Debris Flow in Montana, August 16, 2025

Area Forecast Discussion
National Weather Service Missoula MT
1232 PM MDT Sat Aug 16 2025

.DISCUSSION...

KEY MESSAGES:

-  Thunderstorms capable of gusty outflow winds and brief heavy
   rain this weekend.

-  Above normal temperatures, with typical hot, dry, summer
   pattern by mid next week.

GOES Water Vapor imagery early this afternoon shows moisture
streaming from the Pacific ocean off the northern California coast
through the ID/MT border. Widespread cloud cover and shower
activity has been observed within this moisture fetch, where
precipitable water values are near 200% of normal. This area of
shower activity will shift eastward into central Idaho and
northwest Montana through this evening; however, widespread
wetting rains are not expected as moisture is currently focused in
the mid-upper levels of the atmosphere. Further east, mostly
sunny skies will allow for the atmosphere to destabilize, with
high resolution models developing convection between 200-400PM
across Lemhi County, ID and southwest Montana (generally south and
east of Missoula). Outflow winds are the main concern, with
stronger cells capable of producing gusts near 50 mph.
Thunderstorms will also produce brief heavy rain under the
stronger cores, with a 10% chance for excessive rainfall leading
to localized flooding to burns scars and flood prone areas across
Ravalli, Granite, and Powell Counties.

Models have been consistent with highlighting nocturnal showers
and isolated thunderstorms tonight. Activity will develop late
this evening across northeast OR as an upper level wave moves into
the region within southwest flow. This overnight development will
generally focus across north-central Idaho into west-central
Montana.

By Sunday, showers and thunderstorms will focus across southwest
Montana, generally along and south of I-90. Forecast soundings
suggest deeper instability will be present, with CAPE climbing
near 750-1000 J/kg by the afternoon. Forecast models suggest
higher probabilities for thunderstorms, with 4-hr chances for
lightning peaking at 70-80% along the divide from 300-600 PM MDT.
Storms will once again be capable of outflow winds and localized
heavy rainfall. Flood prone areas will be monitored for an outside
chance for flash flooding.

Looking ahead to early next week, the region will remain under
the influence of southwesterly flow, as high pressure strengthens
over the Four Corners region. Pacific moisture will fuel high-
based convection Monday into Tuesday, with lesser coverage
compared to this weekend. By Wednesday into Friday, a familiar
mid-August pattern will be in place, with hot and dry conditions.

&&

.AVIATION...Regional radar imagery shows a line of showers moving
through eastern Washington and the Idaho Panhandle early this
afternoon. Showers will continue eastward into northwest Montana
and Idaho/Clearwater Counties through this evening, with limited
visibility impacts given the elevated nature of activity. Further
east, thunderstorms will develop between 16/2000-2200Z south and
east of the I-90 corridor. Thunderstorms will develop in the
vicinity of KSMN, KHRF, and KBTM through this evening, with
outflow winds and brief heavy rain being the primary threats. The
highest probabilities for outflow winds greater than 35 mph
will focus along the divide, near KBTM, reaching 70% from
300-600PM.

&&

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