563 
FXUS65 KBOI 201848
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
1248 PM MDT Sun Sep 20 2026

.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.

&&

$$

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AVIATION.....KA
SHORT TERM...JM
LONG TERM....JM