textproduct: Boise

KEY MESSAGES

- Intermittent showers and embedded thunderstorms across much of southeast Oregon and west-central Idaho through Friday with significant rainfall totals possible this evening.

- Temperatures 10 to 20 degrees below normal this week.

- Cool and unsettled conditions continue into early next week.

SHORT TERM /Through Saturday Night/

Periods of rain will continue in varied coverage this afternoon through tonight, as the upper low remains positioned over the Pacific Northwest coast. A cold front approaching from west- central Oregon this afternoon will cross the forecast area overnight into Friday morning. This front will bring another wave of widespread rain and embedded thunderstorms tonight, along with breezy westerly winds. The front will push out east early Friday, clearing cloud coverage by the afternoon. The resulting surface heating will introduce weak daytime instability, triggering another round of isolated to scattered thunderstorms on Friday.

The aforementioned afternoon/evening storms on Friday will be capable of producing locally heavy downpours especially over higher terrain areas, elevating flash flood potential over recent burn scars across southeast Oregon and the central Idaho mountains, where Flash Flood Watches remain in effect (including the Wapiti burn scar). Total rainfall accumulations through Friday evening are forecast to range from 1.0 to 1.5 inches in mountain areas, with localized higher totals in core storm centers. Peak temperatures through Friday will stay 10 to 20 degrees below normal.

On Saturday, the upper low center will remain quasi-stationary over the Pacific northwest coast. While lingering atmospheric moisture will support ongoing shower chances, spatial coverage will decrease as dynamic forcing temporarily weakens from the cooler temperatures. Temperatures on Saturday will begin a slight recovery, warming back closer to seasonal averages under persistent southwest flow aloft.

LONG TERM /Sunday through Thursday/

The large Pacific upper low will slowly steer inland onto the coasts of Washington/Oregon on Sunday and the low core will pass directly over our area through Monday, returning moderate to widespread precipitation late Sunday into Monday. Guidance continues to track the low steering moisture from Hurricane Marie into the Upper Great Basin and near our area come Monday. The moisture will remain close enough for south-central Idaho to experience heightened chances for heavier rainfall, especially the central mountains. Snow levels are still expected to drop low enough over the highest peaks for a rain and snow mix, mainly on Monday.

Below-normal temperatures will persist through early next week due to upper troughing and widespread cloud cover. Latest long-range guidance suggests another Pacific upper low steering into the region Tuesday through Thursday, but with considerable uncertainty over its magnitude and behavior towards late week. For now, drier conditions and warmer temperatures are expected Tuesday through late next week. The approaching low may return scattered precipitation on Thursday, depending on the amount of Pacific moisture steered into our area.

AVIATION /18Z Thursday through Friday/

Issued 1021 AM MDT THU SEP 3 2026 Numerous rain showers today, becoming more widespread in evening. MVFR/IFR conditions and mountain obscuration in heavier precip. A 20-30% chance of embedded thunderstorms in E Oregon and w-central ID through late in the evening. Snow levels lowering to 7500 ft MSL in E Oregon tonight. Surface winds: SE- SW 5-15 kt with gusts around 20-25 kt throughout the afternoon, becoming W-NW 10-15 kt with gusts up to 30 kt with frontal passage in the evening after Fri/00Z. Winds aloft at 10kft MSL: S-SW 15-30 kt.

KBOI...Generally VFR. Light rain showers will continue through the afternoon. Then, continuous rain this evening with a 20% chance of MVFR conditions. Surface winds: E-SE 5-15 kt shifting back to the W-NW with gusts to 20 kts by 01z/Fri.

WEEKEND OUTLOOK...Showers to continue through the weekend as the upper level trough moves across the area. Saturday looks to be the driest with showers mainly in the higher terrain. However, areas of fog is likely (60% chance) to develop in many valley areas both Saturday and Sunday mornings. Showers return to much of the area on Sunday especially across the central mountains of Idaho. W-SW surface winds 5-15 kts with gusts 25 kts.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Flash Flood Watch through Friday evening for IDZ011. Flash Flood Watch through Friday evening for IDZ013. OR...Flash Flood Watch through Friday evening for ORZ061>063.