textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- Upper low pressure approaching from ne California will bring showers, scattered strong thunderstorms, and cooler temperatures through Friday.

- Strongest thunderstorms in central Idaho with gusty outflow winds, hail, and brief heavy rain.

- Gradual warming and drying trend beginning Saturday and continuing through at least Tuesday.

SHORT TERM /Through Saturday Night/

Issued 302 PM MDT THU SEP 17 2026 Southwesterly flow ahead of the approaching upper low will keep temperatures warm across the region today, with dry air preventing storm formation over the Treasure Valley and Owyhee Mountains in Idaho and Malheur County in Oregon. Otherwise, showers and thunderstorms are expected to return this afternoon through evening over eastern OR and the west-central ID mountains. Latest satellite imagery shows some clearing and cumulus buildup over the aforementioned areas, aligning with the expected instability for this evening. Gusty outflow winds up to 50-60 mph, small hail, and brief heavy rain are the main concerns. Abundant lightning is also likely with these scattered storms in the afternoon, and fast storm motion will also keep rainfall amounts low, especially over southeast Oregon. This will increase the risk for lightning ignitions, prompting the Red Flag Warning for scattered lightning through 9 PM MDT this evening. Conditions will cool 5-10 degrees Friday as the low pressure system moves overhead. Scattered showers and isolated afternoon thunderstorms will occur again Friday, with increased coverage over the Treasure Valley and Malheur County. Showers will then taper off over the easternmost portions of the CWA on Saturday with high pressure building back in over the region.

LONG TERM /Sunday through Thursday/

Issued 302 PM MDT THU SEP 17 2026 High pressure rebuilding over the region will return a slight warming and drying trend Sunday through Monday, with peak daytime temperatures expected to hover in the lower 80s for the Treasure Valley area. Late Monday through Tuesday, the area will be sandwiched between a shortwave trough steering across the lower Great Basin and a longwave Pacific trough moving into the northwest coast. This pattern will keep us in moderated zonal flow aloft on Tuesday, with seasonable conditions and a chance of showers near the ID-NV border. Come Wednesday, the area will come under a strong southwest flow ahead of the approaching Pacific trough, continuing dry conditions and temperatures 5 to 10 degrees above normal through Thursday.

AVIATION /06Z Friday through Saturday/

Issued 1155 PM MDT THU SEP 17 2026 Generally VFR. Areas of fog overnight resulting in areas of IFR/LIFR, primarily in E-Oregon. Isolated showers and lightning overnight, becoming isolated to scattered Fri/PM. Outflows of around 35 kt possible out of stronger storms. Surface winds outside of storms: SW-NW 6-12 kt overnight. then W-NW 5-15 kt with gusts to 20- 25 kt Fri/PM. Winds aloft at 10kft MSL: Variable 5-15 kt.

KBOI...VFR. A slight chance of rain showers Fri/AM. Isolated to scattered rain showers Fri/PM with a 10% chance of lightning at the terminal. Foothills will be favored for showers (60% chance) and thunder (20% chance). Surface winds: NW 5-15 kt.

Weekend Outlook...Brief MVFR and mountain obscuration in lingering showers near KTWF-KJER on Saturday, with showers tapering off by midday. Expect VFR conditions otherwise through Sunday with variable winds less than 10 kt overnight and in high terrain and NW 5-15 kt in valleys during the afternoon.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Fire Weather Watch from 3 AM MDT Friday through Friday evening for IDZ403-421-423-424-426. OR...None.