textproduct: Boise

KEY MESSAGES

- Showers and thunderstorms with local heavy rains today.

- Areas of valley fog early saturday morning.

- Temperatures 10 to 20 degrees below normal today, then around 10 degrees below normal Saturday and Sunday.

- Warming next week, with an increasing chance of thunderstorms Wednesday afternoon through Friday.

SHORT TERM /Through Sunday Night/

At 1 AM MDT radar showed a moderate to heavy rain band extending from Washington County/ID SSW-ward through Vale, OR, into central Malheur County/OR. Since then the band has shifted eastward into ID and has weakened since late evening when it had heavier rain in eastern OR. Radar showed the heaviest rain just west of Weiser/ID around midnight MDT, and a Flood Advisory was issued there for urban and small stream flooding until 3 AM MDT. Latest hi-res models show the rain band becoming stronger again in the eastern parts of Boise and Valley counties starting around 3 AM MDT, and especially the Wapiti Burn scar. A Flash Flood Watch is in effect on Burn scars through Friday in most of our CWA (except south of the Snake Basin), but greatest concern is on the Wapiti Burn Scar as the band should move right overhead 4-5 AM MDT. Meanwhile, a cold front reached Twin Falls and Jerome just before midnight with westerly gusts to 40 mph. The front will continue east today while colder air comes in from the west. Pre-dawn temps were in the 40s in the valleys and upper 30s in the mountains, with snow reported above 5500 feet in eastern OR. The cold air and pcpn were due to a deep upper low quasi- stationary on the Oregon coast. The low is forecast to gradually fill (i.e., weaken) through Saturday before moving inland Sunday and through our northern zones Sunday night. Back to today, instability will increase this afternoon leading to scattered thunderstorms in eastern Oregon late morning through afternoon, and southwest Idaho (mainly Owyhee County) by mid- afternoon and possibly the Treasure Valley late afternoon. Stronger storms will feature heavy rain and hail. Improving conditions will take place overnight through Saturday evening, but areas of fog will form overnight into Saturday morning after today's rain. Becoming wetter again Saturday night through Sunday night, with a chance of thunderstorms Sunday afternoon/ evening, as the upper low moves through. Temps will stay below normal through the weekend, but Sunday should be a couple degrees warmer than Saturday.

LONG TERM /Monday through Friday/

High pressure aloft will rebuild starting Monday as the upper low exits through Montana into southern Canada. The higher pressure will bring warmer air back into our CWA the rest of the week, with temps rising back to normal or several degrees above Wednesday and beyond. The next upper low is forecast to stay off the Oregon coast, but latest models (GFS more than ECMWF) show southwesterly flow aloft tapping into moisture from the next westward-moving hurricane in the Pacific subtropics. We therefore have a slight chance of thunderstorms in the forecast as early as Wednesday afternoon, but more likely Thursday and Friday.

AVIATION /06Z Friday through Saturday/

Issued 1142 PM MDT THU SEP 3 2026 Mostly VFR. Numerous rain showers through Friday morning, then scattered showers and isolated thunderstorms Friday afternoon. Areas of MVFR/IFR with heaviest precip. Mountains obscured. Snow levels 6.5k-7.5k feet MSL in E Oregon, and 8k-9k feet in w-central ID. Surface winds: SW-NW 5-15 kt with areas of 20-30 kt gusts, then variable 5-15 kt starting early Friday morning. Winds aloft at 10kft MSL: SW 15-25 kt.

KBOI...Generally VFR. Showers bringing MVFR at times, decreasing after Friday/10Z. Then, a 20% chance of showers and thunderstorms Friday afternoon. Surface winds: NW 5-15 kt. Gusts to 20-25 kt until early Friday morning, becoming light and variable as showers exit mid-morning.

Weekend Outlook...Mostly VFR. Scattered showers Saturday morning. Activity decreasing from SW to NE late morning. Then, showers increasing again Sunday, with highest chances (50-70%) over east Oregon and w-central Idaho. Localized MVFR/IFR and mtn obscuration in heaviest precip. Patchy fog/low stratus development Saturday/ Sunday mornings. SW-NW surface winds 5-10 kt with PM gusts to 20 kt.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Flash Flood Watch through this evening for IDZ011-013. OR...Flash Flood Watch through this evening for ORZ061>063.