textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- Warm Today and Tuesday.

- Chance of thunderstorms in S-central ID Mon, and Central ID Tue afternoons.

- A strong low pressure system expected later this week will bring cooler, windier, and wetter conditions.

SHORT TERM /Through Wednesday Night/

Issued 302 AM MDT MON AUG 31 2026 Transient upper level shortwave ridging builds over the region today, initiating a notable warming trend across southeast Oregon and southwest Idaho. High temperatures today will rebound near seasonal normals, reaching the mid to upper 80s in lower valley locations. The return of southwesterly flow will allow for moisture to spread northward into south-central Idaho this afternoon and evening. A weak shortwave traversing the Great Basin this afternoon will trigger thunderstorm development over the higher terrain south of Twin Falls. Outflow winds up to 40 mph are possible from the strongest storms.

By Tuesday, an expansive upper level low pressure system will settle along the Pacific Northwest coast. As the pressure gradient tightens between this approaching trough and high pressure to our southeast, southwesterly flow aloft will strengthen significantly. This warm advection pattern will push Tuesday afternoon maximums around 5 to 10 degrees above normal, with valley locations climbing into the upper 80s to near 90 degrees. Afternoon surface winds will also become breezy to locally gusty on Tuesday, particularly across open terrain and high elevation zones. This could be problematic for ongoing fires across the BLM lands as the winds will combine with low humidities.

Shortwave energy rotating around the parent trough will shift inland Tuesday night into Wednesday. This will draw monsoonal moisture northward into south-central Idaho while pushing dynamic lift across southeast Oregon and the West Central Mountains. Expect shower and thunderstorm chances to increase across eastern Oregon late Tuesday night, spreading eastward into the Idaho mountains on Wednesday. Southwest Idaho valleys will likely sit within a dry slot between the primary low center to the west and deep monsoonal moisture to the east, limiting widespread precipitation chances for lower elevations. High temperatures will begin their downward trend on Wednesday as cloud cover increases and the cooler air mass begins to impinge from the west. Thunderstorms will be possible again in the mountains of Central Idaho Wednesday afternoon and evening.

LONG TERM /Thursday through Monday/

Issued 302 AM MDT MON AUG 31 2026 Deterministic and ensemble consensus remains strong for a deep long-wave trough and closed low pressure system to stall near the Oregon and Washington coastline through much of the long term period. This positioning keeps our forecast area situated in persistent, moderately unstable southwesterly flow aloft. Embedded shortwaves will periodically orbit the main low center, maintaining persistent shower and thunderstorm chances through Thursday and Friday, particularly over southeast Oregon and the West Central Idaho Mountains. SW Idaho valleys will remain susceptible to dry slotting, though isolated activity cannot be ruled out as atmospheric instability pulses each afternoon.

A substantial cooling trend will take hold starting Thursday as lower heights overspread the area. Temperatures will fall roughly 5 to 10 degrees below seasonal averages, holding valley highs in the mid to upper 70s through the upcoming weekend and into Monday. Afternoon wind gusts will remain elevated daily, favoring typical wind-prone corridors including southeast Oregon and terrain south of the Snake River Plain. The wide variations in precipitation among the ensemble clusters is due to the potential entrainment of tropical moisture from the hurricane forecast to form off the coast of Baja California over the next 48 hours. While the confidence in precipitation amounts is low, high confidence exists for a prolonged cool, windy, and periodically unsettled pattern through Monday.

AVIATION /12Z Monday through Tuesday/

Issued 603 AM MDT MON AUG 31 2026 VFR. Showers and thunderstorms developing south/east of KTWF late this afternoon/evening. Storms capable of outflows to 25-35 kt. Surface winds: light and variable. Winds aloft at 10kft MSL: W 5-15 kt.

KBOI...VFR. Surface winds: Southeast 5-10 kts switching to the west-northwest around 00z Tuesday.

BOI WATCHES/WARNINGS/ADVISORIES

ID...None.

OR...None.