Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
438 PM EDT Sun Oct 22 2023

Valid 00Z Mon Oct 23 2023 - 00Z Thu Oct 26 2023

...Pacific Northwest through the Northern High Plains...
Days 2-3...

Confidence continues to increase for early season heavy snow both
in the Cascades as well as the eastern slopes of the Rockies in
Montana, spreading east over the Plains in MT starting Tuesday.

A shortwave trough currently over the North Slope of Alaska will
amplify/sharpen as it dives south from the Gulf of Alaska Monday
afternoon as ridging sets up over Alaska. This wave closes into a
low over Vancouver Island Tuesday as slows its progression,
reaching the WA/OR border by 00Z Thursday. This evolution is now
in excellent agreement among 12Z guidance with the CMC/UKMET a
little farther east by late Wednesday, but overall the trend
toward the GFS solution of the past several days is nearly
complete.

An initial wave pushes along the southern BC border Monday which
sends a strong cold front south in its wake east of the Rockies of
MT Monday night. This baroclinic zone then sets up over
central/southern MT Tuesday with height falls under the low over
the Pacific Northwest. Here, ascent occurs with enhanced 850-600mb
fgen beneath the strong synoptic lift downstream of the developing
low, which is additionally aided by post-frontal easterly
component/upslope flow - making for a corridor of moderate to
locally heavy snow to develop later Tuesday through Wednesday (and
lingering longer farther south and east).

Snow levels drop under the upper low over the Pacific Northwest -
from around 3000ft over the northern Cascades early Tuesday to
around 2000ft Tuesday night. The strong cold front over MT quickly
allows for snow to ground level with the thermal gradient over
southern MT being a dividing line between snow levels below 1000ft
to the north and 6000ft to the south. This cold air/low snow level
eventually wins/shifts south into WY on Wednesday.

Easterly component upslope flow brings snow to just the eastern
slopes and crest of the northwestern MT Rockies/Glacier NP with
Day 2 (00Z Tue-00Z Wed) snow probs for 6"+ moderately high in the
Lewis Range. Also on Day 2, lowering snow levels with onshore flow
brings similar moderate probs for 6"+ to the WA Cascades and
Olympics. 

Snow probs for 6"+ expand greatly for Day 3 (00Z Wed-00Z Thur)
with moderately high values over central MT over the
Bitterroots/Clearwater Ranges of MT/ID, with highest probabilities
over the ranges of central/southern MT including the Big and
Little Belts and Red Lodge Absarokas where there are moderate
probs for 12"+.

Due to this being an early season snow, extra caution is warranted
around and above the snow level for these areas of terrain and
High Plains.


Jackson