687
FXUS65 KMSO 181828
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
1228 PM MDT Fri Sep 18 2026

.DISCUSSION...

KEY MESSAGES:

- Thunderstorms this afternoon/evening with locally heavy rain,
  minor runoff concerns near burn scars and susceptible terrain.

- Steady precipitation tonight into Saturday south of I-90.

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

This afternoon through Saturday afternoon: Broad-scale lift and
partial clearing north of an upper low over southwest Idaho will
trigger rounds of showers and thunderstorms this afternoon and
evening, primarily from I-90 southward. Weak steering flow
combined with anomalous moisture (precipitable water 150-200% of
normal) will make locally heavy rainfall the primary hazard.
Typical rain rates of 0.25-0.50 in/hr are expected, with localized
cores exceeding 0.75 in/hr. This poses a localized risk for
standing water, rapid runoff, and debris flows over burn scars and
susceptible terrain.

The upper low will slide across southern Idaho tonight into
Saturday, spreading a band of wrap-around stratiform rain with
embedded convection from I-90 south into Lemhi County.
Precipitation will taper off late Saturday 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.

Sunday through next weekend: Clearing skies Saturday night 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...Multiple rounds of showers and thunderstorms will impact
regional terminals south of I-90 this afternoon and evening as
partial afternoon clearing allows instability to increase in
conjunction with broad scale lift. Primary hazards include erratic
convective outflow gusts of 20-30 kt (locally higher with a storm
or two) and brief heavy downpours driving localized MVFR
visibility reductions.

Precipitation turns stratiform tonight through Saturday leading
to intermittent MVFR/IFR ceilings/visibility and terrain
obscurations. Precipitation diminishes on Saturday afternoon with
clearing Saturday night allowing for fog and some low stratus for
valley locations.

&&

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

$$