textproduct: Boise

KEY MESSAGES

- Numerous showers, isolated-to-scattered thunderstorms, and cooler temperatures today. Strongest thunderstorms in central Idaho with gusty outflow winds, hail, and brief heavy rain.

- Showers and thunderstorms continue tonight and tomorrow over the central Idaho mountains.

- Gradual warming and drying trend beginning Saturday and continuing through early next week.

SHORT TERM /Through Sunday Night/

Currently, the upper-level low pressure system is moving directly overhead as of 1400 MDT, causing numerous showers and isolated- to- scattered thunderstorms over southwest Idaho. Concern has increased through the day as we have been observing these cells, with QPE totals (over 2+ inches SE of Glenns Ferry, small area of 3+ inches) instigated by training thunderstorms causing the issuance of a Flood and Flash Flood products. The watch is of particular concern for the Boise mountains, with extra attention on a few burn scars in the area. Isolated thunderstorms are also ongoing over southern Malheur County and moving into northwest Baker County. While burn scars over SE Oregon could also see impacts from these thunderstorms, confidence is not as high as of this forecast issuance.

These areas of showers and thunderstorms are expected to continue through the afternoon into evening for both SE Oregon and SW Idaho. Thunderstorms will be capable of producing gusty winds up to 40 mph, small hail, and brief heavy rain. Abundant lightning is also likely, with rainfall totals varying heavily dependent on where the thunderstorm cores move over. Once late evening rolls around, coverage will decrease greatly compared to the afternoon, becoming largely confined to the Boise and west- central ID mountains. While overall coverage wanes tonight, showers and thunderstorms are expected to continue through Saturday afternoon as moisture wraps around the eastward moving low pressure center. Confidence of showers is high (30-50% chance), but confidence in thunderstorms is low (10-15% chance). Fog is likely in the late overnight to early morning periods tonight and Saturday night, especially in areas where there was a lot of rainfall.

Behind the low, high pressure returns as a ridge axis moves over the region, leading to warming conditions late Saturday and through Sunday. Temperatures warm up to about 5 degrees above normal by Sunday.

LONG TERM /Monday through Friday/

An upper level ridge over the Intermountain West and Rocky Mountains will remain the dominate weather feature through the extended period. Early in the week on Monday and Tuesday, the region will sit under a modest northwesterly flow. Dynamics will favor development of showers in Central and Eastern Idaho Monday and Tuesday afternoons. The chance of precipitation each afternoon is only 10%. Otherwise, that zonal flow to the north moderates temperatures, holding them steady at 5 degrees above normal for most of the long term. Upper level ridge amplifies to the east as a strong Alaskan low pressure system approaches the Pacific Northwest. This will bring well above normal temperatures for Thursday and Friday. Temperatures look to run at least 5 to 10 degrees above seasonal normals. That would mean temperatures in the upper 80s to low 90s in the lower valleys.

AVIATION /18Z Friday through Saturday/

Issued 1138 AM MDT FRI SEP 18 2026 VFR in eastern Oregon. Mainly VFR in southwest Idaho with occasional MVFR through early Sat/AM amidst scattered to widespread showers and isolated thunderstorms. Storms this afternoon could produce outflows gusts up to 35 kt. Surface winds outside of storms: W-NW 5-15 kt with gusts to 20-25 kt Fri/PM. Winds aloft at 10kft MSL: NW-N 5-15 kt.

KBOI...Mainly VFR. Showers developing N and E of KBOI this afternoon. Highest chances for showers over terminal will be late Fri evening, with brief MVFR possible. 10% chance of lightning at the terminal. Clearing early Sat morning with SCT clouds and light winds. 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...Flash Flood Watch until midnight MDT tonight for IDZ013-028. OR...None.