Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
344 PM EST Fri Dec 09 2022

Valid 00Z Sat Dec 10 2022 - 00Z Tue Dec 13 2022

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

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

A vigorous upper trough tracking down along the West Coast will be
responsible for the multi-day onslaught of heavy snowfall from the
higher terrain of the West Coast on inland to the Intermountain
West. Latest NAEFS percentiles shows heights within the
700-500-200 mb layers below the lowest 1% climatological
percentiles by Saturday 18Z, displaying how unusually deep the
upper trough off the Pacific Northwest coast is for this time of
year. This upper trough will direct a stream of Pacific moisture
at California, which will then spill over into the Great Basin and
parts of the Intermountain West. NAEFS does depict >90th
climatological percentile 500-700mb mean specific humidity values
streaming into parts of these regions on Saturday, then pushes the
axis of most anomalous moisture a little farther south into
southern California and the Lower Colorado River Valley on Sunday.
The deep reservoir of moisture is ushered into the West Coast
courtesy of a strong IVT, which is at or above the 99th
climatological percentile over central California at 18Z on
Saturday and then up to 99.5% in southern California on Sunday.

With the upper trough tracking into the West Coast this weekend,
snow levels will drop precipitously up and down the West Coast,
with rising SLRs and strong upslope enhancement further aiding in
the development of heavy snowfall rates. The Sierra Nevada in
particular can expect prolific snowfall rates of ~3"/hr at times
this weekend. Latest WSSI shows not only an extensive "Extreme
Impact" area along the spine of the Sierra Nevada this weekend,
but also in parts of the Shasta mountains of northern California.
It is here where travel will be the most dangerous and disruptions
to daily life will be greatest. By the time the storm is all said
and down along the West Coast, snowfall will be measured in feet
from the Cascade Range on south to the southern Sierra Nevada and
even the higher elevations of the Transverse Range. Localized
snowfall totals greater than 5 feet are expected along the highest
and topographically-favored slopes of the Sierra Nevada.

By Sunday night, the upper low makes its way across the Great
Basin and approaches the central Rockies by Monday morning.
Falling heights will lead to falling snow levels while the upper
trough continues to inject anomalous Pacific moisture into the
Four Corners region. NAEFS shows 500-700mb temps that are in the
lowest 1-2.5 climatological percentile over northern Arizona,
southern Utah, and southern Nevada. This will give the higher
elevations, including ranges such as the Mogollon Rim, the San
Juan of southwest Colorado, and in the higher terrain neighboring
and including Zion National Park in southwest Utah. Latest WPC
probabilities show anywhere from 50-80% odds for snowfall totals
>6" in these areas. The Wasatch on north to the Sawtooth range in
southern Idaho also feature 50-70% probabilities for >6" snowfall
totals as well. 

...Northeast...
Days 2-3...

An amplifying trough traversing the Great Lakes Saturday night
will take on a subtle negative tilt at 500mb, allowing for strong
vertical ascent within the column to spawn a wave of low pressure
near Lake Ontario Saturday night. The low and its associated
precipitation shield then tracks into the interior Northeast on
Sunday. Meanwhile, a strong dome of high pressure anchored over
eastern Canada will force a cold air damming wedge into the
Northeast with sufficiently cold air entrenched over the region.
The initial wave of precipitation will look to fall in the form of
snow via 850-700mb WAA aloft. The heaviest snowfall is currently
expected to occur in the Adirondacks and Catskills with the latest
WPC probabilities showing 20-30% odds for >4" of snowfall. It is
worth noting, given the strong dynamics at play, there could be a
more expansive swath of >2" probabilities at lower elevations,
which is shown on the WPC PWPF. Much of the Hudson Valley and
Berkshires on south into the Poconos have 40-60% probabilities for
>2" of snowfall. One limiting factor for a larger footprint of
snowfall totals >4" is due to storm track and the potential for
dry slotting in the 700-300mb layer on Sunday. It is also worth
mentioning some areas in northern Pennsylvania may witness
slightly milder temperatures at low levels but sub-freezing
temperatures at the surface. This has led to potential for light
ice accumulations being depicted in western and northern
Pennsylvania, but WPC PWPF keeps <10% probabilities for ice totals
<0.1" of ice.


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 reach the Pacific Northwest today then
impact California, the Great Basin, and the Four Corners through
Monday.

--Prolific snowfall rates of 3â€/hr are expected Saturday in the
Sierra Nevada with totals locally more than 5 feet through Sunday.
This will result in extremely dangerous travel, especially across
mountain passes. If you must travel, prepare for rapidly changing
conditions, and carry winter driving supplies.

--Elsewhere, snowfall totals of 1-3 feet are forecast for the
neighboring mountain ranges of the West Coast and Intermountain
West.

--As the system moves east, a strong surface low is expected to
develop in eastern Colorado Monday and then slowly track from the
Central High Plains to the Upper Midwest through the middle of
next week.

--Confidence is unusually high for strong winds and significant
snows to produce hazardous impacts across much of the
Central/Northern Plains and into the Upper Midwest starting Monday
night.


Mullinax