The Treasure Valley will remain warm and mostly dry through Thursday before a low-pressure system brings a slight cooldown and limited shower chances Friday. A beautiful fall weekend then follows, with sunny skies and temperatures climbing back to the low 80s by Sunday and Monday.
Slightly cooler by Friday but a spectacular weekend ahead in Scott Dorval's video forecast
Thursday: Warm with Isolated Storm Chances
Thursday will bring mostly sunny skies and a high near 83 degrees in the Treasure Valley, running well above the normal high of around 81 degrees for mid-September in Boise. The Treasure Valley and Malheur and Owyhee counties will remain generally dry Thursday, with only a 20 percent chance of isolated showers and thunderstorms between 10 a.m. and 3 p.m. Winds will gust up to 22 mph Thursday afternoon.
Central Idaho Mountains: Significant Storm Threat Thursday
While the Treasure Valley stays mostly dry, the central Idaho mountains north of the Snake River Basin will see a much more active weather day Thursday. Monsoon moisture surging northward from Arizona through Utah will curve westward across central Idaho and into the circulation around the Oregon low, fueling showers and scattered thunderstorms across the mountains Thursday.
Rain amounts greater than 0.25 inches are forecast in the Idaho mountains north of the Snake River Basin, with locally up to 0.50 inches possible right under the strongest thunderstorms. Hikers, campers, and outdoor enthusiasts in the central Idaho mountains should be prepared for rapidly changing conditions Thursday and have rain gear readily accessible. Flash flood potential near burn scars will need to be monitored with any heavy rainfall.
Friday: Low Passes Overhead with Shower Chances
The upper-level low will move directly over the region Friday, bringing a more noticeable cooldown with highs near 75 degrees in the Treasure Valley. A slight 20 percent chance of showers and thunderstorms will develop Friday afternoon and evening, favoring the Idaho side. Friday night will partially clear with patchy valley fog possible toward sunrise Saturday.
Weekend Outlook: Outstanding Fall Conditions
After the low exits east of the region Saturday morning, high pressure will rebuild over the western United States, delivering a beautiful fall weekend.
Saturday: Sunny skies and a high near 78 degrees. An excellent day for outdoor activities after the morning fog clears.
Sunday: Sunny skies and a high near 81 degrees. Right at seasonal normals and ideal for any outdoor activity.
This weekend's conditions will be exceptional for outdoor activities across the Treasure Valley and central Idaho mountains:
- Outdoor festivals and community events: Saturday and Sunday will be picture-perfect with sunny skies, temperatures in the upper 70s to low 80s, and light winds.
- Hiking and trail running: The Boise Foothills and surrounding areas will be spectacular. The recent rainfall has improved trail conditions and fall colors are beginning to emerge.
- Fall foliage viewing in the central Idaho mountains: This weekend will be an outstanding time to visit the Sawtooth Valley, Payette River corridor, and the mountains around McCall. Thursday's rainfall will further enhance the fall color change, and temperatures in the mountains will be comfortable in the mid to upper 60s Saturday and climbing to the low 70s Sunday and Monday.
- Golf and cycling: Near-perfect conditions Saturday through Tuesday with sunny skies and temperatures in the upper 70s to low 80s.
The official start of astronomical fall, the autumnal equinox, arrives on Tuesday, September 22 at 8:29 p.m. MDT. The Treasure Valley will celebrate the first full day of fall, Wednesday, under sunny skies, with a high near 83 degrees.
Looking Ahead
There is some uncertainty in the forecast Tuesday through Wednesday as a longwave trough steering into western British Columbia may extend down into the Northwest region. There is slight potential for the central Idaho mountains to see precipitation late Tuesday into Wednesday, as well as a slight area-wide cooldown. Otherwise, the area will stay mostly dry and warm through midweek.
Tonight: Partly cloudy, with a low around 58. Northwest wind 5 to 8 mph becoming calm in the evening.
Thursday: Isolated showers and thunderstorms between 10am and 3pm. Mostly sunny, with a high near 83. Light and variable wind becoming southwest 5 to 10 mph in the afternoon. Winds could gust as high as 22 mph. Chance of precipitation is 20%.
Thursday Night: Increasing clouds, with a low around 55. West wind 5 to 9 mph becoming calm after midnight. Winds could gust as high as 20 mph.
Friday: A slight chance of showers, with thunderstorms also possible after noon. Partly sunny, with a high near 75. Calm wind becoming west northwest around 6 mph in the afternoon. Chance of precipitation is 20%.
Friday Night: A slight chance of showers and thunderstorms before 9pm, then a slight chance of showers between 9pm and midnight. Partly cloudy, with a low around 54. West northwest wind 5 to 7 mph becoming calm in the evening. Chance of precipitation is 20%.
Saturday: Sunny, with a high near 78.
Saturday Night: Clear, with a low around 54.
Sunday: Sunny, with a high near 81.
Sunday Night: Mostly clear, with a low around 55.
Monday: Sunny, with a high near 83.
Monday Night: Mostly clear, with a low around 55.
Tuesday: Sunny, with a high near 83.
Tuesday Night: Mostly clear, with a low around 54.
Wednesday: Sunny, with a high near 83.
