Area Forecast Discussion

KEY MESSAGES

- Rain showers over the higher terrain in Idaho continue into the evening, slowly moving eastward.

- Gradual warming and drying trend beginning Sunday and continuing through the middle of next week.

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

SHORT TERM /Through Monday Night/

An upper level low has moved over eastern Idaho and southwestern Montana this afternoon. This will maintain northerly flow across the region, keeping cloud cover lingering over the higher terrain, particularly along the Idaho/Nevada border this evening. Isolated showers remain possible across the west-central mountains of Idaho through early this evening. 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. Thermal profiles remain warm enough to prevent any precipitation from reaching the surface through Monday night. Overnight lows will remain mild for late September, staying in the upper 30s to upper 40s across lower elevations.

LONG TERM /Tuesday through Saturday/

Ensemble guidance remains in good agreement regarding the overall pattern evolution through midweek, followed by increasing spread heading into the weekend. Tuesday looks to be a transitional day as the upper ridge axis gets pushed into the Northern Rockies by an incoming Pacific trough. High temperatures on Tuesday will stay mild, though slightly cooler than Monday due to mid-level cloud cover and lingering southwesterly flow.

Guidance has shifted toward a broader trough track further north, restricting the strongest moisture and dynamic lift primarily to the Idaho panhandle and Washington. This track leaves our forecast area with breezy conditions and cooler temperatures from Wednesday into the weekend. Thursday is projected to be the warmest day of the long term as the 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. Only a small fraction of ensemble guidance generates measurable precipitation with the trough/frontal passage next weekend, which is likely why NBM has no mention of precipitation in the forecast at this time.

AVIATION /18Z Saturday through Sunday/

Issued 1112 AM MDT SAT SEP 19 2026 Mainly VFR. Isolated showers redeveloping across the west- central and Boise mountains. Patchy fog in mountain valleys late tonight. Surface winds: NW-N 5-15 kt. Winds aloft at 10kft MSL: NW-NE 5-15 kt.

KBOI...Mostly clear skies this afternoon and Sunday. Surface winds: NW 5-15 kt.

Sunday Outlook...VFR conditions through Sunday with variable winds less than 10 kt overnight and in high terrain and NW 5-15 kt in valleys during the afternoon. Patchy fog in mountain valleys Sunday morning clearing as the sun rises.

BOI WATCHES / WARNINGS / ADVISORIES

ID...None.

OR...None.