textproduct: Boise

KEY MESSAGES

- Isolated to scattered thunderstorms this 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 Friday Night/

A negatively tilted upper level trough is pushing through the region this morning, initially triggering mainly virga showers across southeast Oregon due to dry low level air. RH's across southeast Oregon remain in the teens this morning, thus limiting rainfall. However, as the system tracks eastward toward southwest Idaho by sunrise, it will interact with a corridor of deeper atmospheric moisture. This interaction will allow showers to become more organized, increasing precipitation coverage and chances for rain and thunderstorms for lower elevations and mountain zones alike.

By this evening, a large upper level low approaches the Washington coast, sending a broad plume of Pacific moisture over our area. The setup resembles a late fall or early winter weather pattern, characterized by strong southwesterly flow aloft. Downslope winds off surrounding terrain will likely cause some valley shadowing initially, limiting precipitation totals for the Treasure Valley and Baker Valley before lower levels saturate.

Periods of rain continue in varied coverage Thursday into Friday as the parent low remains positioned over the Pacific Northwest coast. All areas should see at least a tenth of an inch from today through Friday except for the Western Magic Valley and Camas Prairie. Mountains should see beneficial rainfall near an inch total with the valley seeing around 0.25'. Embedded thunderstorms could produce locally higher amounts. Although rain looks to be steady through Friday across much of the area, the amount of precipitation could cause some localized flooding issues for the recent burn scars. Lowering heights and cool onshore flow will keep temperatures well below normal through Friday. Southeast Oregon and the West Central Mountains of Idaho will see record to near record low max temperatures Thursday and Friday.

LONG TERM /Saturday through Wednesday/

Model ensembles show the broad upper level low maintaining its influence over the Pacific Northwest through the weekend before gradually shifting eastward early next week. On Saturday, persistent southwesterly flow aloft will maintain a chance of showers, primarily focused over the higher terrain of central Idaho and eastern Oregon, while lower valleys experience lower precipitation odds. The upper level low pushes east across the area on Sunday and this is when it could potentially link up with moisture from Hurricane Marie, increasing the threat for heavier precipitation across south-central Idaho. Overall confidence remains low at this time, but a growing number of ensembles have shifted towards this moisture moving across the Great Basin in the latest runs.

As the main low axis finally crosses the Northern Rockies Monday, cooler temperatures and additional shower activity will sweep through the region. Snow levels may lower enough to bring light snow or mixed precipitation to the highest peaks. By Tuesday and Wednesday, ensemble clusters favor a transition to drier conditions and a subtle warming trend as an upper level ridge attempts to build inland from the Pacific coast. High temperatures throughout the long term period will remain generally near or slightly below seasonal averages.

AVIATION /06Z Wednesday through Thursday/

Issued 1231 AM MDT WED SEP 2 2026 Scattered showers/isolated thunderstorms developing early AM hours over E Oregon, expanding into Treasure Valley and w-central ID through morning. Thunderstorms capable of brief MVFR/IFR heavy rain, hail, and gusty outflows. Then, widespread rain developing Wed evening, with mtns becoming obscured. Surface winds: SW-NW 5-15 kt, gusts to 20-30 kt during early AM precip. Then, SW 10-20 kt with 25- 30 kt gusts Wednesday afternoon. 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, then rain expected Wed evening. Surface winds: SE 5-10 kt, becoming NW or variable around Wed/10Z. Then SW 5- 15 kt, with gusts around 20-25 kt Wed afternoon.

BOI WATCHES/WARNINGS/ADVISORIES

ID...OR...None.