighted by the
ECMWF EFI containing 0.8-0.9 values of anomalous wind speeds in
the central Sierra Nevada. The combination of high snow load and
gusty winds could lead to some downed tree limbs and power lines
in the northern and central Sierra Nevada on Tuesday.

By Tuesday night, an even stronger 250mb jet streak will push in
from the Pacific and force the core of the best 700mb moisture
flux into the Intermountain West. With falling 700-500mb heights
across the region from the upper trough moving in aloft, this will
cause snow levels to drop and snow to fall heavily from the Tetons
and Wasatch on south to the San Juans and the Mogollon Rim. Snow
levels will initially be quite high in the Four Corners region
Tuesday night, but they will fall to 6,000' by Wednesday
afternoon. The slug of Pacific moisture is impressive with NAEFS
showing climatological percentiles >99% in the 500-700mb layer
across north-central Arizona and southwest Colorado. WPC PWPF
shows 50-70% probabilities for >8" of snow for the Wasatch,
Mogollon Rim, and most of the Colorado Rockies west of the
Continental Divide. The San Juans have the best odds of seeing
>12" snowfall totals with probabilities as high as 60-80%. Similar
to their neighbors in the Sierra Nevada, it will be a combination
of Snow Amount and Snow Load that drives the WSSI to indicate
"Moderate" to "Major" impacts in parts of these mountain ranges.
Treacherous travel conditions are expected in these ranges
Wednesday and into Wednesday night.

...Central Plains & Midwest...
Days 2-3...

The moisture associated with the upper trough traversing the
Intermountain West will make its way into the Nation's Heartland
where a wave of low pressure is set to form in lee of the Rockies
by Thursday. There does remain some notable spread in guidance on
snowfall totals and where the axis of heavy snow sets up, but
currently, WPC PWPF does show up to 5-10% probabilities for >4" of
snowfall in western Nebraska. Should this upper trough continue to
support a stronger surface low, snowfall totals could approach
levels closer to warning criteria on Thursday.

Mullinax