textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- A chance of showers will spread northward into the area tonight, then become widespread Thursday and Thursday night. There is a slight chance of thunderstorms this afternoon/evening, then a better chance Thursday afternoon and evening.

- 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/

Issued 453 AM MDT WED OCT 21 2026 Light showers with some embedded thunder have been ongoing overnight over SW Idaho as southerly flow aloft has continued to push moisture into the area. Despite the moisture, the area today will see minimal precipitation chances until evening. Models have been increasing in confidence about a dry slot moving overhead through the afternoon, clearing skies and keeping precipitation chances primarily to the west-central Idaho mountains and south- central Idaho. Even then, chances only range from 10-15%. If a stray shower or thunderstorm were to emerge, the primary threat would be localized heavy rainfall, with a secondary threat of gusty outflow winds. Otherwise, through the day today, warm conditions can be expected.

Once this evening/overnight rolls around, the low pressure system will begin to move onshore into California and Nevada. Precipitation chances will steadily rise in SE Oregon through the night, with high confidence in a band of precipitation moving from south-to-north. Embedded thunderstorms are possible as this band moves north overnight at 10-15% chances. SW Idaho precipitation chances start to increase late morning into the afternoon as the system moves into northern Nevada. Widespread rain and scattered embedded thunderstorms are forecast Thursday and Thursday night, especially on the SW Idaho side. Upwards of 30% chances for thunderstorms are forecast, highest in south- central Idaho and the western Magic Valley. Favorable convective parameters in this area, including MLCAPE near 500 J/kg and effective shear values 20-25 kts, will enable multicell clusters and transient supercells. The primary threat will be outflow wind gusts up to 50 mph, with a secondary threat of hail. The time frame of this threat will begin Thursday afternoon and taper off by evening.

The QPF totals are indicative of the high PWAT values, showing event totals of 0.30 to 0.50 inches in the Treasure and Magic Valleys, and 0.60 to 1.00 inches at higher elevations. Less rain will fall in Oregon on the western flank of the upper low. That area will have 0.20 to 0.30 inches in the valleys and up to 0.70 inches at higher elevations and/or closer to the ID border. Higher totals will be seen where thunderstorms cores move over. With flow aloft continuing from the south through this event, snow levels will remain above 9000 feet, so all precipitation will be rain.

The upper-level low will move overhead Thursday night into Friday, then gradually NE of the area through Friday night. Flow aloft will shift out of the NW, with a cold front moving through Thursday night. Pre-frontal winds will increase out of the SE through the western Snake plain Thursday afternoon, then shift to gusty northwest late Thursday night behind the cold front. Snow levels will lower by Friday afternoon, down to around 5000-7000 ft MSL. Higher elevation passes and ridgetops will likely only see a dusting of snow as precipitation chances gradually taper off west-to-east behind the front. Gusts up of 30-40 mph will be seen across the area, excluding sheltered mountain valleys. The highest speeds will be seen across higher terrain, ridgetops, and south/east of Boise through the western Magic Valley. Once the winds taper off Friday night, valley floors are forecast to cool down into the low-to-mid 30s. Areas of fog will form Friday morning in the wetter and wind- protected northern valleys and again late Friday night.

LONG TERM /Saturday through Wednesday/

Issued 453 AM MDT WED OCT 21 2026 Drying conditions and near normal temperatures will return for the weekend thanks to a building ridge of high pressure over the West Coast. By late Sunday, long range forecast models have once again converged on a closed low pressure system moving onshore and slowly east over the area through the early part of next week. This pattern would bring cooler and wetter conditions yet again by early next week. The highest chances for precipitation would be over the west-central Idaho mountains. With how highly variable and volatile the synoptic pattern has been even just within 24 hours of this forecast, forecaster confidence is low at this time.

AVIATION /12Z Wednesday through Thursday/

Issued 617 AM MDT WED OCT 21 2026

VFR. Generally increasing mid-to-high clouds. Showers ongoing this morning, expected to taper off late this morning. Shower and thunderstorm chances increase again mid afternoon over the west- central ID mnts and ID/NV border. Mountains obscured in precipitation. Patchy smoke near prescribed burns. Showers and thunderstorm chances across entire area Thursday. Surface winds: E-SE 5-12 kt, with gusts 20-25 kt east of KBOI developing late morning through afternoon. Winds aloft at 10kft: S 10-20 kt.

KBOI...VFR. Vicinity showers through mid morning. Surface winds: variable or SE less than 7 kt through Wednesday morning, becoming S-SE 6-11 kt in the afternoon.

BOI WATCHES/WARNINGS/ADVISORIES

ID...None.

OR...None.