217
FXUS65 KBOI 200531
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
1131 PM MDT Sat Sep 19 2026

.UPDATE...Updated aviation 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/...
Issued 232 PM MDT SAT SEP 19 2026
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/...
Issued 232 PM MDT SAT SEP 19 2026
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 /06Z Sunday through Monday/...
Issued 1130 PM MDT SAT SEP 19 2026
Mainly VFR, with clearing skies overnight through Sunday. Patchy fog
in mountain valleys overnight through morning, with localized MVFR-
IFR conditions near KMYL. Surface winds: NW-N 5-15 kt. Winds aloft
at 10kft MSL: NW-NE 5-15 kt.

KBOI...Mostly clear skies through Sunday. Surface winds: variable or
SE 5-10 kt overnight through Sunday morning, then back to light and
variable in the evening.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...None.

OR...None.

&&

$$

www.weather.gov/Boise

Interact with us via social media:
www.facebook.com/NWSBoise
www.x.com/NWSBoise

AVIATION.....JM
SHORT TERM...JDS
LONG TERM....JDS