998
FXUS65 KBOI 092129
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
229 PM MST Tue Dec 9 2025

.SHORT TERM...Tonight through Thursday night...Continued warm
and moist conditions are dominating the region today through
early Wednesday due to an ongoing Atmospheric River (AR) event
impacting much of the Pacific Northwest and the Northern Rocky
Mountains. This system has already produced moderate to locally
heavy rain, particularly across Baker County, Oregon, and the
central Idaho Mountains, where rainfall rates of up to a tenth
of an inch per hour have been observed at some locations. An
additional 0.50 to 0.75 inches of liquid precipitation is
forecast through Wednesday across these areas. With the snow
level expected to remain high, above 8,000 feet Mean Sea Level
(MSL), river or stream flooding is not anticipated due to the
limited snowpack and moderate rainfall intensity. However,
conditions are highly favorable for mud and rock slides,
especially on roads through steep terrain; a significant rock
slide has already closed Highway 21 near Mile Post 87. Travelers
in the Central Idaho Mountains should exercise extreme caution
tonight into Wednesday as additional slides remain possible.

Temperatures will remain well above normal, reaching the low 60s
on Wednesday and the mid-50s on Thursday in the lower valleys.
This warm period will transition as a strengthening valley
inversion builds in earnest on Thursday as the ridge amplifies.
This inversion could limit daytime heating and nighttime cooling
in the lower valleys if stratus and fog develop, which is deemed
likely (60% chance) to develop in the Snake River Plain over the
next couple of days.

.LONG TERM...Friday through Tuesday...High pressure is expected
to continue building over the area Friday through Monday.
Precipitation is not expected over the region until late
Monday/early Tuesday. While the ridge will keep our area dry,
valley inversions may continue to develop under the subsidence
aloft, as indicated by mixing heights below 2000 ft AGL
throughout the long term period from Friday to Tuesday. Remnant
moisture from the atmospheric river event this week will likely
keep fog and low stratus in the valleys Friday-Sunday. Another
consequence of the inverted conditions is increased uncertainty
in regards to temperatures. While a warmer airmass will be in
place, colder air could get trapped in the valleys. For now, am
maintaining temperatures of 10-15 degrees above normal in the
lower to mid 50s for lower elevations and lower to mid 40s for
higher elevations, with a gradual and weak cooldown over the
weekend through Monday. Late Monday through early Tuesday will
see a longwave trough off the Pacific Northwest coast steer into
the region, with considerable guidance uncertainty over its
arrival and pattern. Snow levels are expected to mostly linger
in the 6000-7000 ft range Friday through Sunday, dropping
briefly into the 5000-6000 ft range late Sunday through most of
Monday, then returning into the 6000-7000 ft range Tuesday. With
the snow levels on the higher-end, mainly rain over lower
elevation areas and possibly mixed precipitation over mountains
will be expected Monday and Tuesday.

&&

.AVIATION...Precip continuing across E-Central Oregon and much of SW-
Idaho today. VFR conditions in light precip and MVFR/LIFR ceilings
in heavier rain and high-elevation rain/snow mix. Patchy fog in
mainly in sheltered mountain valleys this afternoon/evening. Snow
Levels: 6.5-7.5 kft MSL over Central Mountains, 9-10 kft MSL
in/south of the Snake Plain. Surface winds: SW-SE 5-15 kt with gusts
to 20-30 kt in Magic Valley and S`rn ridges/mountains. Winds aloft
at 10kft MSL: W-NW 40-55 kt.

KBOI...Generally VFR with light rain, with periods of lowering MVFR
ceilings and vicinity fog possible this afternoon/evening. Surface
winds: N-NE up to 5-10 kt, becoming SE 4-8 kt after 00Z.

&&

.AIR STAGNATION...An upper-level ridge is expected to
strengthen over the area this week, potentially setting up a
prolonged air stagnation event later in the period. Conditions
remain favorable for good mixing and precipitation through at
least Wednesday. However, the ridge will begin to expand across
the Intermountain West on Thursday, leading to mixing heights
around 2,000 feet. The ridge is forecast to reach its peak
amplitude this weekend, which will cause mixing heights to drop
further, between 1,200 and 1,700 feet. Due to this sustained
poor ventilation, an Air Stagnation Advisory may be needed later
this week, specifically covering the period from Friday through
Sunday.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...None.
OR...None.

&&

$$

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