539
FXUS65 KMSO 190740
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
140 AM MDT Sat Sep 19 2026

.DISCUSSION...

KEY MESSAGES:

- Steady precipitation today south of about I-90.

- Warming and drying trend Sunday into the first part of next
  week.


Satellite imagery early this morning indicates a cutoff upper
level low pressure circulation moving across southern Idaho. A
deformation zone north and east of the low pressure center is
bringing rain to north-central Idaho, along with west-central and
southwest Montana. Rainfall is heavy at times, with rates of up to
about 0.30 inches per hour in the Clearwater and Bitterroot
ranges.

The upper low will slide across southern Idaho throughout the day
today, spreading a band of wrap-around stratiform rain with
embedded convection from I-90 south into Lemhi County.
Precipitation will taper off late this afternoon and evening.
Storm total rainfall will range from 0.50-1.50 inches from
northern Lemhi County east to Butte, with isolated totals
exceeding 2.0 inches.

Clearing skies tonight will favor valley fog and low stratus,
particularly in areas that received substantial rainfall. Upper-
level ridging builds across the region Sunday and Monday,
establishing a warming and drying trend.

While deterministic models diverge on the details mid to late
next week, ensemble clusters favor southwesterly flow, mild
temperatures, increased afternoon breezes, and a gradual return of
isolated afternoon thunderstorm chances starting as early as
Tuesday. By Sunday (Sep 27), ensemble guidance (60-70% confidence)
points toward a deeper Pacific Northwest trough pushing inland,
bringing cooler temperatures and higher shower chances.


&&

.AVIATION...Mostly stratiform rain will continue this morning for
terminals along and south of about Interstate 90. Expect decreases
in visibility and ceilings down to IFR, especially during times
of heavier rainfall. Winds at KMSO will become easterly to
southeasterly with the wraparound nature of the precipitation
band. Rain and low stratus will obscure terrain across the
Clearwater, Bitterroot, and mountain ranges of southwest Montana.
Low-level moisture will be conducive to fog formation and low-
level stratus this morning and again on Sunday morning, especially
in valleys that receive ample rainfall early this morning and
today.



&&

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

$$