806
FXUS65 KMSO 232132
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
232 PM MST Sat Nov 23 2024

.DISCUSSION...

KEY MESSAGES:

-  The last push of the west coast Atmospheric River will spread
   anomalously high moisture across the Northern Rockies today.

-  Arctic air largely locked east of the Continental Divide will
   slowly seep over the mountains of Glacier NP and into the
   Flathead Valley this evening.

There are several forecast challenges expected over the next 18
hours with the most significant one affecting a large swath of
Northwest Montana. While the vast majority of arctic air remains
locked east of the Continental Divide, a steady Canadian seep of
the modified air has remained in place across the Yaak eastward
into the Tobacco Root Valley, and then also within the North Fork
of the Flathead River Valley. This has prevented snow from
switching to rain at any point overnight and resulted in very
impressive snowfall accumulations (exceeding a foot in some
places). But wait, that`s not all! Even more snow is expected to
impact these same areas tonight.

Forecast models show a secondary resurgence of cold air seeping
across the mountains following the passage of today`s cold front.
This surge of arctic air will combine with the persistent westerly
moisture fetch while being further enhanced by both a weak
Pacific shortwave moving into Central Idaho/SW Montana and the
exiting upper level trough pushing northeast into Canada. All
these factors when combined will produce an area of intense
convergence that models almost unanimously agree will produce a
period of heavy focused snowfall this evening and overnight. High
resolution models are in strikingly good agreement showing a
swath of very heavy snow developing from Marias Pass west towards
the northern Flathead Valley including Kalispell, West Glacier,
Hungry Horse and Essex. These areas should pick up another 2 to 5
inches of snow this evening before midnight. Other areas across NW
Montana outside this area will pickup additional snowfall
tonight, but only on the order of a few more inches. The
previously issued Winter Weather Advisories have subsequently been
extended through Sunday morning to account for tonight`s
potential impacts.

Meanwhile, the aforementioned shortwave progressing across Central
Idaho and Southwest Montana later this evening will bring a return
of snow to southern Idaho County, Lemhi County, and Southwest
Montana, including the city of Butte. Winter Weather Advisories
have been issued for the Bitterroot/Sapphire Mountains, Lemhi
County, and Butte/Blackfoot region to account of the potential
impacts associated with overnight snow. And while the Bitterroot
and Missoula Valleys may experience some snowflakes tonight, these
areas stand the lowest chance of seeing any major impacts.

Snow will gradually taper off Sunday morning as brief ridging
builds over the Northern Rockies by Monday. On Tuesday forecast
models appear to sync the upper level troughing over western
Canada with the Pacific Northwest cutoff low responsible for the
recent persistent Atmospheric River. This will result in a rather
strong upper level trough passing over Idaho and Montana. There
will be an uptick in convective snow showers, though the potential
for any accumulation of significance is low.

Thereafter, northwesterly flow appears the most reasonable
forecast as we head into the Thanksgiving Holiday Weekend. This
will likely produce cooler than normal temperatures and periodic
chances for mountain snow showers. But as of right now there are
no big weather systems expected to impact the Northern Rockies
through the Thanksgiving Holiday Weekend.

&&

.AVIATION...A cold front moving through the region is
transitioning precipitation from widespread stratiform to
scattered showers. One area where consistent snowfall is projected
remains in far northwest Montana, including the Flathead Valley,
extending north to the Canadian border and eastward into the
Glacier Region. These locations are expected to see continued
snow accumulations through tonight. Meanwhile, showers are
increasing in coverage across eastern Oregon, driven by a
shortwave trough. This system is forecast to reach north-central
Idaho this evening and move through Lemhi County and southwest
Montana overnight. Snowfall associated with the trough is likely
to result in reduced ceilings and visibility, particularly at KSMN
and KBTM, through Sunday morning. Additionally, model cross-
sections indicate increasing moisture in west-central Montana,
which could produce light snow along with lowered ceilings and
visibility by Sunday morning.


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory until 10 AM MST Sunday for
     Flathead/Mission Valleys...Kootenai/Cabinet Region...West
     Glacier Region.

     Winter Weather Advisory until noon MST Sunday for
     Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from midnight tonight to noon MST Sunday
     for Butte/Blackfoot Region.

ID...Winter Weather Advisory until 11 AM PST Sunday for Southern
     Clearwater Mountains.

     Winter Weather Advisory from 6 PM this evening to noon MST
     Sunday for Eastern Lemhi County...Western Lemhi County.

&&

$$