910
FXUS65 KBOI 100344
AFDBOI

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

.DISCUSSION...The potent atmospheric river continues to spread
precipitation across northeast Oregon and west-central/central
Idaho this evening, and is expected to last through mid-day
Wednesday before decreasing from south to north. The past 24
hour liquid precipitation totals are approximately 0.3 to 1.00
inches for the higher terrain in those areas. Meanwhile, a few
hundredths of rain were observed in the western Snake Plain,
while it remained mostly dry south and west of the Snake Plain.
Four temperature records were tied today due to the mild airmass
with this event, which included Burns and Ontario in Oregon,
and Twin Falls and Jerome in Idaho. Mild temperatures are
expected to last through at least Saturday. See previous
discussion for more.

&&

.AVIATION...Rain continuing across NE Oregon and the central Idaho
mountains through Wednesday morning, dry elsewhere. Mainly VFR
outside of precip, with MVFR to LIFR conditions in the mountains.
Snow levels 7.5-9.5 kft MSL. Mountains obscured. Patchy fog in
sheltered valleys overnight into Wednesday morning. Surface winds:
SW-SE 5-15 kt with gusts 15-30 kt, except lighter in the Snake Basin
outside of the Magic Valley. Winds aloft at 10kft MSL: W-NW 30-50
kt.

KBOI...VFR under mid-level clouds. Clouds decreasing late Wednesday
morning. Surface winds: Mainly SE 4-8 kt.

&&

.AIR STAGNATION...An upper-level ridge is expected to strengthen
over the area late this week and into the weekend, which may
create a multi-day air stagnation event in the valleys.
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 AGL. 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 AGL. Due
to this sustained poor ventilation, an Air Stagnation Advisory
may be needed later this week, specifically covering the period
from Friday through Sunday.

&&

.PREV DISCUSSION...
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.

&&

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

&&

$$

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