109
FXUS65 KBOI 111649
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
949 AM MST Thu Dec 11 2025

.DISCUSSION...Today through Friday afternoon...High pressure
over the region will continue through the remainder of the week,
keeping warm and dry conditions over most areas. Temperatures
are expected to remain on the warmer end today and tomorrow,
with peak valley temperatures climbing in the mid to upper 50s
today and in the lower to mid 50s tomorrow. The West-Central ID
mountains (near McCall) may see up to 20% chance of light rain
this afternoon. Winds will stay on the weaker end from the
Southeast/Southwest for most areas, except in the Magic Valley
where Southwest gusts up to and around 20 mph are likely this
afternoon. Patchy morning fog possible in sheltered mountain
valleys Friday morning. Low mixing heights are still expected
tonight through the weekend, so an air stagnation advisory
remains intact Friday 12Z through Sunday 12Z.


&&

.AVIATION...Mainly VFR with high clouds. Patchy fog will
persist across sheltered valleys of SE Oregon and SW Idaho
through 19z. Scattered light showers returning to the west-
central Idaho mountains this afternoon, 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. Surface winds: SE less than 10 kt, becoming NW 4-8 kt
this afternoon.

Weekend Outlook...Mostly VFR with low confidence in low stratus and
fog in sheltered valleys each morning. Winds will generally be from
W-SW less than 10 kt.


&&

.AIR STAGNATION...A broad upper-level ridge will continue, with
mixing heights lowering to 1,500 to 2,500 feet AGL today and to
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 will
remain in effect Friday through Sunday. A system early next week
may provide increased mixing and winds, ending the air
stagnation, with an active pattern expected beyond.


&&

.PREV DISCUSSION...
SHORT TERM...Today through Saturday night...Continued warm and
dry several more days under a broad long wave upper ridge anchored
over the eastern Pacific and western US. The atmospheric river in
the north Pacific has shifted north and now comes inland across
WA and northern ID. A surface inversion, along with light winds,
has created air stagnation in the valleys that will continue
through at least Sunday night. High temps will continue 15 to
20 degrees above normal, but not quite as warm as yesterday.
Low temps should get gradually colder in the valleys as the
inversion persists, but not as cold as it would be with snow
cover. We should also have less valley fog than Wednesday
morning as the air mass continues to slowly dry out.

LONG TERM...Sunday through Thursday...Above normal temperatures
with clearing skies will continue air stagnation concerns as high
pressure remains over the Intermountain West through Monday. The
ridge axis will gradually move east on Sunday, bringing a slight
reduction in temperatures and an increase in clouds as a weak low
skirts to our south over southern Nevada. This low will bring a
slight uptick in westerly winds, but likely won`t bring enough
mixing to clear out any lingering low clouds or stagnant air in
sheltered valleys. That pattern shift looks to come on Tuesday
with another atmospheric river. This plume of moisture is poised
to move into the Pacific Northwest on Tuesday, bringing high
terrain showers primarily in southeast Oregon and central Idaho.
Snow levels will likely remain elevated around 7000-8500 feet
MSL thanks to warm southwest flow initially, but could lower
back to 5000-6000 feet MSL by Wednesday. There is slight
disagreement among long range models in the strength of the
Pacific High, with some ensemble solutions showing the ridge
maintaining strength over the region and keeping showers to the
north and west of southeast Oregon and southwest Idaho Tuesday
through Thursday.

With some lingering uncertainty in the evolution of the pattern,
current forecast chances for any significant accumulation of rain
or snow are minimal over our region. Temperatures look to remain
around 10-20 degrees above normal through the long term period
with later weekdays showing the best potential for significant
cooling.

&&

.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...JY
AVIATION.....JDS
AIR STAGNATION...JY
SHORT TERM...LC
LONG TERM....SA