textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- Strong thunderstorm-outflow wind event moving northward through Idaho this evening.

- Heat Advisory until 9 PM MDT in the Treasure Valley, western Magic Valley, and Upper Weiser River Valley.

- Weak Pacific cold front will bring more thunderstorms to the area Saturday afternoon and evening.

SHORT TERM /Through Sunday Night/

Issued 246 PM MDT FRI AUG 21 2026 The big issue early this evening will be strong thunderstorm-outflow winds entering southwest Idaho ahead of a northward surge of monsoon moisture currently in Nevada. HRRR is the most aggressive of all models but it tends to do well in these situations. HRRR has shown this outflow event consistently in hour-by-hour updates since last night, moving across the NV/ID border around 6 PM MDT this evening, through the Snake Basin 8-9 PM MDT, then weakening as it gets into the ID mountains around 9 PM but still holding together all the way past McCall around midnight MDT. A Wind Advisory has been issued for winds 20-35 mph with gusts to 55 mph, i.e., not as intense as HRRR as that model may have over-forecast it. The full force of the HRRR forecast would bring gusts 60 to 75 mph through the Snake Basin, likely causing damage, not to mention blowing dust and severely restricted visibility. Even so, areas of blowing dust and reduced visibility are expected. Strongest winds should last less than one hour at any given location. The timing of the event looks more definite, though, with the event entirely north of the Snake Basin by 10 PM MDT as it continues to move north while steadily weakening. By the time it gets to McCall, max gusts should be less than 40 mph. As stated earlier, these winds are due to outflow ahead of thunderstorms in a northward surge of monsoon moisture, so that after the winds die down there will be scattered showers and thunderstorms, but with little rain reaching the ground tonight.

Meanwhile, we still have a Heat Advisory in effect through 9 PM MDT this evening. Saturday should be 5 to 10 degrees cooler than today as a Pacific cold front moves inland and through our area. The front will bring gusty west winds to Oregon and breezy northwest winds to western Idaho. With monsoon moisture still present on the Idaho side, the approaching front will help develop more showers and storms Saturday afternoon and Saturday night, with a better chance of rain than tonight's storms. Sunday will be slightly cooler again, with another chance of late-day showers and storms on the Idaho side. Nighttime temps will be warm tonight and Saturday night, then cooler Sunday night.

LONG TERM /Monday through Friday/

Issued 246 PM MDT FRI AUG 21 2026 Southwest flow aloft will continue through the long term as the Desert Southwest upper-level ridge is tempered by semi-persistent troughing over the Pacific NW. Enough moisture will be in place to carry shower and thunderstorms chances through at least Tuesday night, primarily over southwest ID, with greater uncertainty Wednesday onward. Models show a potent shortwave trough moving over the area on Monday afternoon, with a 20 to 50 percent chance of showers and thunderstorms over southern ID. In far southeastern OR, there is a 15 to 30 percent chance Monday afternoon and evening as well. The upper-level low pressure system is forecast to move inland mid-to-late next week, causing dry, breezy, and slightly cooler temperatures Wednesday through Friday. Temperatures will be around 2-5 degrees above normal through the period.

AVIATION /00Z Saturday through Sunday/

Issued 540 PM MDT FRI AUG 21 2026

Generally VFR. High density altitude from heat through evening. Areas of wildfire smoke. Scattered showers/thunderstorms from S to N through late tonight. Activity capable of erratic outflows to 40-55 kt, and blowing dust reducing vis. Surface winds outside of storms: variable 5-15 kt. Winds aloft at 10kft MSL: S 10-20 kt.

KBOI...VFR. A 40% chance of showers and thunderstorms impacting KBOI terminal, this evening through 22/05Z. Moderate confidence of strong outflow wind gusts and blowing dust reducing vis at the terminal from nearby storms. Surface winds: variable 5-15 kt, gusty storm outflow winds 35-50 kt possible this evening from the southeast-southwest.

Weekend Outlook...Generally VFR. Areas of wildfire smoke. Scattered showers and thunderstorms returning Saturday, especially far E Oregon and SW Idaho. Gusty outflow winds 35-55 kt and locally heavy rain from storms. High density altitude from heat Sat afternoon. Drier Sunday. Surface winds: Gusty SW winds Saturday afternoon. Breezy W-NW winds all areas on Sunday.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Red Flag Warning from noon Saturday to midnight MDT Saturday night for IDZ401>403-421-424-426. Heat Advisory until 9 PM MDT this evening for IDZ012-014-016- 033. Wind Advisory until 10 PM MDT this evening for IDZ012-014>016- 029-030. OR...Heat Advisory until 9 PM MDT /8 PM PDT/ this evening for ORZ064. Wind Advisory until 10 PM MDT /9 PM PDT/ this evening for ORZ064. Red Flag Warning from 11 AM to 8 PM PDT Saturday for ORZ670>672.