531
FXUS65 KBOI 211148
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
548 AM MDT Mon Sep 21 2026

.UPDATE...Updated aviation discussion.

&&

.KEY MESSAGES...
- Mostly dry with warming temperatures through Thursday.

- Chance of showers and thunderstorms Tuesday, mostly across
  higher terrain of southwest Idaho.

- Cooler temperatures heading into this weekend with a chance
  of precipitation across SE Oregon and portions of SW Idaho
  Friday.

&&

.SHORT TERM /Through Wednesday Night/...
Issued 245 AM MDT MON SEP 21 2026
Broad upper ridging will keep warm and dry conditions over the
region on Monday. This morning, some mountain valley fog is
possible in Central Idaho high terrain. But as the sun rises in
clear skies temperatures reach 5-10 degrees above normal, or
about 5 degrees warmer than Sunday. An eastward shift in the
ridge axis on Tuesday will allow for a closed low to lift
northeastward through the Great Basin. This will bring a 20-30%
chance of showers and thunderstorms to higher terrain along the
Idaho/Nevada border, the Central Idaho mountains and the
mountains in Baker County Tuesday afternoon. Temperatures may be
a degree or two cooler on Tuesday compared to Monday, but a
general warming trend continues into Wednesday as the cut off
low moves east, heights rise, and stronger warm southwesterly
flow aloft moves in.

&&

.LONG TERM /Thursday through Monday/...
Issued 245 AM MDT MON SEP 21 2026
A ridge will remain over the area on Thursday. Thursday will be
the hottest day in the 7-day period, with temperatures near 90
degrees in lower elevations and 80 degrees in higher elevations.
A trough moves into the Pacific NW Friday/Saturday with cooler
temperatures and increased chances (15 to 25 percent) of
precipitation this weekend. Precipitation chances will be
highest in Northeastern Oregon and West-Central Idaho, with
generally dry conditions as one approaches the Nevada border.
Temperatures fall to near normal on Friday, and 5 to 10 degrees
below normal on Saturday and Sunday. The temperature drop may
shift fully to Friday or Saturday as synoptic models come into
better agreement on the cold front passage, causing a sharper
cool off on Friday or prolonged warmness on Friday respectively.
Cooler temperatures remain through the weekend into next Monday
as a trough axis entrenches itself along the coast and the jet
stream stays overhead.

&&

.AVIATION /12Z Monday through Tuesday/...
Issued 440 AM MDT MON SEP 21 2026
VFR. Sparse mountain valley fog in Central Idaho this morning
quickly clears around sunrise. High clouds moving in this
afternoon. Surface winds: mainly variable or S-SE 5-10kt. Winds
aloft at 10kft MSL: S 5-10kt.

KBOI...VFR with clear skies, high clouds move in this afternoon. Surface
winds: SE 5-10kt, brief light NW this afternoon before shifting
back to SE late this evening.

&&

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

OR...None.

&&

$$

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