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