textproduct: Boise

UPDATE

Updated aviation discussion.

KEY MESSAGES

- Gradual warming and drying trend beginning Sunday and continuing through this week.

- Next trough and cold frontal passage possible late next week.

SHORT TERM /Through Tuesday Night/

Issued 250 AM MDT SUN SEP 20 2026 Clearing skies tonight combined with recent rainfall will promote efficient radiational cooling, leading to patchy mountain valley fog development, particularly in Long Valley.

A weak upper level ridge building across the area Sunday will support stable atmospheric conditions, clear to partly cloudy skies, and well above normal temperatures through Monday. High temperatures will range from the upper 70s to lower 80s across lower valleys, with 60s to near 70 in mountain locations, sitting roughly 5 degrees above seasonal norms. Weak surface pressure gradients will keep winds light and diurnal through Sunday.

By Monday afternoon, the ridge axis shifts slightly eastward as a weakening shortwave trough approaches the Pacific Northwest coastline. Mid and high level moisture will gradually increase ahead of this system, increasing high cloudiness across southeast Oregon and west-central Idaho. Surface winds will transition to a southwesterly orientation Monday afternoon, with localized gusts of 15 to 20 mph over exposed ridge tops and open valley areas. There is still a slight chance of precipitation Tuesday evening as a Californian low moves up into the region. The bulk of the moisture stays south, but just enough creeps up for some showers to be possible in South- Central Idaho.

LONG TERM /Wednesday through Sunday/

Issued 250 AM MDT SUN SEP 20 2026 The warming trend tapers off behind the Tuesday low pressure system, but continues to rise by a few degrees late this week. Thursday is projected to be the warmest day of the long term as a trough axis and associated cold front approach, allowing valley temperatures to reach the mid to upper 80s. Temperatures drop sharply behind the frontal passage Friday and Saturday, falling below seasonal normals. Precipitation chances amongst guidance is still lower overall, but we'll likely see this change as they come into better agreement on the timing of the front.

AVIATION /18Z Sunday through Monday/

Issued 1232 PM MDT SUN SEP 20 2026 VFR. Patchy mountain valley fog overnight. Surface winds: variable 5-10kt. Winds aloft at 10kft MSL: variable 5-15kt.

KBOI...VFR. Surface winds: NW 5-10kt becoming SE 5-10kt after sunset.

BOI WATCHES/WARNINGS/ADVISORIES

ID...None.

OR...None.