textproduct: Boise

KEY MESSAGES

- A chance of showers will spread northward into the area tonight, then become widespread Thursday and Thursday night, along with a chance of thunderstorms.

- Thunderstorms Thursday afternoon into evening will be capable of isolated severe wind gusts/hail in Southwest Idaho.

- Becoming windy late Thursday night behind a cold front, with much cooler conditions Friday.

SHORT TERM /Through Friday Night/

Upper low off central California is forecast to move inland across southern California Thursday morning, then northeastward through Nevada later Thursday, and into southern Idaho by Friday morning, an ideal track for rain in our CWA. Latest models have more QPF than the previous cycle, especially in Idaho, and we have issued a Flash Flood Watch for the Wapiti and Crooked Burn Scars for Thursday night and Friday morning. WPC has a marginal risk of excessive rainfall in our CWA Thursday and Thursday night. At that time lift will be enhanced by a surface cold front, and instability will increase ahead of the incoming upper low, as also indicated by SPC's thunderstorm forecast. We estimate total rainfall through Friday at 1.00 to 1.25 inch in Idaho north of the Snake Basin, .50 to .75 inch in the Snake Basin, and .35 to .60 inch in eastern OR. These values are about double what models showed yesterday. Rainfall should also last longer, well into Friday on the Idaho side, but ending from west to east in Oregon as colder and drier air comes in behind the cold front. Rain may briefly change to snow before ending in Harney County Friday morning.

Later Friday part of the upper low will continue to move north- eastward and join up with a trough moving across western Canada, while the rest of the low splits southward and moves to Arizona. Our CWA will then dry out and become significantly colder, with low temps Saturday morning in the teens and 20s in the mountains and lower to mid 30s in the valleys, while high temps stay only in the upper 40s and 50s.

Winds will stay light through Thursday morning, then increase from the southeast Thursday afternoon, especially in and south of the Snake Basin. Gusts to 35 mph will develop between Boise and Twin Falls Thursday afternoon. Winds will shift to west or northwest Thursday night behind the cold front, and continue 15 to 25 mph with gusts to 35 mph Friday.

LONG TERM /Saturday through Wednesday/

Low pressure will stay southeast of the region for the weekend, allowing a short-lived ridge of high pressure to build in and bring dry conditions and a slight warming trend through Sunday. Temperatures will climb to about 5-10 degrees above normal for Sunday and Monday, with cooler and wetter conditions returning thanks to a Gulf of Alaska low pressure system moving inland from the northeastern Pacific. Lingering forecast uncertainty exists for the timing and track of this low pressure system, with disagreement present in ensemble solutions and between deterministic models. Current forecast shows this cut off low bringing much cooler, below normal temperatures and a 30-50% chance of precipitation over higher terrain from Tuesday to late Wednesday. Snow levels will likely range from 5500-6500 feet, depending on the track of the low pressure, with light mountain snow accumulation by early Thursday.

AVIATION /18Z Wednesday through Thursday/

Issued 1138 AM MDT WED OCT 21 2026

Generally VFR. Localized MVFR/IFR in smoke from prescribed burns today. Scattered rain showers and isolated t-storms moving from south to north late tonight with precipitation becoming widespread tomorrow. Gusty outflow winds up to 50 mph, small hail, and heavy rain in stronger t-storms tomorrow. Mountains obscured in heavy rain and low clouds. Surface winds: E-SE 5-10 kt except NE-E 10-20 kt near KMUO to KGNG, then variable less than 10 kt. Winds aloft at 10kft: SE-S 15-30 kt.

KBOI...VFR. A 30% chance of a brief shower or thunderstorm overnight, with widespread rain showers and isolated embedded thunderstorms tomorrow evening. Brief heavy rain, small hail, and gusty outflow winds up to 35 mph possible in showers and thunderstorms overnight. Surface winds: S-SE 5-10 kt.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Flash Flood Watch from Thursday evening through Friday morning for IDZ013. OR...None.