textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- One more day of above-normal temperatures on Monday.

- Thunderstorms return to the Northern Rockies Monday and Tuesday.

- A cool and wet pattern arrives Tuesday, lowering temperatures and bringing widespread rain through Friday.

A brief period of warm and dry weather driven by a high-pressure system overhead will last through Monday afternoon. Afternoon temperatures will rise to around 15 degrees above the seasonal average. However, the latest forecast models show thunderstorms developing earlier in the day ahead of an approaching cold front. As a result, multiple locations across western Montana and north- central Idaho will see thunderstorms develop as early as 2:00 PM.

These showers and thunderstorms will expand into northwest Montana by the evening hours. Current high-resolution models indicate thunderstorms will begin in Lemhi County and the southern Bitterroot Valley between 1:00 PM and 2:00 PM, and track from south to north into west-central Montana between 4:00 PM and 6:00 PM. This storm track historically brings strong, gusty winds to areas in the Bitterroot and Missoula Valleys. There is a 30 percent chance that gusts will exceed 40 mph, and a 10 to 15 percent chance they will reach 50 mph with these thunderstorms. Keep a plan in mind to seek shelter if you have outdoor recreation plans Monday afternoon between 1:00 PM and 5:00 PM for locations south of Interstate 90, and from 5:00 PM to sunset for locations north of Interstate 90 in western Montana. These thunderstorms are expected to reach the Flathead Lake area between 5:00 PM and 6:00 PM. In addition to sudden, strong wind gusts, frequent lightning and small hail are expected.

The low-pressure storm system responsible for the moist southwest winds triggering Monday's thunderstorms will move further inland, tracking over eastern Oregon and southern Idaho on Tuesday. This will bring a sharp drop in temperatures on Tuesday, dropping daytime highs to around 5 degrees below seasonal averages.

Uncertainty remains regarding exactly how much rain will fall. Total amounts will depend heavily on the exact speed and path of the low-pressure storm system. Current guidance favors widespread rainfall for western Montana and north-central Idaho, with the highest totals likely focused south of Interstate 90, where 0.25 to 0.75 inches of rain could fall by the end of the day Wednesday. The Hells Canyon area could see up to 1 inch of rain.

Rain chances remain elevated through Friday, with daily showers and thunderstorms developing across the Northern Rockies. Long- range forecast models agree that this slow-moving low-pressure system will drift northeast into eastern Idaho and southwest Montana by Friday. This would bring the threat of heavy rainfall to western Montana, which could create a localized flooding threat for isolated areas. Confidence is currently low on exactly where the heaviest rain will develop, but the chance for at least 0.50 inches of rain is highest (50 to 60 percent) south of Interstate 90 in western Montana and along Highway 12 in north-central Idaho. Stay tuned for further updates.

AVIATION

A high-pressure system will maintain mostly clear skies and favorable flying conditions across the Northern Rockies through Monday morning. An active weather pattern returns to the area Monday afternoon, with scattered thunderstorms likely to impact all regional terminals. Thunderstorms will develop over Lemhi County and KSMN around 1:00 PM MDT Monday (25/1900Z) and will move north through the afternoon. The storm system is expected near the KHRF and KBTM around 3:00 PM MDT Monday (25/2100Z). Thunderstorms will reach KMSO by 5:00 PM MDT Monday (25/2300Z) and KGPI by 8:00 PM MDT Monday (26/0200Z). The primary hazards from these storms will be strong, erratic wind gusts, frequent lightning, and small hail.

HYDROLOGY

Several days of precipitation along with mountain snow melt this week, will increase river levels throughout western Montana and north-central Idaho. Current forecasts have the Flathead River at Columbia Falls and the Swan River at Big Fork nearing action stage by the end of the week. The Clark Fork River Above Missoula may near the 6.0 foot mark by this coming weekend.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.