478
FXUS65 KBOI 110406
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
906 PM MST Wed Dec 10 2025

.DISCUSSION...Dry conditions are expected overnight as the
atmospheric river remains north of our area. However, lingering
moisture near the surface will allow patchy fog to develop in
sheltered valleys of southeast Oregon and the southwest Idaho
mountains. On Thursday, the moisture plume will shift slightly
to the south as the flow aloft becomes more northwesterly,
bringing a 20-60% chance of showers to the west-central Idaho
mountains but little chance elsewhere. Snow levels will remain
very high at around 7500 to 8500 feet. Highs on Thursday will
not be quite as mild as today, but will still be 15-20 degrees
above normal.

&&

.AVIATION...Mainly VFR with high clouds overnight. However,
patchy fog (possibly dense) will develop overnight across
sheltered valleys of SE Oregon and mountain valleys of SW
Idaho. Highest chance of fog is near KBKE/KONO/KMYL. Scattered
light showers returning to the west-central Idaho mountains
Thursday, with MVFR/IFR conditions and mountain obscuration in
precipitation. Snow levels 7500-8500 ft MSL. Surface winds:
W-SE 5-15 kt. Winds aloft at 10kft MSL: W-NW 25-40 kt.

KBOI...VFR expected with fog/stratus unlikely (less than 10%
chance) through Thursday morning. Surface winds: SE less than 10
kt, becoming NW 4-8 kt Thursday afternoon.

&&

.AIR STAGNATION...A broad upper-level ridge over the eastern
Pacific and western US is expected to strengthen over our area
for the remainder of the week and into the weekend, causing
mixing heights to lower to 1,500 to 2,500 feet AGL Thursday and
1,000 to 2,000 feet AGL Friday through Sunday. At the same
time, winds are expected to be less than 10 mph resulting in
sustained poor ventilation. An Air Stagnation Advisory is in
effect Friday through Sunday. A system early next week may
provide increased mixing and winds, with an active pattern
expected beyond.

&&

.PREV DISCUSSION...
SHORT TERM...Tonight through Friday night...High pressure will
continue building over the area as the warm front over the
Southern end steers Northward, continuing warmer than normal
temperatures and drier conditions Thursday and Friday. Though
not as warm as today, valley daytime temperatures are expected
to remain in the upper 50s Thursday and Friday. Small chances of
precipitation are only expected in the Northernmost mountains
of our area (near McCall and Baker City) throughout Thursday.
Patchy fog remains likely overnight tonight and Thursday
morning, especially in sheltered mountain valleys. A cooling
trend is expected Saturday, bringing daytime valley temperatures
into the lower 50s and mountain temperatures into the upper
40s. Winds will remain on the calmer side from the
Southeast/Southwest under 10 mph today through Friday for most
areas except the Magic Valley, which is still gusting 20-25 mph
this afternoon and possibly return stronger breezes up to 20 mph
Thursday and Friday. High pressure and daytime clearing with
weak winds will favor valley inversions for the remainder of the
week and over the weekend.

LONG TERM...Saturday through Wednesday...An upper-level ridge
is expected to reach peak amplification on Saturday, leading to
the greatest potential for warming aloft and the continuation of
the valley inversion throughout the Snake River Plain. This
setup will increase the chances for fog and stratus development
in the valley, and slightly colder temperatures will be observed
as cold air becomes trapped near the surface. These stagnant
conditions will remain in place through Sunday. By Monday, the
ridge is forecast to weaken, and cloud cover will increase ahead
of the next incoming cold front. Moisture appears limited with
this frontal passage, and while the ECMWF shows the highest
precipitation amounts, overall totals across guidance remain
light. Precipitation chances are similar to previous ensemble
guidance, starting Monday afternoon and increasing to a 55-70%
chance north of the Snake Plain and a 25-50% chance in and south
of the Snake Plain by Monday evening. While the front looks
strong enough to weaken the inversion, it may not be quite
strong enough to fully break it up.

Looking further ahead, another Atmospheric River (AR) event
could return to the Pacific Northwest as early as Wednesday,
setting the stage for another round of widespread precipitation.
The forecast area looks to remain on the warm side of the jet
stream, with snow levels hovering between 5,000 and 8,000 feet
MSL from Monday through Wednesday. There is high confidence in
this system affecting the region late next week, though the
timing could be later (Thursday or Friday). Temperatures
throughout the extended period are expected to remain 10-15
degrees above normal.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Air Stagnation Advisory from 5 AM Friday to 5 AM MST Monday
     IDZ012-014-016-033.
OR...Air Stagnation Advisory from 5 AM MST /4 AM PST/ Friday to 5
     AM MST /4 AM PST/ Monday ORZ061>064.

&&

$$

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DISCUSSION...ST
AVIATION.....ST
AIR STAGNATION...ST/JY
SHORT TERM...JY
LONG TERM....JDS