textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- Hot temperatures this week, hottest today with a Heat Advisory in the Snake Plain and high desert.

- Isolated to scattered showers and thunderstorms near the Idaho/Nevada border today.

- Showers/thunderstorms increase through midweek with heavy rainfall possible especially on Wednesday.

- Areas of smoke through the week.

SHORT TERM /Through Wednesday Night/

Issued 225 PM MDT MON JUL 20 2026 The central U.S. upper-level ridge will continue to build northwest across the Great Basin today, causing the hottest temperatures across the region through the short term period. Highs will reach the upper 90s to 105 degrees in the lower valleys, and a Heat Advisory remains in effect for much of the lower elevations across the Snake Plain and high desert until 9 pm MDT tonight. Southerly flow aloft will continue to transport monsoonal moisture into the region, with widespread PWAT values above the 95th percentile (over 1.00") today. Isolated to scattered showers and thunderstorms will develop near the Idaho/Nevada border this afternoon and evening, with outflow wind gusts to 40 mph possible. Outside of storms, breezy conditions will develop late today with gusts 15-30 mph, locally up to 40 mph in eastern Oregon this evening. Overnight, remnants from these thunderstorms are expected to move north across SW Idaho as weak showers that are mostly virga. There is a 10-15% chance that these intensify to thunderstorms as they move over the lower Snake Plain overnight around 2 a.m. MDT and further into the Boise Mountains.

On Tuesday, shower and thunderstorm potential will expand across higher elevations, especially the west-central Idaho mountains and ID/NV border. PWATs increasing further Tuesday as well, with widespread values near the 97th percentile (near 1.15"). Continued monsoonal moisture streaming in will also create light rain / virga showers over SE Oregon, with minimal QPF. Temperatures will be a few degrees cooler than Monday.

By Wednesday, precipitable water values will rise to around 1.25 inches, near the 99th percentile. An embedded shortwave trough will interact with this moisture, increasing shower and thunderstorm coverage, especially across eastern areas where precipitation chances will reach 30-70%. The higher terrain across southeast Oregon and far southwest Idaho will see a 20-40% chance of precipitation with a less than 20% chance in the lower valleys. With deep moisture in place and slow expected storm motion, thunderstorms will be capable of heavy rain and flash flooding, especially over burn scars and steep terrain. Current long-range forecast models have QPF bullseyes of near 0.5" in a 3 hour period, which is quite intense. With convective allowing models coming into range tomorrow, the threat timing and intensity will become clearer, however the concern level is already high. Temperatures will be similar to that on Tuesday.

With southerly flow aloft, smoke should improve each afternoon, but northwest surface winds are expected to continually transport the smoke back into SE Oregon and SW Idaho.

LONG TERM /Thursday through Monday/

Issued 225 PM MDT MON JUL 20 2026 Drier Thursday as upper flow changes from SSW to WSW and shifts the monsoon flow eastward and out of our CWA. The main upper high over the interior western US will also expand westward out into the Pacific effectively preventing more monsoon moisture from reaching our area. On Monday an upper trough off the northwest coast will come inland across WA and northern OR beginning a breezy and cooler period in our CWA.

AVIATION /06Z Tuesday through Wednesday/

Issued 1146 PM MDT MON JUL 20 2026 Generally VFR. Areas of MVFR-IFR visibility due to wildfire smoke. Isolated/scattered showers will spread north through morning and afternoon. Thunderstorms re-develop near NV-ID border in the afternoon, with brief heavy rain and outflows to 40 kt. High density altitude during afternoon heat. Surface winds: generally SW-NW 5-15 kt with localized gusts around 25 kt. Winds aloft at 10kft MSL: variable 5-10 kt.

KBOI...Generally VFR. Potential for MVFR visibility in the early morning hours due to wildfire smoke. A 15% chance of light rain after Tue/09Z. High density altitude during afternoon heat. Surface winds: variable around 6 kt through morning, then NW 10-15 kt with gusts 20-25 kt in the afternoon.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Air Quality Alert until 3 PM MDT Wednesday for IDZ011-012-014- 033. OR...Air Quality Alert until 1 PM MDT /noon PDT/ Wednesday for ORZ061>064.