439
FXUS65 KMSO 212027
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
227 PM MDT Mon Jul 21 2025

.DISCUSSION...

KEY MESSAGES:

-  Storms today could produce moderate to heavy showers with
   concerns for localized flooding/ debris flows along US-95 and
   US-12 in north-central ID

-  Widespread showers and thunderstorms on Tuesday, some moderate
   to heavy north of I-90, especially along the Canadian border.

Showers have begun in north-central ID and are spreading into
Western MT, just west of Missoula. As expected they are forming
along a tight and stationary boundary. As an example, Grangeville
ID has received amounts of 0.25 to 0.50" around the city while
Craigmont ID just 20 miles northwest has received 0.04" through 1
pm MDT. This boundary is expected to move eastward and be centered
southwest to northeast from Salmon ID to Butte MT tomorrow
afternoon. Locations south of I-90 will likely only receive
measurable precipitation during that boundary`s passage, keeping
storm totals highly dependent on timing and speed of passage. A
slower passage during daylight would equal higher precipitation
amounts, whereas a quicker overnight passage would result in
lighter amounts.

Northwest MT will see shower activity increase this evening and
will benefit tomorrow by proximity to a mid-level low. Expect
moderate to heavy showers at times throughout Tuesday. While
receiving a 0.25" or more is highly probable Polson and northward,
the highest amounts will depend on being under a stronger core
and orographic uplift of higher terrain. At least 1 model suggests
the highest terrain of the Purcell, Salish, and Cabinet Mountains
could receive rainfall rates of 3/4 to 1" per hour. If you are
recreating in the mountains of northwest MT Tuesday, flash
flooding and debris flows are very low probability but you should
have a plan.

Wednesday onward, there is a drying and warming trend as a ridge
builds over the central US and the Pacific trough contracts. By
Friday, almost all the models are in agreement with this scenario
and temperatures will return to just slightly warmer than normal.
We also expect southwesterly flow to develop which could increase
instability and allow a return of afternoon thunderstorms.

&&

.AVIATION...The most persistent showers are currently along and
north of a Riggins-Lolo Pass-Condon-Essex line. Obscured terrain
and brief MVFR conditions are possible in heavier showers. High-
RES models suggest isolated thunderstorms this afternoon and
evening, with locally gusty winds and brief visibility reductions,
mainly in southwest MT in the vicinity of KBTM between 22/00z and
22/03z.

On Tuesday, as the trough lifts northeast, more widespread showers
and isolated thunderstorms are expected, especially north of I-90.
MVFR ceilings and visibilities will be more common.


&&

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

$$