Divide, Montana Weather Forecast Discussion

933
FXUS65 KMSO 290801
AFDMSO
Area Forecast Discussion
National Weather Service Missoula MT
201 AM MDT Mon Sep 29 2025
.DISCUSSION...
KEY MESSAGES:
- Strong, gusty outflow from showers and thunderstorms are the
  main threat today.
- Widespread rain Tuesday evolves into an unsettled and
  progressively cooler pattern for the rest of the week.
The pattern change commences today as a Pacific trough to the
west drives southerly flow over western Montana and north central
Idaho. Increasing moisture will phase with a wave of energy to
produce showers and thunderstorms, with activity peaking between
2pm and 9pm MDT this evening. Despite increasing moisture in the
mid and upper levels of the atmosphere, relatively dry air at the
surface will create an environment favorable for high-based
thunderstorms.The primary threats, mainly along and south of I-90,
will be strong outflow gusts up to 45 mph, small hail, and brief
heavy rain.
Shower and thunderstorm coverage will expand Monday night
into Tuesday morning as an upper-level shortwave and cold front
arrive. Widespread, beneficial precipitation is expected to
continue through Tuesday before the focus for precipitation shifts
toward northwest Montana on Wednesday. Probabilities for
receiving at least 0.25" of rain through Wednesday evening are
highest across north-central Idaho (70-80%) and decrease eastward
across western Montana, from ~80% near the Idaho border to ~50%
farther east.
Looking ahead, ensemble guidance is in strong agreement
on a persistently cool and unsettled pattern for the late week
and weekend. Forecast uncertainty now centers on the trough`s
evolution- whether a series of progressive shortwaves will bring
daily shower chances, or if a deeper, cutoff low brings more
significant precipitation and accumulating snow to elevations near
6000 ft. While the progressive shortwave solution is favored,
about 20% of ensemble members suggest the higher-impact cutoff low
scenario, which bears watching. Regardless of the exact
evolution, valley temperatures will cool into the 60s by mid-week
and the 50s for the weekend. Backcountry users should prepare for
cold, wet, windy, and potentially snowy conditions this weekend.
&&
.AVIATION...A significant pattern change will commence today as a
Pacific trough drives southerly flow and increasing moisture into
western Montana and north-central Idaho. While conditions will be
quiet this morning, this instability is expected to generate
showers and thunderstorms, with peak activity anticipated between
29/2000z and 30/0300z. Relatively dry air at the surface will
create an environment favorable for high-based thunderstorms,
making strong outflow gusts up to 40 kts the primary threat,
particularly for areas along and south of I-90. In addition to the
wind, these storms may also produce small hail and brief heavy
rain. Airfields KSMN and KHRF have the highest potential for
thunderstorms. This activity marks the beginning of the shift to
a wetter pattern, as shower and thunderstorm coverage is expected
to expand and become more widespread overnight into Tuesday.
Anticipate obscurations of the higher terrain to increase due to
lowering clouds cover and rain Monday afternoon into Tuesday
morning.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&
$$

NWS MSO Office Area Forecast Discussion