Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 PM EST Sat Nov 16 2024

Valid 00Z Sun Nov 17 2024 - 00Z Wed Nov 20 2024

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

A sharp trough sweeping southeast from the Gulf of Alaska will
reach the PacNW coast Sunday morning before crossing the northern
Rockies through Monday. Deep low pressure develops over the Gulf of
Alaska Sunday night in the wake of the trough and a reinforcing
trough around this new low looks to rapidly develop Tuesday west of
Vancouver island and will go a long ways to maintaining a rather
active pattern over the Northwest through the next week.

A rich plume of moisture streaming ahead of the approaching trough
will quickly raise snow levels tonight above 5,000ft in the north
WA Cascades and over 8,000ft in the OR Cascades. This robust
moisture reaches the northern Rockies late tonight with snow levels
around 5,000ft there through Sunday. Meanwhile, the sharp cold
front will cross WA/OR through Sunday and cause snow levels to
plummet to 2,000ft in the WA Cascades and 2,500ft in the Oregon
Cascades by Sunday evening. Snow levels fall to around 2,000ft in
the northern Rockies Sunday night. Continued onshore flow under
this trough and ahead of the next developing system will maintain
moderate snow rates over this terrain through Monday. Day 1 PWPF
for >6" snow are categorical for all the WA and higher OR Cascades,
Blue Mtns of OR and Salmon River Mtns of ID north through the
Bitterroots. The lower snow levels and more expansive moisture
brings categorical Day 2 PWPF for >8" along the WA/OR Cascades and
Northern Rockies down to the Tetons.

One note about the onshore flow tonight is the moisture overriding
cold surface conditions over the Columbia Basin tonight into Sunday
to cause pockets of freezing rain. Day 1 PWPF for >0.1" ice are up
to 20% in the Columbia Basin.

A lull from brief ridging Monday night quickly gives way to the
next surge of Pacific moisture later Tuesday into the PacNW. Snow
levels will rise on the initial plume of SWly flow and from there
is quite a bit of uncertainty from a wintry weather perspective
with the 12Z suite featuring a stalled low just offshore through
the rest of next week. An active pattern to say the least.


...Northern Plains...
Day 3...

Low pressure closes off tonight/Sunday south of AZ which tracks
over west TX into the central High Plains Monday. This portion is
rather warm and should produce rain where last week had the
historic heavy snow. This low rapidly shifts up the Plains Monday
night as a potent low with comma head/snow banding on the NW side
not developing until over North Dakota later Tuesday. Day 3 PWPF
for >4" are 10-40% over north-central WY terrain, the Black Hills,
and much of western/northern ND. A trend to a wetter comma head was
made today in the forecast, so banding similar to what occurred
this morning over eastern MT may be at stake a bit farther east on
Tuesday.


Jackson