343
FXUS65 KBOI 031200
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
600 AM MDT Thu Sep 3 2026

.UPDATE...Updated aviation discussion.

&&

.KEY MESSAGES...
- Intermittent showers and embedded thunderstorms across much of
  southeast Oregon and west-central Idaho through Friday with
  significant rainfall totals possible.

- Much cooler this week, with highs 10-20 degrees below normal
  by Thursday with periods of gusty winds.

- Cooler than normal weather anticipated late this weekend into
  early next week.

&&

.SHORT TERM /Through Saturday Night/...
Issued 312 AM MDT THU SEP 3 2026
A large upper level low remains positioned off the Pacific
Northwest coast, maintaining a moist southwest flow aloft across
southeast Oregon and southwest Idaho. A plume of Pacific
moisture interacting with synoptic forcing and upper level
energy will keep a broad band of stratiform rain with periods of
embedded convection ongoing across the region through Friday
night. Satellite and radar show a broad area of precipitation
early this morning associated with an embedded shortwave moving
northeast across Washington State. As the shortwave exits the
region this afternoon, widespread precipitation will transition
to scattered showers, allowing for occasional breaks in cloud
cover.

A cold front approaching from the west later this afternoon
will cross the forecast area overnight into Friday morning. This
front will bring another wave of widespread rain and embedded
thunderstorms, along with increased westerly winds, though wind
speeds will remain below headline thresholds. The front exits
early Friday, helping clear out cloud cover during the morning.
The resulting surface heating will introduce weak daytime
instability, triggering another round of isolated to scattered
afternoon thunderstorms.

The primary focus through Friday remains the potential for
heavy rainfall with embedded thunderstorms. Instability will be
sufficient for robust storm development, particularly during the
afternoon and evening hours. Heavy downpours could produce
locally high precipitation rates, elevating flash flood
potential over recent burn scars across southeast Oregon and the
central Idaho mountains, where Flash Flood Watches remain in
effect (including the Wapiti burn scar). Total rainfall
accumulations through Friday evening are forecast to range from
1.0 to 1.5 inches in mountain areas, with localized higher
totals in core storm centers. High temperatures through Friday
will stay 10 to 20 degrees below normal, with record low maximum
temperatures possible in portions of southeast Oregon and west-
central Idaho.

By Saturday, the upper low center remains stacked near the
Pacific Northwest coast. While lingering atmospheric moisture
will support ongoing shower chances, spatial coverage will
decrease as dynamic forcing temporarily weakens. Temperatures on
Saturday will begin a slight recovery, warming back closer to
seasonal averages under persistent southwest flow aloft.

&&

.LONG TERM /Sunday through Thursday/...
Issued 312 AM MDT THU SEP 3 2026
The large upper level low shifts slowly eastward directly over
the Pacific Northwest on Sunday. As the system pivots inland, it
will pull additional Pacific moisture into the region, leading
to an uptick in precipitation coverage and shower chances late
Sunday into Monday. Guidance continues to track moisture from
Hurricane Marie northward through the Great Basin into the
Northern Rockies. The majority of this tropical moisture is
expected to track east of the forecast area as the trough axis
sits directly overhead. However, it will remain close enough for
south- central Idaho to experience heightened chances for
heavier rainfall, with snow levels dropping low enough over the
highest peaks for a rain and snow mix.

Below-normal temperatures will persist through early next week
due to upper troughing and widespread cloud cover. Heading into
Tuesday and Wednesday, ensemble guidance suggests the low will
exit to the east, allowing an upper level ridge over the eastern
Pacific to build inland. This pattern shift will bring drier
conditions and a gradual warming trend back toward near-normal
seasonal temperatures by the middle to end of next week.

&&

.AVIATION /12Z Thursday through Friday/...
Issued 553 AM MDT THU SEP 3 2026
Numerous rain showers today, becoming widespread Thu evening.
MVFR/IFR conditions and mountain obscuration in heavier precip.
A 15% chance of embedded thunderstorms in E Oregon and w-
central ID through late this evening. Snow levels lowering to
7500 ft MSL in E Oregon tonight. Surface winds: variable 5-15
kt, then SE- SW 5-15 kt with gusts around 20-25 kt by late
morning. Winds aloft at 10kft MSL: S-SW 15-30 kt.

KBOI...Generally VFR. Light rain showers will continue through
the afternoon. Then, continuous rain this evening with a 20%
chance of MVFR. Surface winds: E-SE 5-15 kt shifting back to the
W-NW with gusts to 20 kts by 01z/Fri.

WEEKEND OUTLOOK...Showers to continue through the weekend as the
upper level trough moves across the area. Saturday looks to be
the driest with showers mainly in the higher terrain. However,
areas of fog is likely (60% chance) to develop in many of the
valley both Saturday and Sunday mornings. Showers return to much
of the area on Sunday especially across the central mountains
of Idaho. W-SW surface winds 5-15 kts with gusts 25 kts.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Flash Flood Watch from 3 PM MDT this afternoon through Friday
     evening for IDZ011.
     Flash Flood Watch through Friday evening for IDZ013.
OR...Flash Flood Watch from 3 PM MDT /2 PM PDT/ this afternoon
     through Friday evening for ORZ061>063.
&&

$$

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AVIATION.....JDS
SHORT TERM...JDS
LONG TERM....JDS