Area Forecast Discussion

UPDATE

Updated aviation discussion.

KEY MESSAGES

- Isolated to scattered showers and thunderstorms today, mainly over the higher terrain of the ID/NV border and west-central ID mountains.

- Shower/thunderstorm coverage and chances 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 211 PM MDT TUE JUL 21 2026 Light rain / virga showers have been ongoing through the morning into early afternoon, with some areas picking up a couple hundreths. Southerly flow aloft will be maintained today as the upper-level ridge remains in the south-central CONUS and a low pressure system moves north along the Pacific NW coast. Monsoonal moisture will continue to increase over the entire area today, and even more on 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. Instability this afternoon and evening will be limited and focused over the higher terrain, where there is a 20-40% chance of showers and thunderstorms developing. Thunderstorms may produce brief heavy rain and gusts 40-50 mph. Showers and isolated thunderstorms 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 compared to yesterday, 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.

Southerly flow aloft will continue Wednesday, with a shortwave trough expected to aid convection in the afternoon and evening across the central Idaho mountains, where the chance of showers and thunderstorms will rise to 40-90%. 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. QPF totals near the Wapiti have bullseyes of near 0.5" in 3 hours, and near 0.2" in 1 hour. A Flash Flood watch has been issued for the Wapiti burn area from 12 pm MDT to 9 pm MDT Wednesday to match the confidence for this threat. Models also indicate the potential for gusty outflow winds as cold pools from thunderstorms descend over the mountains, with wind gusts from 40-50 mph possible. 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.

The upper-level low moving up the Pacific NW coast Tuesday and Wednesday will begin to makes its way onshore Thursday, shifting 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 211 PM MDT TUE JUL 21 2026 Friday looks very hot as the upper high over the Great Basin builds to maximum amplitude while an upper trough stays off the northwest coast. High temps will reach 102 to 106 degrees in the Treasure Valley and southern Malheur County, with 90s in the mountains as high as 5000 feet. By then most of the monsoon moisture will be east of our CWA but eastern Owyhee County and Twin Falls County will still be in it with a slight chance of thunderstorms. On Saturday, upper winds will become more westerly as part of the offshore trough comes inland. This means slightly cooler temps although still hot, but also drier. Hot, dry weather will continue through Tuesday.

AVIATION /00Z Wednesday through Thursday/

Issued 529 PM MDT TUE JUL 21 2026 Generally VFR, with areas of MVFR/IFR due to wildfire smoke. Visibility will be at a minimum overnight, with improvement during the peak of daytime heating. Showers and thunderstorms will be possible this evening and overnight near the Nevada border, with more widespread activity Wednesday afternoon. High density altitude due to continued hot temperatures. Thunderstorms Wednesday afternoon could bring torrential rain, wind gusts to 40 kt and terrain obscuration. The most likely impacted area will be in central Idaho. Surface winds: W-NW around 10 kt, becoming variable less than 10 kt overnight. Winds aloft at 10kft MSL: WSW around 10 kt.

KBOI...VFR with mid/high clouds, then visibility reductions due to wildfire smoke overnight into early Wednesday. High density altitude Wednesday afternoon. Showers will try to make it to the terminal Wednesday afternoon, and there is potential for gusty outflow winds Wednesday evening. Surface winds: W-NW up to 15 kt with higher gusts this evening, becoming variable less than 10 kt overnight. Swinging back to the WNW Wednesday afternoon.

BOI WATCHES / WARNINGS / ADVISORIES

ID...Air Quality Alert until 3 PM MDT Wednesday for IDZ011-012-014- 033. Flash Flood Watch from Wednesday afternoon through Wednesday evening for IDZ013. OR...Air Quality Alert until 1 PM MDT /noon PDT/ Wednesday for ORZ061>064.