283
FXUS65 KMSO 171921
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
121 PM MDT Thu Sep 17 2026

.DISCUSSION...

KEY MESSAGES:

-  Showers and thunderstorms this afternoon through Saturday,
   focused I-90 southward. Gusty outflow winds today, locally
   heavy rain concern on Friday.

-  Warming and drying Sunday through early next week.


This Afternoon through Saturday: An upper-level low spinning over
Northern California will draw deep moisture into the region
through this evening, driving precipitable water (PWAT) values to
150-200% of normal. Morning cloud cover has kept instability
tempered thus far but partial clearing is still expected to allow
for multiple rounds of showers and thunderstorms focused I-90
southward. Across Lemhi and southern Idaho counties, 30-40 kt of
deep- layer shear will support a few strong thunderstorms capable
of producing hail and damaging winds. Outside of these strong
cells, general storms will frequently produce 25-35 mph outflow
gusts and brief heavy rain.

Elevated instability and broad synoptic lift will sustain showers
and isolated thunderstorms Friday morning, concentrated south of
I-90. Additional convective rounds will redevelop Friday afternoon
and evening. Heavy rainfall will be the primary threat on Friday,
with localized rates exceeding 0.75 in/hr. This poses a threat
for localized runoff and debris flows over burn scars and other
vulnerable terrain. One complicating factor could be cloud cover
muting instability but broad scale lift will lead to considerable
showers and at least some thunderstorm activity.

The upper low will slide through southern Idaho Friday night into
Saturday, delivering a period of wrap-around stratiform rain
along the I-90 corridor southward to Lemhi County, before
precipitation tapers off late Saturday.

Sunday through Mid-Next Week: Ensemble guidance shows high
confidence (90-100%) in upper-level ridging building across the
region Sunday and Monday, bringing a notable warming and drying
trend. Although detail guidance diverges Tuesday through late next
week, the broad ensemble pattern favors southwesterly flow, mild
temperatures, and a gradual reintroduction of thunderstorm
chances.



&&

.AVIATION... Multiple rounds of showers and thunderstorms will impact
regional terminals south of I-90 this afternoon and evening as
partial clearing allows instability to increase. 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. Elevated instability and
widespread synoptic lift will result increasing showers and
thunderstorms beginning on Friday morning (primarily south of
I-90) ahead of peak heating, followed by renewed afternoon
convective coverage. Expect intermittent MVFR ceilings and terrain
obscurations near heavier precipitation cores.

&&

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

$$