108
FXUS65 KBOI 212046
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
246 PM MDT Fri Aug 21 2026

.KEY MESSAGES...

- Strong thunderstorm-outflow wind event moving northward
  through Idaho this evening.

- Heat Advisory until 9 PM MDT in the Treasure Valley, western
  Magic Valley, and Upper Weiser River Valley.

- Weak Pacific cold front will bring more thunderstorms to the
  area Saturday afternoon and evening.

&&

.SHORT TERM /Through Sunday Night/...
The big issue early this evening will be strong thunderstorm-outflow
winds entering southwest Idaho ahead of a northward surge of monsoon
moisture currently in Nevada. HRRR is the most aggressive of
all models but it tends to do well in these situations. HRRR has
shown this outflow event consistently in hour-by-hour updates
since last night, moving across the NV/ID border around 6 PM MDT
this evening, through the Snake Basin 8-9 PM MDT, then
weakening as it gets into the ID mountains around 9 PM but still
holding together all the way past McCall around midnight MDT. A
Wind Advisory has been issued for winds 20-35 mph with gusts to
55 mph, i.e., not as intense as HRRR as that model may have
over-forecast it. The full force of the HRRR forecast would
bring gusts 60 to 75 mph through the Snake Basin, likely causing
damage, not to mention blowing dust and severely restricted
visibility. Even so, areas of blowing dust and reduced
visibility are expected. Strongest winds should last less than
one hour at any given location. The timing of the event looks
more definite, though, with the event entirely north of the
Snake Basin by 10 PM MDT as it continues to move north while
steadily weakening. By the time it gets to McCall, max gusts
should be less than 40 mph. As stated earlier, these winds are
due to outflow ahead of thunderstorms in a northward surge of
monsoon moisture, so that after the winds die down there will be
scattered showers and thunderstorms, but with little rain
reaching the ground tonight.

Meanwhile, we still have a Heat Advisory in effect through 9 PM
MDT this evening.  Saturday should be 5 to 10 degrees cooler than
today as a Pacific cold front moves inland and through our area.
The front will bring gusty west winds to Oregon and breezy
northwest winds to western Idaho. With monsoon moisture still
present on the Idaho side, the approaching front will help
develop more showers and storms Saturday afternoon and Saturday
night, with a better chance of rain than tonight`s storms.
Sunday will be slightly cooler again, with another chance of
late-day showers and storms on the Idaho side. Nighttime temps
will be warm tonight and Saturday night, then cooler Sunday night.

&&

.LONG TERM /Monday through Friday/...
Southwest flow aloft will continue through the long term as the
Desert Southwest upper-level ridge is tempered by semi-persistent
troughing over the Pacific NW. Enough moisture will be in place to
carry shower and thunderstorms chances through at least Tuesday
night, primarily over southwest ID, with greater uncertainty
Wednesday onward. Models show a potent shortwave trough moving
over the area on Monday afternoon, with a 20 to 50 percent
chance of showers and thunderstorms over southern ID. In far
southeastern OR, there is a 15 to 30 percent chance Monday
afternoon and evening as well. The upper-level low pressure
system is forecast to move inland mid-to-late next week,
causing dry, breezy, and slightly cooler temperatures Wednesday
through Friday. Temperatures will be around 2-5 degrees above
normal through the period.

&&

.AVIATION /18Z Friday through Saturday/...
Issued 1145 AM MDT FRI AUG 21 2026

VFR. High density altitude this afternoon from heat. Patchy
wildfire smoke. Increasing high clouds with scattered
showers/thunderstorms late this afternoon into this evening.
Surface winds: variable 5-10kt, showers and storms capable of
gusty erratic outflows up to 60 kt this evening. Winds aloft at
10kft MSL: S 10-20 kt.

KBOI...VFR. 10-20% chance of showers and thunderstorms over
terminal after Sat/00Z, but high confidence with strong outflow
wind gusts impacting the terminal. Surface winds:
variable 5-10kt, gusty outflow winds 30-50kt possible this
evening from the southwest-southeast.

Weekend Outlook...Scattered showers and thunderstorms continuing
Saturday. Gusty outflow winds 30-50 kt and locally heavy rain
from storms. Drier Sunday. Areas of wildfire smoke. Gusty SW
winds Saturday afternoon. Breezy W-NW winds all areas on Sunday.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Heat Advisory until 9 PM MDT this evening for IDZ012-014-016-
     033.
     Wind Advisory from 6 PM to 10 PM MDT this evening for IDZ012-
     014-015-029.
OR...Heat Advisory until 9 PM MDT /8 PM PDT/ this evening for
     ORZ064.
&&

$$

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SHORT TERM...LC
LONG TERM....CH
AVIATION.....CH