131
FXUS65 KBOI 182017
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
217 PM MDT Fri Sep 18 2026

.KEY MESSAGES...
- Numerous showers, isolated-to-scattered thunderstorms, and
  cooler temperatures today. Strongest thunderstorms in central
  Idaho with gusty outflow winds, hail, and brief heavy rain.

- Showers and thunderstorms continue tonight and tomorrow over
  the central Idaho mountains.

- Gradual warming and drying trend beginning Saturday and
  continuing through early next week.

&&

.SHORT TERM /Through Sunday Night/...
Currently, the upper-level low pressure system is moving
directly overhead as of 1400 MDT, causing numerous showers and
isolated- to- scattered thunderstorms over southwest Idaho.
Concern has increased through the day as we have been observing
these cells, with QPE totals (over 2+ inches SE of Glenns Ferry,
small area of 3+ inches) instigated by training thunderstorms
causing the issuance of a Flood and Flash Flood products. The
watch is of particular concern for the Boise mountains, with
extra attention on a few burn scars in the area. Isolated
thunderstorms are also ongoing over southern Malheur County and
moving into northwest Baker County. While burn scars over SE
Oregon could also see impacts from these thunderstorms,
confidence is not as high as of this forecast issuance.

These areas of showers and thunderstorms are expected to
continue through the afternoon into evening for both SE Oregon
and SW Idaho. Thunderstorms will be capable of producing gusty
winds up to 40 mph, small hail, and brief heavy rain. Abundant
lightning is also likely, with rainfall totals varying heavily
dependent on where the thunderstorm cores move over. Once late
evening rolls around, coverage will decrease greatly compared to
the afternoon, becoming largely confined to the Boise and west-
central ID mountains. While overall coverage wanes tonight,
showers and thunderstorms are expected to continue through
Saturday afternoon as moisture wraps around the eastward moving
low pressure center. Confidence of showers is high (30-50%
chance), but confidence in thunderstorms is low (10-15% chance).
Fog is likely in the late overnight to early morning periods
tonight and Saturday night, especially in areas where there was
a lot of rainfall.

Behind the low, high pressure returns as a ridge axis moves
over the region, leading to warming conditions late Saturday and
through Sunday. Temperatures warm up to about 5 degrees above
normal by Sunday.

&&

.LONG TERM /Monday through Friday/...
An upper level ridge over the Intermountain West and Rocky Mountains
will remain the dominate weather feature through the extended
period. Early in the week on Monday and Tuesday, the region will sit
under a modest northwesterly flow. Dynamics will favor development
of showers in Central and Eastern Idaho Monday and Tuesday
afternoons. The chance of precipitation each afternoon is only 10%.
Otherwise, that zonal flow to the north moderates temperatures,
holding them steady at 5 degrees above normal for most of the long
term. Upper level ridge amplifies to the east as a strong Alaskan
low pressure system approaches the Pacific Northwest. This will
bring well above normal temperatures for Thursday and Friday.
Temperatures look to run at least 5 to 10 degrees above seasonal
normals. That would mean temperatures in the upper 80s to low 90s in
the lower valleys.

&&

.AVIATION /18Z Friday through Saturday/...
Issued 1138 AM MDT FRI SEP 18 2026
VFR in eastern Oregon. Mainly VFR in southwest Idaho with
occasional MVFR through early Sat/AM amidst scattered to
widespread showers and isolated thunderstorms. Storms this
afternoon could produce outflows gusts up to 35 kt. Surface
winds outside of storms: W-NW 5-15 kt with gusts to 20-25 kt
Fri/PM. Winds aloft at 10kft MSL: NW-N 5-15 kt.

KBOI...Mainly VFR. Showers developing N and E of KBOI this
afternoon. Highest chances for showers over terminal will be
late Fri evening, with brief MVFR possible. 10% chance of
lightning at the terminal. Clearing early Sat morning with SCT
clouds and light winds. Surface winds: NW 5-15 kt.

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...Flash Flood Watch until midnight MDT tonight for IDZ013-028.
OR...None.

&&

$$

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SHORT TERM...CH
LONG TERM....JDS
AVIATION.....JR