982
FXUS65 KBOI 280220
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
820 PM MDT Wed Aug 27 2025

.DISCUSSION...Thunderstorms have not been as numerous today
as they were yesterday, but those that have formed were still
producing locally heavy rain. Precipitable water at Boise this
evening was 1.22 inch as measured by the balloon sounding, vs
1.53 inch early this morning, and 1.57 inch Tuesday evening.
The main activity this evening has been a line of thunderstorms
in eastern Malheur County that moved over Lake Owyhee around 7
PM MDT and was entering Canyon and northwestern Owyhee Counties
at 8 PM MDT. Although the storms were moving faster than storms
yesterday, they were still dropping rain at the rate of 2-4
inches per hour, intense enough to trigger debris flows on burn
scars, and loosen rocks and mud on steep slopes. We issued a
Flash Flood warning this evening for Leslie Gulch in eastern
Malheur County, an area notorious for flash flooding, even
though heavier rains were north of there. Latest hi-res models
take the line of storms southeastward through Owyhee County
between 9 and 10 PM MDT, with the Treasure Valley on the
northern fringe. Thunderstorm outflow winds have been reported
up to 35 mph. Relatively little activity was occurring elsewhere
in our CWA this evening but we plan to continue the Flash Flood
Watch in western Idaho until the current storms die down, or
midnight at the latest.

&&

.AVIATION...Isolated showers and thunderstorms continue this
evening southwest of the Snake River Plain, ending by
Thursday/04Z. Outflow gusts to 35 kt and heavy rain in
stronger showers. Localized MVFR/IFR in low clouds and patchy
fog Thursday morning, primarily in mountain valleys. Mountains
obscured in low clouds and heavy rain. Surface winds: variable
5-15 kt with gusts to 25 kt. Winds aloft at 10kft MSL: W-NW
15-25 kt.

KBOI...Periods of MVFR/IFR in low clouds and patchy fog near the
foothills, ending by late morning. Otherwise VFR. Surface winds:
southeast 5-8 kt becoming west-northwest 10 kt or less by
Thursday/18Z.

&&

.PREV DISCUSSION...
SHORT TERM...Tonight through Friday night...
A very moist airmass remains over the forecast area but drier
air is starting to make its way into eastern Oregon. Thunderstorms
are developing again with daytime heating and increased atmospheric
instability. Greatest potential for heavy rain is across SW Idaho
and will let the flash flood watch continue for Idaho zones into
this evening. Less storm coverage and increased storm motion has
diminished the flash flood threat enough across eastern Oregon
to cancel the watch for that area.

Showers and thunderstorms will diminish tonight as an elongated
shortwave trough axis extending from the central mountains west
along the OR/WA border lifts north. Still a chance (20-30%) of
showers and thunderstorms Thursday and Friday along the Nevada
border as an upper level ridge builds over the area. Otherwise,
dry conditions will prevail and temperatures will trend up to
around seasonal normals.

LONG TERM...Saturday through Wednesday...A broad upper ridge over
the southwest United States will build north over Idaho Saturday
through Monday, bringing a slight warming trend over the weekend.
Lower valley daytime high temperatures will be in the upper 80s
and lower 90s throughout the extended forecast period. There is
better long-range guidance agreement on an upper low off the
Pacific Northwest coast taking an odd wobbling and quasi-
stationary behavior. The system is projected to stall off the
coast of British Columbia and Washington. For now, a 20-30
percent chance of afternoon/evening showers/thunderstorms
remains in the forecast for eastern fringes of the forecast
area and along the Nevada border. Otherwise dry conditions
will prevail.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Flash Flood Watch until midnight MDT tonight
     IDZ011>016-028>030-033.
OR...Flash Flood Watch until midnight MDT /11 PM PDT/ tonight Periods of MVFR/IFR in low clouds and patchy fog near the
foothills, ending by late morning. Otherwise VFR. Surface winds:
southeast 5-8 kt becoming west-northwest 10 kt or less by
Thurs/18Z.ORZ064.

&&

$$

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DISCUSSION...LC
AVIATION.....SA
SHORT TERM...TL
LONG TERM....TL