789
FXUS65 KMSO 152054
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
254 PM MDT Tue Sep 15 2026

.DISCUSSION...

KEY MESSAGES:

-  Cool and showery conditions are expected from Yaak to Glacier
   National Park through this evening.

-  Warming and drying trend expected through Wednesday.

-  Shower and Thunderstorm pattern develops Thursday and continues
   into Saturday.

-  Another warming and drying trend Sunday through early next
   week.

Satellite imagery this afternoon shows a large upper level low
digging southwards from Vancouver Island towards northern
California. To the east of this feature, a steady stream of clouds
can be seen tracking over northern Idaho into northwest Montana. A
shortwave trough will slowly track eastwards along the Canadian
Border and continue to produce showers through this evening. So
if you`re planning on being outside from Yaak to Eureka to Glacier,
you may want to bring a jacket. Despite the cloud cover,
temperatures are generally 5 to 10 degrees warmer than they were
at this time yesterday. Highs will warm into the low 60s across
northwest Montana. Further south the clouds are thinner or
nonexistent, so expect warmer temperatures rising into the 60s
and 70s, especially over central Idaho and Lemhi County.

On Wednesday, the trough moving eastwards towards central Montana
and induce a 1026 millibar high near Cut Bank. This feature
combined with lower pressure over southern Idaho will produce a
northeast wind gradient across the region. Generally 10 to 20 mph
are expected during the day. By the evening time, the gradient
will tighten a bit and easterly gusts exceeding 25 mph could be
found (70-80% chance) in the channeled areas along and west of the
Continental Divide. There is a 82% chance that winds could exceed
30 mph between Salmon and Tendoy along Highway 28 during the
evening hours. Other than the winds, highs will be near seasonable
levels...70s across western Montana, and 80s in Idaho.

On Thursday the upper low that we mentioned in the beginning will
start to wobble northeastwards towards southwest southern Idaho.
It will draw up over 200 percent of normal, monsoon moisture from
the Four Corners region. Wind shear has trended to be stronger as
this system could bring a 45 knot mid-level jet streak by Thursday
night. This means that not only will any thunderstorms produce a
lot of rain, they could contain hail. Initially guidance is
showing less than 300 J/KG CAPE/instability in the hail growth
zone on Thursday which could support small hail. On Friday,
depending on how much heating we`re able to get, the hail
potential seems to be greater with the GFS showing much higher
instability values that could support hail to one inch or greater.
Lightning, gusty winds will be other threats with the storms. The
NBM, ensemble and AI guidance are depicting the area from central
Idaho east and north including west-central and southwest Montana
as being the areas with the highest probability for thunderstorms.

As the low pulls away from the region on Saturday, the focus for
showers and thunderstorms shifts eastwards from US-93 corridor
eastwards into central Montana. We could have lingering clouds
over central Idaho but the lower valleys should remain mostly dry
with moderating temperatures.

Model guidance continue to show large upper ridging developing
Sunday into early next week. We could have patchy fog developing
along river bottoms each morning. If you take the full potential,
highs could crack the the 80 degree mark in Missoula, which would
be around 10 degrees above average. The highest probabilities for
80s are in the lower valleys of Idaho (80-90%). This ridge would
bring us back to summer-like conditions for 2-3 days. There are
hints that there could be a frontal system that could temper
things a bit by mid- week.


&&

.AVIATION...A wave will bring a chance for showers across far
northwest Montana through daybreak Wednesday. Expect periodic
ceiling changes and mountain obscuration from Yaak to Glacier
National Park. The showers are expected to remain more over the
mountains near GPI, but couldn`t rule out occasional sprinkles.
Other locations will have less middle cloud cover. A northerly
wind gradient may help to enhance the early evening north winds at
KMSO, and KGPI may experience more variable wind rather than the
typical southerly lake breeze. A stronger northeast wind gradient
will develop on Wednesday, generally ranging between 10 to 20
knots during the day. Channeled terrain along and west of the
Divide could have higher amounts. Other than the wind, VFR
conditions are expected.


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&

$$