Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 AM EST Fri Dec 01 2023

Valid 12Z Fri Dec 01 2023 - 12Z Mon Dec 04 2023

...Cascades through the Northern Rockies...
Days 1-3...

The northwestern U.S. can expect an active weather pattern to
close out the work-week and persist into the weekend thanks to an
unusually strong North Pacific jet extension delivering a
seemingly endless supply of Pacific moisture and upper level
disturbances. After the first round of wintry weather on Thursday,
a second and more impactful upper trough will track through the
Pacific Northwest Friday morning. It will then race southeast
towards the Wasatch Friday evening and over the central Rockies
Friday night. A steady barrage of 850-700mb moisture flux and
sufficiently cold temps >3,000ft will result in heavy snow in the
Olympics and Cascades this morning, followed up by an even more
potent frontal system that arrives late Friday night. The steady
onshore flow will maximize upslope flow into the Olympics and
Cascades, allowing for excessive snowfall rates of 2-3"/hr in some
cases. Meanwhile, as moisture over the Pacific Northwest advances
farther inland, heavy mountain snow will also be common in ranges
such as the Blue Mountains of eastern OR, the Bitterroots and
Boise of ID, the Tetons of western WY, the Independence and Ruby
Mountains of northern NV, and the northern Wasatch of UT. By
Saturday night, the next Pacific storm system is set to track
farther north towards British Columbia. The storm system's warm
front will lift through the Pacific Northwest Saturday night
causing snow levels to rise through Sunday morning. The surge of
moisture will still result in additional heavy snowfall in the
Northern Rockies, but the primary precipitation type in western
WA/OR will quickly become rain, which is likely to cause rapid
snow melt and increase the potential for minor to moderate river
flooding to close out the weekend.

WPC probabilities continue to depict high chances (>80%) for
snowfall totals >24" in the Cascades, as well as the Blue
Mountains of eastern OR, the Boise Mountains, and as far inland as
the Tetons and northern Wasatch. There is also a high chance for
snowfall totals >12" in the Olympics, Blue, Boise, and Bitterroots
ranges. Latest WSSI shows Major impacts as a result of this
multi-day winter storm are anticipated in the Olympics, Cascades,
Blue, Boise, Bitterroots, Tetons, and Wasatch. This also includes
the Ruby and Independence Mountains of northern NV. The WSSI's
Major Impacts are driven primarily by Snow Amount, but there is
also some Moderate Impacts being depicted in these mountain ranges
due to Blowing Snow. These impacts include the major passes of WA
and OR, likely leading to treacherous or even impossible travel at
times.

...Central & Southern Rockies...
Days 1-3...

Broad upper troughing over the Four Corners region containing a
series of embedded shortwave troughs will keep snow in the
forecast over the Central and Southern Rockies today. As the first
round of upper level disturbances track east into the Great Plains
on Saturday, the next upper level trough will be hot in its heels,
tracking into the central Rockies Saturday night. This particular
disturbance will deliver a higher concentration of 700mb moisture
flux to the CO Rockies and the southern WY ranges (Sierra Madre
and Medicine Bow specifically). WPC probabilities feature
moderate-high chances (50-70%) for >6" of snow in the San Juans of
NM today. Meanwhile, the CO Rockies will feature the longest
duration of heavy snowfall that lasts through the weekend. WPC
probabilities show a high chance (>80%) for >12" of snowfall at
elevations >10,000ft. WSSI shows Minor impacts are the most common
impact, especially at elevations >7,000ft. Some Moderate impacts
are anticipated in the tallest peaks of central CO, northern NM,
and southern WY.

...Northern New England...
Day 3...

A complex setup involving a several upper level disturbances
tracking through the Nation's Heartland and a dome of high
pressure to the north will lay the ground work for the next winter
storm in northern New England Sunday and into early Monday. The
ECMWF shows a more southerly storm track with a quicker reforming
coastal low. This would suppress any protruding warm nose of >0C
air at low levels and negate a dry slot intrusion aloft.
Meanwhile, the GFS/CMC camp is slower on the transfer to the
coastal low forming and the initial low over th Great Lakes tracks
farther north. While the ECMWF solution cannot be dismissed with
over 72 hours before the start of the event, members of the ECMWF
EPS also show quite a spread in snowfall across northern New
England with many members in the less snowy GEFS/GEPS camp. In
addition, while there is a dome of Canadian high pressure anchored
to the north, the air-mass itself is rather marginal. This
suggests potential heavy snowfall will be more confined to the
higher elevations for now, particularly above 1,500ft. The latest
WPC probabilities for Sunday show low-moderate chances (30-50%)
for snowfall >4" in the Adirondacks and White Mountains.


***Key Messages for Pacific Northwest Winter Storm***
--Prolonged Winter Storm in the Northwest
A long-duration winter storm along and west of the Cascades
continues through this weekend. Periodic heavy precipitation with
rain for the valleys and snow at many mountain passes can be
expected..

--Heaviest Snowfall in the Cascades
Through Sunday, there is a high chance (>90%) of at least 24" of
snow for Cascade passes, including I-90. Snow levels remain below
4000 feet through Saturday before rising above pass level Saturday
night. Travel will be difficult and hazardous due to both heavy
and blowing snow.

--Mountain Snow through the Northern Rockies
Periods of heavy snow are will produce as much as 1-3 feet to
higher terrain in the northern Intermountain West and Northern
Rockies through Sunday.

--Potential for River Flooding this Weekend
Several inches of rain in valleys and the rising snow levels
Saturday night into Sunday will likely produce minor to moderate
river flooding late this weekend into next week.

Mullinax