Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 PM EST Thu Dec 23 2021

Valid 00Z Fri Dec 24 2021 - 00Z Mon Dec 27 2021

...Repeating rounds of heavy snow continue for western terrain
with multiple additional feet expected while snow levels decrease
from north to south...

...Western U.S...
Days 1-3...
A highly-amplified, positively-tilted upper trough shifting south
down the West Coast will close off into a low off CA this evening
before opening back up and ejecting east across southern CA Friday
morning and to the south-central Great Plains by Friday evening.
Deep southwesterly flow ahead of this low will continue to shift a
strong moisture surge from central to southern CA and spread
across the Desert Southwest tonight, with highly anomalous
precipitable water values extending east through the southern
Great Basin and Four Corner Region by this evening. This moisture
influx, along with orographic lift and broad scale lift over from
upper jet forcing over a frontal boundary will continue to support
widespread precipitation, including mountain snows, with
orographically focused moderate to heavy amounts across the
Southwest. The heaviest snow through Friday is expected to fall
across central and southern Sierra Nevada, the highest Transverse
Ranges of SoCal, the mountains of northern AZ/NM and most of the
ranges of UT where Day 1 snow probabilities are high for a foot or
more with 2-4ft likely in portions of the southern Sierra and the
higher San Juan/Sawatch Mtns of CO.

Meanwhile, the next system is currently off the AK Panhandle and
will amplify/close into a low over western WA with another
positively tilted trough that extends the full latitude of the
West Coast by late Saturday before its southern end (which may
become a closed low near the south-central CA coast Saturday
evening) ejects east across the Great Basin and Rockies into
Sunday. Strong onshore flow and moderate Pacific moisture
maintains moderate precip rates with high Day 1 snow probabilities
for 8 or more inches over the WA/OR Cascades and moderate for the
Klamath/Siskiyous/Trinity Alps as well as inland over the
Sawtooth, and the western Wyoming ranges.

A focused area of heavy precip on the leading edge of the second
trough is likely to develop over the OR coast to Cascades Friday
and spread south down through central CA through Saturday and
potentially weaken for southern CA Saturday night depending on how
far south the trough can dig. Snow levels will drop to sea level
over WA/OR under this trough by late Saturday and decrease below
2000ft in CA. This brings moderate Day 2 probabilities for 8 or
more inches to the Olympics and WA Cascades (down to about
2000ft), moderate probabilities for a foot or more for the OR
Cascades the Klamath and high probabilities for northern Sierra
(which then expands to the central and southern Sierra again for
Day 2.5).

A deep, cold core low over southern BC/northern WA develops Sunday
with maintained snow levels at sea level for Portland and Seattle,
though generally light onshore flow under this low should limit
snow rates. Day 3 snow probabilities are high for a foot or more
for the OR Cascades and yet another trough developing offshore and
swinging across CA is anticipated for Monday which should result
in several more feet of snow for the Sierra with the potential for
10 or more feet storm total for the High Sierra.


...Northeast...
Days 2/3...

Uncertainty still reigns with the track and development of the
leading shortwave from the trough currently off the CA coast. The
GFS continues to be the strongest with its development as it
tracks from the OH Valley across the Northeast Friday night into
Saturday night. The 00Z ECMWF/ECENS offered some support with a
weaker version of a similar system, but the 12Z ECM has backed off
to a weaker/faster solution with less of a heavy snow threat. So
the GFS and 12Z NAM are the only ones with a noted low, while the
12Z ECM and CMC are weaker and the 12Z UKMET is much weaker. WPC
preference remains with a more suppressed solution than what the
GFS/GEFS suggest, though some percentage of those were included in
the Day 2/3 QPF in the northeast today. This weaker solution would
be less dynamic, faster, and would feature less banded snow,
though the warm air advection ahead of the wave would bring
freezing rain potential from the PA/NY border to central New
England where Day 2/2.5 probabilities for a tenth inch of ice are
10 to 30 percent. Any low that develops would promote a changeover
to snow across northern NY/New England as it moves off the coast
Saturday night. 

For days 1-3, the probability of ice accumulation greater than
0.25" is less than 10 percent.


~~~Key Messages for Western U.S. Heavy Snow Through Early Next
Week~~~

Multiple waves of low pressure will spread precipitation across
the West through early next week with periods of heavy snowfall
likely for terrain along the Pacific Coast each day through
Tuesday, with snow at times spreading across much of the Great
Basin and Intermountain West to the Rockies.

Heavy snow accumulating to multiple feet is likely during the next
several days in many of the mountain ranges, with more than 7
additional feet likely in the Sierra. Light to moderate snow
accumulations spread over several days are likely even at sea
level over the Pacific Northwest.

Travel will become dangerous and is discouraged, especially along
mountain passes where long duration closures are likely. Whiteout
conditions, downed trees, and power outages are all possible.
Dangerous avalanches are likely in the Sierra, Washington
Cascades, and Northern Rockies.

Extremely cold temperatures will overspread the Northwest early
next week, with some record cold possible especially over the
Pacific Northwest. Low temperatures below 0F will be common east
of the Cascades through the Northern Plains.



Jackson