textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- Isolated to scattered showers and thunderstorms today, mainly over the higher terrain.

- Showers/thunderstorms increase Wednesday with heavy rainfall possible especially over the central Idaho mountains.

- Hot temperatures and areas of smoke through the week.

SHORT TERM /Through Thursday Night/

Issued 329 AM MDT TUE JUL 21 2026 Weak southerly flow aloft associated with an upper ridge centered over the south-central U.S. will continue to transport monsoonal moisture into our area today and Wednesday. Precipitable water will climb well over 1 inch, rising above the 95th percentile today and closer to the 99th percentile on Wednesday, indicating a very moist airmass. A band of light showers will move north across our area this morning with about a 10% chance of embedded lightning. Despite the moisture, instability this afternoon and evening will be limited and focused over the higher terrain, where a 20-40% chance of showers and thunderstorms will develop. Storms may produce brief heavy rain and gusts to 45 mph. Showers and isolated storms will continue tonight near the Nevada border as an impulse lifts north. The increasing moisture and cloud cover will bring a few degrees of cooling today in most areas, but it will still be hot. Similar to yesterday, smoke should decrease in the afternoon as daytime heating mixes out smoke near the surface. Outside of storms, winds will be locally breezy with gusts 15-25 mph. Northwest surface winds will transport smoke from wildfires in Oregon back across the area tonight.

On Wednesday, a shortwave trough will aid convection in the afternoon and evening across the central Idaho mountains, where the chance of showers and thunderstorms will rise to 40-80%. Deep moisture and expected slow storm motion will result in the potential for heavy rain and flash flooding, especially over burn scars and steep terrain, including the Wapiti burn area. Models also indicate the potential for gusty outflow winds as a cold pool forms over the mountains. Elsewhere, a 20-40% chance of showers and thunderstorms will develop across the higher terrain with a less than 20% chance in the lower elevations. Increased clouds/showers will cool most areas slightly, with up to 10-15 degrees of cooling across the central Idaho mountains.

An upper level trough off the Pacific Northwest coast on Thursday will transition the flow aloft to southwesterly. This will begin to push monsoonal moisture to the east, but there will still be enough moisture and instability for a 10-20% chance of showers and thunderstorms. Decreasing moisture will allow temperatures to rise with lower valley highs once again near 100 degrees. Breezy conditions will develop across southeast Oregon in response to the southwest flow. Smoke may continue to impact the area Wednesday and Thursday with improvement each afternoon.

LONG TERM /Friday through Tuesday/

Issued 329 AM MDT TUE JUL 21 2026 An upper level trough will remain in place along the Pacific Northwest coast through the period with an upper level ridge to the east. The resulting southwest flow will continue to usher in a much drier airmass. Shower/storm chances will generally be less than 20% and confined to the higher terrain and along the Idaho/Nevada border. The pattern will bring hot, dry, and breezy conditions through the weekend and into early next week. Lower valley highs will remain in the upper 90s to around 100 degrees. Winds each day will gust 15-30 mph, strongest in open, elevated areas including the high desert of southeast Oregon and southwest Idaho south of the Snake River. Smoke may impact the area through the period, especially across southeast Oregon.

AVIATION /12Z Tuesday through Wednesday/

Issued 614 AM MDT TUE JUL 21 2026 Generally VFR. Areas of MVFR-IFR visibility due to wildfire smoke, improving in the afternoon. Scattered showers and isolated thunderstorms will spread north through the day. Scattered thunderstorms re-develop near NV-ID border in the afternoon and evening, with brief heavy rain and outflows to 40 kt. High density altitude during afternoon heat. Surface winds: W-NW 5-15 kt with localized gusts around 25 kt. Winds aloft at 10kft MSL: variable 5-10 kt.

KBOI...Generally VFR, except for brief MVFR visibility possible this morning due to wildfire smoke. A 15% chance of light rain this morning. High density altitude during afternoon heat. Surface winds: variable less than 8 kt this 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.