textproduct: Boise

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/

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/

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 /18Z Tuesday through Wednesday/

Issued 1133 AM MDT TUE SEP 1 2026 Mid level moisture streaming northward across the region ahead of an approaching trough will set the stage for isolated to scattered showers and thunderstorms through this evening. Gusty winds will accompany any storms that develop due to dry low levels. VFR conditions should be dominant, except near stronger storms. Another round of showers will be possible overnight, with a strong signal of precipitation and potential for MVFR/IFR conditions mid to late week as a trough settles over the region. Surface winds generally ESE 10-15 kt with higher gusts, shifting to the SW/W after 20Z. SW wind becoming dominant after 12Z Wednesday. Winds aloft at 10kft SSW 20-30 kt.

KBOI...VFR with isolated showers and thunderstorms possible through 00Z this evening. Little precipitation expected, but variable gusty winds and a few lightning strikes are possible (15-20% chance). Southeast surface winds should dominate, but outflow winds from storms will make for variable winds at times. Guidance suggests a turn to the WSW after 20Z, but storm outflow will complicate any shift. SE winds to return overnight, before SW winds begin to dominate by late morning Wednesday. Another round of showers (VFR) possible early Wednesday.

BOI WATCHES/WARNINGS/ADVISORIES

ID...OR...None.