textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

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

- Much cooler this week, with highs 10-20 degrees below normal by Thursday with periods of gusty winds.

- Cooler than normal weather anticipated late this weekend into early next week.

SHORT TERM /Through Saturday Night/

Issued 312 AM MDT THU SEP 3 2026 A large upper level low remains positioned off the Pacific Northwest coast, maintaining a moist southwest flow aloft across southeast Oregon and southwest Idaho. A plume of Pacific moisture interacting with synoptic forcing and upper level energy will keep a broad band of stratiform rain with periods of embedded convection ongoing across the region through Friday night. Satellite and radar show a broad area of precipitation early this morning associated with an embedded shortwave moving northeast across Washington State. As the shortwave exits the region this afternoon, widespread precipitation will transition to scattered showers, allowing for occasional breaks in cloud cover.

A cold front approaching from the west later this afternoon will cross the forecast area overnight into Friday morning. This front will bring another wave of widespread rain and embedded thunderstorms, along with increased westerly winds, though wind speeds will remain below headline thresholds. The front exits early Friday, helping clear out cloud cover during the morning. The resulting surface heating will introduce weak daytime instability, triggering another round of isolated to scattered afternoon thunderstorms.

The primary focus through Friday remains the potential for heavy rainfall with embedded thunderstorms. Instability will be sufficient for robust storm development, particularly during the afternoon and evening hours. Heavy downpours could produce locally high precipitation rates, 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. High temperatures through Friday will stay 10 to 20 degrees below normal, with record low maximum temperatures possible in portions of southeast Oregon and west- central Idaho.

By Saturday, the upper low center remains stacked near the Pacific Northwest coast. While lingering atmospheric moisture will support ongoing shower chances, spatial coverage will decrease as dynamic forcing temporarily weakens. Temperatures on Saturday will begin a slight recovery, warming back closer to seasonal averages under persistent southwest flow aloft.

LONG TERM /Sunday through Thursday/

Issued 312 AM MDT THU SEP 3 2026 The large upper level low shifts slowly eastward directly over the Pacific Northwest on Sunday. As the system pivots inland, it will pull additional Pacific moisture into the region, leading to an uptick in precipitation coverage and shower chances late Sunday into Monday. Guidance continues to track moisture from Hurricane Marie northward through the Great Basin into the Northern Rockies. The majority of this tropical moisture is expected to track east of the forecast area as the trough axis sits directly overhead. However, it will remain close enough for south- central Idaho to experience heightened chances for heavier rainfall, with snow levels dropping low enough over the highest peaks for a rain and snow mix.

Below-normal temperatures will persist through early next week due to upper troughing and widespread cloud cover. Heading into Tuesday and Wednesday, ensemble guidance suggests the low will exit to the east, allowing an upper level ridge over the eastern Pacific to build inland. This pattern shift will bring drier conditions and a gradual warming trend back toward near-normal seasonal temperatures by the middle to end of next week.

AVIATION /12Z Thursday through Friday/

Issued 553 AM MDT THU SEP 3 2026 Numerous rain showers today, becoming widespread Thu evening. MVFR/IFR conditions and mountain obscuration in heavier precip. A 15% chance of embedded thunderstorms in E Oregon and w- central ID through late this evening. Snow levels lowering to 7500 ft MSL in E Oregon tonight. Surface winds: variable 5-15 kt, then SE- SW 5-15 kt with gusts around 20-25 kt by late morning. 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. 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 of the valley 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 from 3 PM MDT this afternoon through Friday evening for IDZ011. Flash Flood Watch through Friday evening for IDZ013. OR...Flash Flood Watch from 3 PM MDT /2 PM PDT/ this afternoon through Friday evening for ORZ061>063.