217
FXUS65 KBOI 132207
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
307 PM MST Sat Dec 13 2025

.SHORT TERM...Tonight through Monday night...A ridge of high
pressure will keep conditions dry and stable through early
Monday. Patchy fog will redevelop tonight, mainly in the Lower
Treasure Valley, mountain valleys, and sheltered valleys across
southeast Oregon. Temperatures on Sunday will be similar or
slightly cooler than today as the ridge axis shifts east and
temperatures aloft begin to cool. Additionally, a valley
inversion will remain in place, supporting temperatures trending
a couple degrees lower than today. Winds overall will be light
with a slight breeze across higher elevations and the Snake
Basin east of Boise. On Monday, an atmospheric river will move
into the Pacific Northwest but will weaken as it moves inland.
This will bring a 20-50% chance of rain in the lower valleys
Monday night, and a 50-80% chance in the mountains. Rain amounts
will be light, and snow will be limited to elevations above
7000-8000 feet.

.LONG TERM...Tuesday through Saturday...Unsettled weather conditions
permeate the long term. To begin the period, Tuesday will start with
semi-zonal flow as an upper-level disturbance rounds the Gulf of
Alaska low and looks to come onshore into the Pacific NW. Another
bout of high moisture levels from the subtropics will follow this
disturbance inland, providing the first round of high PoPs Tuesday
night into Wednesday. A potent cold front will also accompany this
trough feature Tuesday night into Wednesday, greatly increasing
winds and dropping temperatures 5-10 degrees, although temperatures
will remain 10-15 degrees above normal. Snow levels will drop behind
the cold front, but the coolest air will remain north of the area.
This will cause snow levels to drop from 6500-8000 ft MSL before the
front to 3000-5000 ft MSL after the front, even with continued cold
air advection through Wednesday evening. Still, snowfall will
accumulate in the mountains and some foothills.

A brief respite from precipitation will come late Wednesday into
early Thursday from a short-wave ridge, however, it will be short
lived. Another longer-wave trough (when compared to the previous on
Tuesday) will bring a strong fetch of moisture inland across the
Pacific Northwest on Thursday. This trough will be associated with a
more prominent atmospheric river, distributing another round of
hefty precipitation across the region. Snow levels across E
Oregon/SW Idaho will initially begin around 4-6 kft MSL Thursday,
rising briefly overnight Thursday into Friday to 5.5-7 kft MSL, then
gradually lowering from 4-6 kft MSL midday Friday to 2.5-4.5 kft MSL
overnight Friday into Saturday behind a cold front. Another boost in
surface winds is also expected Thursday night into Friday with this
system.

Cooler, wet, and breezy conditions will persist into the weekend as
this progressive, moist pattern stays locked in. Precipitation
totals throughout this extended period may exceed 2" in some of the
higher terrain, with 0.25 to 1" possible for lower elevations.
Snowfall totals of over 2 feet are certainly possible for the
mountains, though the oscillating snow levels will undoubtedly
affect accumulations...causing the snow accumulation forecast to be
much more uncertain.

&&

.AVIATION...VFR, except in patchy valley fog/mist tonight into
Sunday morning, including at/around KBKE, KBNO, KMYL, and KONO-KEUL.
Surface winds: variable around 10 kt or less. Winds aloft at 10kft
MSL: W 10-20 kt, becoming SW 15-25 kt tonight.

KBOI...VFR, high clouds. Surface winds: light/variable, becoming SE
4-7 kts overnight.


&&

.AIR STAGNATION...An upper level ridge will remain over the
area through Sunday, resulting in stagnant conditions. Mixing
heights will peak Sunday afternoon at around 1,000 to 2,000 feet
AGL, and winds will generally remain less than 10 mph resulting
in poor ventilation. An Air Stagnation Advisory remains in
effect through Sunday. There is potential for stagnant
conditions to persist into Monday before an active pattern
brings increased mixing and winds, as well as precipitation, on
Tuesday and Wednesday.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Air Stagnation Advisory until 5 AM MST Monday
     IDZ011>014-016-028-033.
OR...Air Stagnation Advisory until 5 AM MST /4 AM PST/ Monday
     ORZ061>064.

&&

$$

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SHORT TERM...ST
LONG TERM....CH
AVIATION.....CH
AIR STAGNATION...ST