240
FXUS65 KBOI 180022
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
622 PM MDT Thu Sep 17 2026

.UPDATE...Updated aviation discussion.

&&

.KEY MESSAGES...
- Upper low pressure approaching from ne California will bring
  showers, scattered strong thunderstorms, and cooler
  temperatures through Friday.

- Strongest thunderstorms in central Idaho with gusty outflow
  winds, hail, and brief heavy rain.

- Gradual warming and drying trend beginning Saturday and
  continuing through at least Tuesday.

&&

.SHORT TERM /Through Saturday Night/...
Issued 302 PM MDT THU SEP 17 2026
Southwesterly flow ahead of the approaching upper low will keep
temperatures warm across the region today, with dry air
preventing storm formation over the Treasure Valley and Owyhee
Mountains in Idaho and Malheur County in Oregon. Otherwise,
showers and thunderstorms are expected to return this afternoon
through evening over eastern OR and the west-central ID
mountains. Latest satellite imagery shows some clearing and
cumulus buildup over the aforementioned areas, aligning with the
expected instability for this evening. Gusty outflow winds up
to 50-60 mph, small hail, and brief heavy rain are the main
concerns. Abundant lightning is also likely with these scattered
storms in the afternoon, and fast storm motion will also keep
rainfall amounts low, especially over southeast Oregon. This
will increase the risk for lightning ignitions, prompting the
Red Flag Warning for scattered lightning through 9 PM MDT this
evening. Conditions will cool 5-10 degrees Friday as the low
pressure system moves overhead. Scattered showers and isolated
afternoon thunderstorms will occur again Friday, with increased
coverage over the Treasure Valley and Malheur County. Showers
will then taper off over the easternmost portions of the CWA on
Saturday with high pressure building back in over the region.

&&

.LONG TERM /Sunday through Thursday/...
Issued 302 PM MDT THU SEP 17 2026
High pressure rebuilding over the region will return a slight
warming and drying trend Sunday through Monday, with peak
daytime temperatures expected to hover in the lower 80s for the
Treasure Valley area. Late Monday through Tuesday, the area will
be sandwiched between a shortwave trough steering across the
lower Great Basin and a longwave Pacific trough moving into the
northwest coast. This pattern will keep us in moderated zonal
flow aloft on Tuesday, with seasonable conditions and a chance
of showers near the ID-NV border. Come Wednesday, the area will
come under a strong southwest flow ahead of the approaching
Pacific trough, continuing dry conditions and temperatures 5 to
10 degrees above normal through Thursday.

&&

.AVIATION /00Z Friday through Saturday/...
Issued 622 PM MDT THU SEP 17 2026

VFR. Isolated rain showers overnight, with isolated to scattered
showers and thunderstorms developing again Fri/PM, primarily
over higher terrain. Thunder capable of outflows to around 35
kt. Surface winds outside of storms: SW-NW 6-12 kt overnight.
then W-NW 5-15 kt with gusts to 20-25 kt Fri/PM. Winds aloft at
10kft MSL: S-SW 5-15 kt.

KBOI...VFR. A slight chance of rain showers Fri/AM, with
uncertainty in where the narrow band of precip will set up.
Surface winds: NW 6-12 kt, with gusts to around 18kt Fri/PM.

Weekend Outlook...Brief MVFR and mountain obscuration in
lingering showers near KTWF-KJER on Saturday, with showers
tapering off by midday. Expect VFR conditions otherwise through
Sunday with variable winds less than 10 kt overnight and in
high terrain and NW 5-15 kt in valleys during the afternoon.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Red Flag Warning until 9 PM MDT this evening for IDZ403-421.
     Fire Weather Watch from 3 AM MDT Friday through Friday evening
     for IDZ403-421-423-424-426.
OR...Red Flag Warning until 8 PM PDT this evening for ORZ670-671-
     674-675.
&&

$$

www.weather.gov/Boise

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

AVIATION.....NF
SHORT TERM...JY/LC
LONG TERM....JY