521
FXUS65 KMSO 062006
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
206 PM MDT Sun Sep 6 2026

.DISCUSSION...

KEY MESSAGES:

-  A few strong thunderstorms are possible this afternoon and
   evening. Main threats are intense rainfall rates and hail.

-  Significant rain event for much of the Northern Rockies tonight
   through Tuesday morning.

-  Gradual drying trend develops mid-week alongside morning
   valley fog.

-  70 percent of ensemble solutions depict another weather-maker
   to impact the Northern Rockies next weekend.

A closed low pressure system is currently visible on satellite
imagery along the Washington/ Oregon border. This features will
shift eastwards into western Montana by early tomorrow morning and
into north central Montana by tomorrow evening.

Ahead of the low this afternoon, waves of energy and moisture
continue to be pumped into western Montana and north central
Idaho, producing areas of showers and embedded thunderstorms.
Dynamic upper level forcing ahead of the low will continue to
increase through the late afternoon and evening, allowing for
increasing chances of thunderstorms, most likely between 3pm and
9pm this evening. Upper level dynamics and wind shear will create
a favorable environment for a few strong to possibly severe
thunderstorms, primarily across southwest Montana where less cloud
cover has allowed for greater surface heating. The main threats
with storms will be intense rainfall rates of 0.75" or more per
hour, hail up to 1 inch in diameter and wind gusts up to 50 mph.
Storm coverage may become more widespread through the evening as
the upper level low nudges eastwards. While flash flooding is a
low probability, flood prone areas and burn scars will be
vulnerable if they take a direct hit from a stronger storm.

Thunderstorm coverage may lull briefly late this evening, but high
resolution forecast models show another round developing around
midnight tonight in southern Idaho county, shifting into southwest
and west central Montana early Monday morning. This next wave
will be enhanced by an incoming jet stream aloft and a passing
mid- level cold front. Storms will again be capable of brief heavy
rainfall, abundant lightning and even small hail.

The bulk of heavy precipitation with the low pressure system will
come Monday into early Tuesday morning. Widespread precipitation
will develop tomorrow morning in north central Idaho, shifting
into northwest Montana by Monday afternoon. The majority of the
region (other than far southwest Montana) have high chances (80 to
100 percent) of seeing at least half an inch of rainfall. The one
exception will be Lemhi County and far southwest Montana where
lighter amounts are anticipated. Northwest Montana, especially
near the Continental Divide is still anticipated to received the
heaviest amounts, with 1 to 3 inches of additional rainfall
anticipated in the 24 hrs ending 6am Tuesday. Localized areas of 3
to 4 inches are not out of the question. A flood watch has been
issued to account for the potential of lowland flooding in poor
drainage areas, rapid rises on streams and creeks, possibly
reaching bankfull, and debris fall on roadways.

Showers will persist on Tuesday mainly across northwest Montana
while other areas will start to moderate. Lingering boundary-layer
moisture will support overnight and early morning fog and low
stratus clouds on Wednesday and possibly Thursday.

Temperatures will rebound Wednesday through Friday, getting back
to near seasonal levels for this time of year...70s and 80s for
the lower elevations of Idaho.

For the longer range, including next weekend, a lot hinges on what
evolves well upstream of us! One of the deepest troughs so far
this season across eastern Russia will support a very amplified
ridge over Alaska later this week. This will in turn support a
deepening trough over western Canada. This kind of setup can lead
to another trough digging over the Pacific Northwest which could
bring unsettled and cooler weather for us. Some of the guidance is
depicting a colder trough scenario which could bring lower snow
levels. In fact, the NBM is starting to show 20 to 30% chance for
snow above 7,000 feet along the Divide next Saturday!


&&

.AVIATION...Cloud cover and periods of thunderstorms will dominate
Sunday. Showers and thunderstorms will affect KSMN, KHRF, KMSO,
and KGPI through 07/0200z. These storms will be capable of heavy
downpours rapidly reducing visibility, erratic outflow winds up to
40kts, stronger winds south and east of KMSO, and hail up to 1
inch in diameter. Around 07/0600z through 07/1200z, another round
of thunderstorms is possible at all terminals. These storms will
be capable of greatly reduced ceilings and visibility, prolific
lightning, and small hail. Monday expect widespread IFR conditions
due to moderate to heavy rain reducing both visibility and
ceilings. KBTM may remain in showers as opposed to stratiform rain
and not experience prolonged periods of IFR.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Flood Watch through Tuesday morning for Flathead/Mission
     Valleys...Potomac/Seeley Lake Region...West Glacier Region.

     Red Flag Warning until 9 PM MDT this evening for East Beaverhead.

ID...None.
&&

$$