Butte-Silver Bow, Montana 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

457
FXUS65 KMSO 282005
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
205 PM MDT Sun Jun 28 2026


KEY MESSAGES:

-  Flash Flood Watch: Localized heavy rain is expected to develop
   later this afternoon and evening from the Glacier Region to
   the Mission Mountains. Swollen streams, isolated mudslides, and
   increased runoff is possible.

-  Hypothermia risk with falling snow above 6500 feet and cold
   backcountry temperatures continues from the southern
   Bitterroot Mountains east towards the Divide through Monday.

-  Strong south to west winds between 20 to 30 MPH are expected
   through tonight at Flathead Lake. These winds and choppy
   conditions are dangerous for small-craft.

-  Scattered heavy showers or thunderstorms are possible across
   northwest Montana, generally north of Interstate 90 Monday
   afternoon and evening. These could cause localized stream,
   rockfall, and/or debris flow issues.

-  Near seasonable temperatures along with daily thunderstorm
   chances are possible Tuesday through Friday of next week.

.DISCUSSION...

A very unusual synoptic pattern to discuss this afternoon. The
MRMS radar loop shows a massive area of precipitation wrapped
around a low in southern Alberta, Canada. This area of heavier
rainfall, possibly a "seclusion", is making its way south and
west towards northwest Montana. This system will bring in 200% of
normal of atmospheric moisture and set the stage for heavy
mountain rainfall in the Glacier Region south and west to the
Mission Mountains.

Due to the strong pressure/wind gradient of 18 millibars between
the 987 millibar surface low pressure in southeast Alberta and
1005 millibars at Glacier Park International, the winds have been
sustained over 20 mph at Flathead Lake. Because of this, there is
a Flathead Lake Wind advisory in effect through late tonight.

Winter storm warnings continue to be in effect for the Bitterroot
Mountains and southwest Montana locations through Monday morning.
Recent model trends continue to show snowfall in these area. The
intensities may increase once the slug of moisture from Canada
arrives late tonight.

By Tuesday, winds will relax and we will transition into a weak
ridge of high pressure with slightly unstable air. Given the
widespread precipitation, fog and low clouds could cause travel
issues across the region Tuesday morning. The lingering moisture
will also support scattered showers later in the day.

A moderating trend is expected later this week with possible
thunderstorms each day. With the added boundary layer moisture
from all the rain, this could enhanced the instability each day.


&&

.AVIATION...A large upper low over Montana will bring in heavier
rainfall from northwest Montana by this evening 29/0000Z/6PM MDT
and spread southwards with time. There will be plenty of low
clouds around, potentially IFR (Instrument Flight Rules) at times
throughout the night tonight into Monday. Moderate to heavy rain
intensities will be possible and with the added moisture, reduced
visibility from rain and/or fog could be an issue. GPI has the
greatest chance for heavy rain rates this evening. MSO could
experience occasional heavy rain rates later tonight. Other
terminals will experience steady rain at times into Monday. Gusty
south to west winds to 25 knots are possible across western
Montana this afternoon and evening due to the tight 10+ millibar
gradient. The elevated moisture levels continue on Monday there
could be moderate showers and a few thunderstorms capable of very
heavy rainfall across the region. The higher probability for any
thunderstorms will be north of I-90 to northwest Montana.

&&

.HYDROLOGY...Anomalously high moisture content (200% of normal)
will fuel heavy precipitation through Monday across western
Montana, especially northwest Montana tonight through Monday.
Snow levels rising toward 8000 feet will support efficient, heavy
rainfall. When combined with south- to-westerly winds picking up
extra moisture off Flathead Lake, rain intensities will increase.
High-resolution weather models show possible rain rates over 0.30
inches per hour in the Lewis, Northern Swan, and Mission Ranges
tonight into Monday morning. Fortunately, existing snowpack is
minimal, so this will translate purely to increased stream runoff.
Areas prone to rockfalls or mudslidesincluding upper elevations
of the Going-to-the-Sun Roadcould be vulnerable depending on
exactly where the heaviest rain sets up. Campers and RVs parked
close to streams or the Hungry Horse Reservoir should consider
repositioning to higher ground today. Flash flood watches were
hoisted for both Flathead and Lake Counties to address the heavy
rain potential.

By Monday afternoon, more moderate to heavy showers could impact
the Lincoln, Sanders, and Lake County areas. High resolution
models depict possible 0.50 to 1.00 per hour rain-rates primarily
in the foothills and mountains in these areas. This has been a
recent change and we are contemplating issuing further watches if
run-to-run consistency continues.


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning until 9 AM MDT Monday for
     Bitterroot/Sapphire Mountains...Butte/Blackfoot Region.

     Flash Flood Watch through late tonight from the Glacier Park
     Region to the foothills and Mission Mountains.

     Lake Wind Advisory until 3 AM MDT Monday for Flathead/Mission
     Valleys.

ID...Winter Storm Warning until 9 AM MDT Monday for Eastern Lemhi
     County...Western Lemhi County.

&&

$$