Ferdinand, Idaho Weather Forecast Discussion

566
FXUS65 KMSO 281920
AFDMSO
Area Forecast Discussion
National Weather Service Missoula MT
120 PM MDT Sun Sep 28 2025
.DISCUSSION...
KEY MESSAGES:
-  Warm and hazy today; big changes arrive starting Monday.
-  Strong, gusty outflow from showers and thunderstorms are the
   main threat on Monday.
-  Widespread rain Tuesday evolves into an unsettled and
   progressively cooler pattern for the rest of the week.
The upper-level ridge overhead will provide one more full day of
dry, unseasonably warm conditions. A stable airmass has trapped
wildfire smoke in the valleys of northwestern Montana, and this
trend of poor air quality will likely continue with only minor
improvement through midday Monday.
The pattern change begins Monday as a Pacific trough moves
onshore, advecting moisture over a dry surface airmass. This
environment is favorable for high-based convection capable of
producing strong outflow winds. Scattered showers and
thunderstorms will develop from southwest to northeast as early as
lunchtime, with peak activity between 2-9 PM MDT. Potential
threats, primarily along and south of I-90, include wind gusts up
to 45 mph, small hail, and briefly heavy rainfall.
Monday night through Tuesday morning, an upper-level shortwave and
increasing precipitable water will expand coverage of showers and
thunderstorms, marking the peak timeframe for any excessive runoff
risks. The latest 12z CAMs show the highest potential for intense
rainfall rates over southeastern Idaho County, northwestern Lemhi
County, and up into the southern Bitterroot/Sapphire region,
especially if storms train over the same area. However, the
overall concern for severity of excessive rainfall has trended
downward due to poor phasing between daytime heating, peak
moisture, and synoptic lift availability.
As the atmosphere continues to moisten on Tuesday, multiple rounds
of beneficial showers and thunderstorms will continue to increase
in coverage. Through Wednesday evening, areas along and west of
US-93 gave a >80% chance for at least 0.10" and a 50% chance for
at least 0.25" of rainfall. These odds increase in the mountains
of north-central Idaho and the Cabinet Range, but decrease back to
~50% for 0.25" near Grangeville and the Nez Perce. While the
strong to severe thunderstorm gust threat will diminish after
Monday, background southwesterly winds will ramp up, especially
in higher terrain, through Tuesday and Wednesday.
Shower chances continue into the late week as winds aloft become
more westerly. Model ensembles now strongly favor a persistently
cool and unsettled pattern for the late week and weekend, all but
entirely eliminating prior solutions of a returning ridge. The
primary uncertainty now is whether the pattern will feature
progressive shortwaves (bringing daily chances for showers and
reinforcing cool temperatures) or a deeper trough/cutoff low
(bringing more significant precipitation, progressively colder
temps, and accumulating mountain snow to ~6000 ft). Regardless,
temperatures will likely cool into the 60s mid-week and 50s by
Friday and Saturday. High elevation backcountry users should
begin to prepare for cold, wet, windy, raw, and potentially snowy
conditions for the weekend ahead.
&&
.AVIATION...VFR weather conditions prevail through Monday morning,
but smoke trapped in valleys of northwestern Montana will continue
to produce periods of lowered visibility (3-6SM) and terrain
obscurations potentially affecting areas near KGPI. Scattered
showers and thunderstorms develop near KGIC/KSMN around and after
29/1900Z tracking northeast. Primary threats will be localized
strong outflow gusts to 40 knots causing low-level wind shear.
Storms will also be capable of producing localized brief IFR
weather conditions in heavy rain or small hail cores (TSRA/TSGR).
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&
$$

NWS MSO Office Area Forecast Discussion