415
FXUS65 KBOI 170043
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
643 PM MDT Wed Sep 16 2026

.UPDATE...Updated aviation discussion.

&&

.KEY MESSAGES...
- Continued warm and dry through tonight.

- A low pressure system from southwestern Oregon will bring
  showers, scattered thunderstorms, and cooler temperatures
  Thursday through Friday.

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

&&

.SHORT TERM /Through Friday Night/...
Issued 222 PM MDT WED SEP 16 2026
An upper level low over southwest Oregon will circulate warm,
dry southerly flow over our CWA this afternoon and tonight.
Showers and thunderstorms will develop near the low tonight
only as far east as central Oregon, while monsoon moisture in
Arizona surges northward through Utah into se Idaho, but stays
east of our CWA.  By morning the monsoon surge will curve
westward across central Idaho and into the circulation around
the OR low. Showers and scattered thunderstorms will develop
in those parts of our CWA Thursday, leaving Malheur and Owyhee
Counties generally dry.  Rain amounts greater than 0.25 inch
are forecast in the Idaho mountains north of the Snake Basin,
and locally up to 0.50 inch right under the strongest
thunderstorms, but should stay light elsewhere.  The low
itself is forecast to move eastward across OR Thursday, then
directly over our CWA Friday and Friday night, then exit east
of our CWA Saturday morning.  Temperatures will cool only
slightly Thursday while upper flow is still southerly, but
more significantly on Friday when the low passes overhead.
Showers will continue over our CWA Friday, with a 20-35% chance
of thunderstorms Friday afternoon and evening favoring the
Idaho side.  Friday night will clear in OR and partially clear
in ID, and should be cooler than tonight or Thursday night.
Patchy valley fog is expected toward sunrise Saturday.

&&

.LONG TERM /Saturday through Wednesday/...
Issued 222 PM MDT WED SEP 16 2026
The upper low will shift eastward out of our area on Saturday,
continuing remnant isolated showers/thunderstorms over the
easternmost portions of our CWA. Otherwise, high pressure will
rebuild over the western US by Sunday, bringing a slight warming
and drying trend to the region through midweek. Temperatures
will reach back to near seasonal normals, with highs in the low
80s for the Treasure Valley Monday through Tuesday. Tuesday
through Wednesday, there is considerable guidance uncertainty
over how far a longwave trough steering into western BC extends
down into the northwest region. There is slight potential for
the central ID mountains to see precipitation late Tuesday into
Wednesday, as well as a slight area-wide cooldown. Otherwise,
the area will stay mostly dry and warm through midweek.

&&

.AVIATION /00Z Thursday through Friday/...
Issued 643 PM MDT WED SEP 16 2026

VFR. Increasing clouds this afternoon and tonight. Showers
increasing from south to north overnight, isolated lightning
early Thu/AM mainly across higher terrain. Showers and isolated
lightning Thu/PM, mainly north of a KBNO/KONO/KTWF line.
Outflows of around 30 kt near storms and heavier showers.
Surface winds outside of storms: Generally variable up to 10 kt
overnight. Then S-SW 5-15 kt with gusts to around 25 kt early
Thu/PM. Winds aloft at 10kft MSL: E-SE 10-20 kt.

KBOI...VFR. A 10-15% chance of showers early Thu/AM into Thu/PM.
Surface winds: NW under 10 kt this evening. Becoming SW-SE 7-12
kt with gusts to around 20 kt late Thu/AM.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Fire Weather Watch from Thursday afternoon through Thursday
     evening for IDZ403-421.
OR...Fire Weather Watch from Thursday morning through Thursday
     evening for ORZ670-671-674-675.
&&

$$

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