textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- Warm temperatures through Thursday, with values 10 to 15 degrees above normal Thursday.

- Chance of showers and thunderstorms Tuesday, mostly across higher terrain of southwest Idaho.

- Cooler temperatures heading into this weekend with a chance of precipitation across SE Oregon and portions of SW Idaho Friday.

SHORT TERM /Through Thursday Night/

Issued 246 AM MDT TUE SEP 22 2026 A cut-off low over California moves towards Montana today, cooling and moistening mid-levels. The destabilization of mid- levels supports the development of showers and thunderstorms this afternoon. On the east side of the low, in Idaho, higher atmospheric moisture will produce more widespread cloud cover inhibiting instability. So while precipitation is likely in S-Central Idaho, the Snake Plain, and Central Idaho mountains: it will likely be a shallower storm mode. Shallow development supports widespread rain showers with weak embedded thunderstorms in this area.

Moisture wrapping around the center of the low will move into a less unstable environment in Southeast Oregon, but more sparse cloud cover will still support convective initiation. A more discrete storm mode is expected here, but the limited instability will keep thunderstorms weak. In all areas, the primary hazard with this system will be gusty outflow winds near precipitation. Outflow winds up to 45 mph are possible, alongside lightning, small hail, and brief moderate- heavy rainfall. Precipitation will dissipate around sunset as surface heating ends and the low moves east.

The cut-off low transitions to a long wave trough as it rejoins the jet stream positioned north of our area. In turn, this brings the jet slightly further south, putting us in the boundary zone of building heights to our south and warm southwesterly flow directly overhead and to the north. The southwesterly flow strengthens and becomes more southerly through Thursday as a strong longwave trough develops over the West Coast. Thus, temperatures will warm slightly Tuesday into Wednesday, and sharply Wednesday into Thursday making Thursday the hottest day in the 7-day forecast. Temperatures rise to 10-15 degrees above normal on Thursday, with many lower valleys forecast to see maximum temperatures in the low-mid 90s. This forecast has trended upwards the past few days as high- resolution models begin to capture winds on Thursday. Ahead of that building coastal trough, winds in the Snake Plain rise to 15 to 25 mph with wind gusts up to 35 mph. With winds coming out of the southeast, or downslope in the Snake Plain, we could see the temperature forecast continue to trend upwards on Thursday despite increasing clouds ahead of the trough. Elevated fire weather conditions develop due to the combination of strong winds and hot temperatures that afternoon.

LONG TERM /Friday through Tuesday/

Issued 246 AM MDT TUE SEP 22 2026 Sharply cooling temperatures, gusty winds, and precipitation to the west and north are all slated for later this week and weekend as a strong coastal trough and associated cold front migrate inland. The cold frontal passage itself will occur early Friday afternoon in the latest synoptic model runs, with good agreement even amongst forecast systems and physical/AI cores. Pre- frontal winds gust 25 to 35 mph across the area that morning, winds slightly weaken around sunrise, and then grow even stronger in peak afternoon heating and in the post-frontal environment with gusts up to 30 to 45 mp. Winds remain strong 30 to 40 mph through Friday night in the Snake Plain. For most of the area, the strong frontal winds are the primary forecast concern. Showers along and behind the front will be limited to south- central Oregon, northeastern Oregon, and north-central Idaho. Southwestern Idaho remains drier due to the trough swinging northward while it moves inland. In areas expected to see showers, weak embedded thunderstorms are possible too. Notable rainfall amounts are limited to higher terrain near the Blue Mountains in Oregon and the West-Central Mountains in Idaho, around 0.1 inches through the frontal passage. Forecast amounts could rise if the troughs track moves further south, or if instability increases and showers grow stronger. Temperatures cool about 5-10 degrees as the front moves through on Friday, and about 10-15 degrees Saturday behind the front.

Cooler temperatures stick around through the weekend and into early next week as a trailing trough amplifies off the coast. Maximum temperatures Saturday through Tuesday will be around room temperature in lower elevations and in the low 60s to high 50s in higher terrain towns. Some mild northwesterly gustiness is expected to persist through the weekend each afternoon. The trailing trough arrives Sunday evening, looking much weaker than the trough Friday. Precipitation is possible Sunday through Monday in NE Oregon and N-central Idaho again, with 20 to 30 mph gusts possible in the Snake Plain at the same time. Models diverge beyond Tuesday.

AVIATION /12Z Tuesday through Wednesday/

Issued 526 AM MDT TUE SEP 22 2026 VFR with high clouds today. Isolated showers/thunderstorms this afternoon, mainly over central ID mtns, SE OR highlands, and south-central ID. Storms will generally be embedded in showers, capable of producing gusty winds, lightning, small hail, and moderate rainfall. Surface winds: S-SE 5-10 kt, becoming NW 5-15 kt this afternoon. Outflow gusts near storms 30 to 40 kts possible. Winds aloft at 10kft MSL: Variable 5-10 kt

KBOI...VFR with high clouds. Surface winds: generally light SE this morning, becoming 5-10 kt NW this afternoon.

BOI WATCHES/WARNINGS/ADVISORIES

ID...None.

OR...None.