textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- Convective potential increases today and Wednesday with isolated to scattered thunderstorms mainly across the mountains of central Idaho and eastern Oregon.

- A strong low pressure system later this week will bring cooler and windier conditions with periods of showers and thunderstorms, especially to Oregon and west-central Idaho.

SHORT TERM /Through Thursday Night/

Issued 315 AM MDT TUE SEP 1 2026 An active and progressive pattern will unfold across southeast Oregon and southwest Idaho through the short term period as an expansive upper level low pressure system moves toward the Pacific Northwest coast. Southwesterly flow aloft has brought increasing Pacific moisture. This combined with an approaching shortwave, at 2 AM MDT near central California, should initiate mostly virga showers across southwest Oregon through the remaining morning hours. Given the very dry boundary layer, rainfall will be limited. However, gusty outflow winds are possible with the stronger showers. As the shortwave continues north-northeast today, dynamic lift combined with daytime heating will trigger convective development by this afternoon. The highest thunderstorm potential will remain focused over the higher terrain of central Idaho and the Western Magic Valley. Again, given the dry boundary layer, storms today will be capable of producing localized gusty outflow winds up to 45 mph and small hail. Valleys in southwest Idaho may end up largely dry-slotted, though mid and high level cloudiness will increase. Temperatures will peak around 5 degrees above seasonal normals, with valley highs touching the upper 80s to near 90 degrees.

A brief break is expected late Tuesday evening before the next trough ejects northeastward from the main low circulation Wednesday morning. This negatively tilted trough is stronger than today's, which should be enough for some isolated showers and thunderstorms Wednesday morning, though the lack of surface instability will be a limiting factor. Better instability should exist across the central Idaho mountains early Wednesday afternoon, which could allow for increased thunderstorm coverage capable of heavy rain and small hail. Cooler air will begin to nudge into eastern Oregon, which will limit afternoon thunderstorm potential there.

The primary change in the weather pattern arrives late Wednesday night into Thursday as a deeper upper low moves onshore into the Pacific Northwest. Dynamic forcing increases dramatically Thursday into Thursday night as deep moisture streams into the region from the south and southwest. Expect widespread precipitation coverage to build from west to east. Precipitable water values will climb into the 70th to 75th percentile range, setting the stage for heavier rain, especially across southeast Oregon and the central Idaho mountains. There remains quite a bit of uncertainty with precipitation amounts Thursday, as PWs shown in the models do not match model rainfall rates given the southwest flow pattern. Thunderstorms embedded within the broader precipitation shield Thursday afternoon could enhance local rainfall rates, but models appear to be overestimating precipitation in this fall pattern. High temperatures on Thursday will drop sharply, running 10 to 15 degrees cooler than Wednesday.

LONG TERM /Friday through Tuesday/

Issued 315 AM MDT TUE SEP 1 2026 The upper level low looks to stall or drift slowly across the Pacific Northwest through Friday before gradually weakening over the weekend.

A prolonged period of rainfall through Friday evening is possible as the main core of upper dynamics and plume of Pacific moisture cross southeast Oregon and southwest Idaho. Showers and embedded thunderstorms will remain widespread. Close to half an inch of rainfall is possible, with locally higher amounts Thursday through Friday. Persistent cloud cover and precipitation will keep Friday temperatures well below normal, with valley highs only in the upper 60s to lower 70s.

For Saturday through Tuesday, the closed low will transition into an open trough and move eastward toward the northern Rockies. While the heaviest and most widespread rain will shift east of our area by early Saturday, unsettled conditions will persist. Secondary shortwaves rotating through the trough will maintain daily chances (20 to 40 percent) for afternoon showers and isolated thunderstorms through early next week, primarily over mountainous terrain. Temperatures will begin a steady recovery over the weekend and into Tuesday, though remaining slightly below normal for early September.

AVIATION /12Z Tuesday through Wednesday/

Issued 604 AM MDT TUE SEP 1 2026 VFR. Locally reduce visibility from a wildfire located southwest of KBKE as well as another large wildfire in the Boise National Forest NE of KBOI. Isolated mainly virga showers this morning across southeast Oregon. A 40 percent for showers and thunderstorms with gusty winds that could impact KMYL, KJER and KTWF this afternoon. Surface winds: South-southeast 10-15 kt gusts up to 25 kts. Gusty outflow winds to up 35 kts.

KBOI...VFR. A 15% chance for showers and thunderstorms near the terminal between 20z/Tues and 01Z/Wed. Surface winds: Southeast 10-15 kts today. Gusts up to 30 kts near showers/storms.

BOI WATCHES/WARNINGS/ADVISORIES

ID...OR...None.