011
FXUS65 KBOI 041827
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
1227 PM MDT Fri Sep 4 2026

.UPDATE...Updated aviation discussion.

&&

.KEY MESSAGES...
- Showers and thunderstorms with local heavy rain today.

- Areas of valley fog late tonight and early Saturday morning.

- Temperatures 10 to 20 degrees below normal today, then around 10
  degrees below normal Saturday and Sunday.

- Cool Monday, then warming next week, with an increasing
  chance of thunderstorms Wednesday afternoon through Friday.

&&

.SHORT TERM /Through Sunday Night/...
Issued 218 AM MDT FRI SEP 4 2026
At 1 AM MDT radar showed a moderate to heavy rain band extending
from Washington County/ID SSW-ward through Vale, OR, into central
Malheur County/OR. Since then the band has shifted eastward into ID
and has weakened since late evening when it had heavier rain in
eastern OR. Radar showed the heaviest rain just west of Weiser/ID
around midnight MDT, and a Flood Advisory was issued there for urban
and small stream flooding until 3 AM MDT. Latest hi-res models show
the rain band becoming stronger again in the eastern parts of Boise
and Valley counties starting around 3 AM MDT, and especially the
Wapiti Burn scar. A Flash Flood Watch is in effect on Burn scars
through Friday in most of our CWA (except south of the Snake Basin),
but greatest concern is on the Wapiti Burn Scar as the band should
move right overhead 4-5 AM MDT. Meanwhile, a cold front reached Twin
Falls and Jerome just before midnight with westerly gusts to 40 mph.
The front will continue east today while colder air comes in from
the west. Pre-dawn temps were in the 40s in the valleys and upper
30s in the mountains, with snow reported above 5500 feet in eastern
OR. The cold air and pcpn were due to a deep upper low quasi-
stationary on the Oregon coast. The low is forecast to gradually
fill (i.e., weaken) through Saturday before moving inland Sunday and
through our northern zones Sunday night. Back to today, instability
will increase this afternoon leading to scattered thunderstorms in
eastern Oregon late morning through afternoon, and southwest Idaho
(mainly Owyhee County) by mid- afternoon and possibly the Treasure
Valley late afternoon. Stronger storms will feature heavy rain and
hail. Improving conditions will take place overnight through
Saturday evening, but areas of fog will form overnight into Saturday
morning after today`s rain. Becoming wetter again Saturday night
through Sunday night, with a chance of thunderstorms Sunday
afternoon/ evening, as the upper low moves through. Temps will stay
below normal through the weekend, but Sunday should be a couple
degrees warmer than Saturday.

&&

.LONG TERM /Monday through Friday/...
Issued 218 AM MDT FRI SEP 4 2026
High pressure aloft will rebuild starting Monday as the upper
low exits through Montana into southern Canada. The higher
pressure will bring warmer air back into our CWA the rest of
the week, with temps rising back to normal or several degrees
above Wednesday and beyond. The next upper low is forecast to
stay off the Oregon coast, but latest models (GFS more than
ECMWF) show southwesterly flow aloft tapping into moisture from
the next westward-moving hurricane in the Pacific subtropics.
We therefore have a slight chance of thunderstorms in the
forecast as early as Wednesday afternoon, but more likely
Thursday and Friday.

&&

.AVIATION /18Z Friday through Saturday/...
Issued 1158 AM MDT FRI SEP 4 2026
Mostly VFR. Scattered showers and isolated thunderstorms this
afternoon and tonight. Areas of MVFR/IFR and mountain
obscuration in heaviest pcpn. Areas of MVFR/IFR in valley fog
after 05/06z. Snow level 7.5k-9.0k feet MSL in E Oregon, and
8.5-10k feet in w-central ID. Surface winds: SW-NW 5-15 kt.
Gusts to 25 kt this afternoon. Winds aloft at 10kft MSL:
SW 15-25 kt.

KBOI...VFR with scattered showers this afternoon. A 20-30%
chance of thunderstorms from 21Z-24Z. VFR tonight with a 30-40%
chance of showers. Surface winds: NW 5-15 kt. Gusts to 20 kt
this afternoon. Winds variable less than 5 kts after 05/06Z
with areas of fog in the vicinity.

Weekend Outlook...Mostly VFR. Scattered showers Saturday morning.
Activity decreasing from SW to NE late morning. Then, showers
increasing again Sunday, with highest chances (50-70%) over east
Oregon and w-central Idaho. Localized MVFR/IFR and mountain
obscuration in heaviest pcpn. Areas of valley/low stratus
Saturday/ Sunday mornings. SW-NW surface winds 5-10 kt with PM
gusts to 20 kt.&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Flash Flood Watch until 7 PM MDT this evening for IDZ011-013.
OR...Flash Flood Watch until 7 PM MDT /6 PM PDT/ this evening for
     ORZ061>063.
&&

$$

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AVIATION.....TL
SHORT TERM...LC
LONG TERM....LC