Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
316 PM EST Fri Nov 15 2024

Valid 00Z Sat Nov 16 2024 - 00Z Tue Nov 19 2024

...Western U.S....
Days 1-3...

A large longwave trough over the Western U.S. with very energetic
jet streaks on both sides of it remains the primary driver of the
active winter weather across the Pacific Northwest this afternoon.

A weak low that will be primarily supported by the left exit region
of the impressive southwesterly jet will cause areas of snow to
form from the Sierra Nevada and northern Great Basin northeast
through Yellowstone and into northeastern Montana and northwestern
North Dakota through Saturday afternoon. The heaviest totals from
this low will generally be across southwestern Montana where the
low tracking across eastern Wyoming will be slower moving and
strengthening. Thus, southwestern Montana will be in the favored
comma-head region of the low for the longest period of time through
tonight. Lesser amounts of snow are expected northeast of there
into northeast Montana and northwestern North Dakota due to the low
moving faster and having fully matured, however localized banding
could form within the broader precipitation shield, which could
result in heavier amounts in the valleys where any bands are the
most persistent. Meanwhile heavier snow further south and west
tonight from the Sierras through southeast Idaho will not be quite
as favored for as long, so most snow totals above 6 inches will be
relegated to the highest peaks in those regions. Once again,
localized banding here as well could cause higher snow amounts in
the valleys.

The arrival of a second much stronger longwave trough into the
Pacific Northwest late Saturday night through Monday will be
characterized by the arrival of much colder air and a full mid-
latitude Pacific fetch of moisture. As usual with this prolonged
period of maritime polar air, the Washington Cascades will receive
by far the most snowfall through Monday, with multi-day totals in
the northern Washington Cascades potentially reaching into the 4 to
6 feet range. As multiple energetic shortwaves also move ashore,
heavy snow will spread south down the Oregon Cascades as well as
well inland across much of the mountains of Idaho, western Montana,
and Wyoming. Expect 2 to 3 feet of accumulation into the Oregon
Cascades and 1 to 2 feet for the Idaho, Montana, and Wyoming
mountains. The arrival of the renewed round of cold air will also
send snow levels crashing right to the valley floors inland, while
west of the Cascades they'll get to 1,000-2,000 ft by Monday.

Expect Major impacts in the WSSI fields for the Washington Cascades
and far northern Idaho primarily due to snow amount. Minor impacts
are expected through Monday for most of Idaho, Montana, and
Wyoming. Up to a 50% chance of extreme impacts are expected for
the Washington Cascades from Snoqualmie Pass north starting Sunday
night. Meanwhile chances for Major impacts have increased above
90% for that same time period. Over 80% probabilities of Major
impacts are expected for the Oregon Cascades starting Monday
morning.

Wegman