textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- Isolated to scattered thunderstorms tonight through Wednesday morning, mainly across the mountains of central Idaho.

- Widespread showers and embedded thunderstorms across much of southeast Oregon and west-central Idaho Wednesday night through Thursday with significant rainfall totals possible.

- Much cooler this week, with highs 10-20 degrees below normal by Thursday with periods of gusty winds.

- Additional wet and cool weather anticipated late this weekend into early next week.

SHORT TERM /Through Thursday Night/

Issued 237 PM MDT TUE SEP 1 2026 A deep upper level low will approach the Pacific Northwest tonight into Wednesday, placing our area in southerly flow aloft. Monsoonal moisture in place over Idaho and a weak shortwave trough will bring scattered showers and thunderstorms to the Magic Valley north through the central Idaho mountains through this evening. Storms may produce outflow gusts to 50 mph, brief heavy rain, and small hail.

After a brief break in storm activity, a shortwave trough ahead of the main upper low will rotate across our area late tonight through Wednesday morning. There will be enough moisture and instability in place across west- central Idaho and east-central Oregon for showers and a 10-30% chance of thunderstorms. The highest rainfall totals and best chance of thunderstorms will be across the central Idaho mountains where 0.10-0.25" of rain is expected, except higher amounts possible in thunderstorms. Temperatures will be around 10 degrees lower than today as cooler air accompanies the shortwave passage. Winds will be gusty tonight south of the Snake River, with gusts 20-35 mph expanding to most higher elevation locations by Wednesday afternoon. Locally low relative humidity near the Idaho/Nevada border and gusty winds will lead to elevated fire weather conditions.

As the low center approaches the coast, moist southwesterly flow will set up across southeast Oregon and west-central Idaho Wednesday night into Thursday. Showers will become widespread in these areas, but precipitation chances will be lower to the east with a less than 10% chance of rain in the Magic Valley. Weak instability will support a 10-20% chance of thunderstorms, highest Thursday afternoon with the addition of daytime heating. Sustained moist southwest flow, orographic lift, and a neutral to weakly unstable atmosphere in addition to dynamic forcing ahead of the upper low will produce significant rainfall totals across much of southeast Oregon into west- central Idaho. Amounts of 0.25-0.75" are expected across much of southeast Oregon (tapering to near zero near the Oregon/Idaho/Nevada triple point), with 0.5-1.5" across the west-central Idaho and east- central Oregon mountains. Locally heavier amounts are possible from thunderstorms. The Treasure Valley looks to pick up less than a tenth of an inch of rain, except slightly higher towards the west end of the valley. Winds will remain gusty near the Nevada border, and temperatures will dip 5-10 degrees cooler than Wednesday where rain is persistent, with highs 10-20 degrees below normal across the area.

LONG TERM /Friday through Tuesday/

Issued 237 PM MDT TUE SEP 1 2026 The pattern will remain cooler and relatively wet through early next week as the upper low wobbles across the Pacific Northwest. A 20-60% chance of showers and 10-20% chance of thunderstorms will continue into Friday as the low center lifts to the north. Precipitation chances then decrease Saturday into early Sunday as the low pivots back to the west, but will still remain around 10-30%. The low is then favored to move to the east late Sunday into Monday, increasing precipitation chances to 30-60% once again before decreasing Tuesday. However, forecast confidence is lower during this period as tropical activity off the coast of Baja California will have downstream forecast implications. Even if tropical activity stays offshore, there is potential for entrainment of tropical moisture into the interior West, which would increase the risk for heavy rainfall. Temperatures will gradually warm to only a few degrees below normal by Saturday, then cool once again early next week as showers increase.

AVIATION /00Z Wednesday through Thursday/

Issued 619 PM MDT TUE SEP 1 2026

Scattered showers/thunderstorms over central Idaho mtns producing MVFR-IFR in heavy rain and hail under the cores, and gusts up to 40 kt. Cells will exit to the N-NE late this evening, then scattered showers/isolated thunderstorms regenerating in early morning hrs across east Oregon and w-central Idaho. Surface winds: SE-SW 5-15 kt, areas of gusts 20-25 kt. Winds aloft at 10kft MSL: S-SW 10-20 kt.

KBOI...VFR. A 30 percent chance of light rain showers during early to mid Wed morning hours. Surface winds: NW 5-10 kt, then SE 5-10 kt after Wed/03Z. Returning to light NW or variable around Wed/10Z.

BOI WATCHES/WARNINGS/ADVISORIES

ID...OR...None.