102
FXUS65 KBOI 220402
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
902 PM MST Fri Nov 21 2025

.DISCUSSION...High clouds are currently passing over the
forecast area through early morning, which may act as a limiting
factor for fog and low stratus growth tonight until they exit
to the southeast. In addition, the clearing of low clouds this
afternoon, along with the lack of antecedent precipitation, has
led to slightly drier conditions this evening than this time
yesterday. Latest hi-res models have picked up on this, reducing
dense fog coverage in the Snake Plain and delaying the onset of
fog/stratus compared to yesterday. However, the best chance for
impact will be south-central Idaho, including the Magic Valley,
where stratus current resides and dewpoint depressions remain
minimal. With further cooling, the Magic Valley will become
susceptible to areas of dense fog throughout the night. Some
dense fog is still a concern in portions of the Treasure Valley,
with latest HREF run indicating the best chance of occurrence
to be mid to late morning. Currently, dense fog chances across
the western Snake Plain are 30-50%, but the outliers will be the
Boise metro at 10% chance and the area between Boise and
Mountain Home at 60% chance. Elsewhere, valleys and basins in
eastern Oregon and west-central Idaho are still set to become
impacted by areas of fog or low stratus beginning in the next
few hours. Drying south to east winds are expected to increase
Saturday afternoon, which will help lift and dissipate
fog/stratus and yield mostly sunny conditions for most.

&&

.AVIATION...Patches of MVFR stratus in the mountains and near the
Nevada border this evening. Areas of LIFR-IFR fog/stratus will
redevelop in the valleys overnight, improving late Saturday morning.
Coverage of fog/low stratus may be lower than last night. Surface
winds: light and variable, becoming E-SE 5-10 kt Saturday morning.
Winds aloft at 10kft MSL: W-SW 5-10 kt.

KBOI...VFR conditions are expected overnight. A less than 30% chance
of LIFR-IFR fog or low stratus will develop at the terminal Saturday
morning 12-17Z, with higher chances southwest of the terminal.
Surface winds SE around 5 kt.

Sunday Outlook...Areas of valley fog/stratus Sunday morning with
improvement in the afternoon. A cold front late Sunday will bring
light precipitation mainly to northern areas and mountain
obscuration. Snow levels 5500-6500 ft MSL Sunday evening, falling to
3000-4500 ft MSL by Monday morning. Surface winds: SE-SW less than
10 kt early Sunday, then SW-NW 5-15 kt Sunday night.

&&

.AIR STAGNATION...High pressure aloft will continue through
Sunday night, with daytime mixing heights peaking between 1500
and 2500 feet AGL in the valleys. Relatively light winds and a
low-level inversion will continue to limit ventilation in most
valleys. A cold front late Sunday into early Monday morning
will bring increased west to northwest winds, improving mixing
and ventilation.

&&

.PREV DISCUSSION...
SHORT TERM...Tonight through Sunday night...Latest forecast
models all indicate persistence through Sunday, and that means
another Dense Fog Advisory for the Magic Valley and Treasure
Valley in Idaho late tonight through Saturday morning. Another
advisory may be needed Saturday night through Sunday morning,
but will address that possibility on Saturday. Little day to
day change is expected through Sunday in temperature, wind,
humidity, or sky cover. Changes will begin Sunday night as
a cold front approaches from the northwest, beginning with
increasing clouds then improved low-level mixing via light
southeast wind. A 30-40 percent chance of light rain and snow
will spread across Baker County and the west-central Idaho
mountains by midnight MST Sunday night, then a 40-70 percent
chance overnight as far south as the Boise Mountains and highest
southern peaks. Snow level will be 4000 to 4500 feet MSL north
and 4500-5200 feet south, with new snow up to two two inches.

LONG TERM...Monday through Friday...A broad Alaskan low moves
across Canada and the Northern Rockies Monday through Tuesday.
While it will bring a 20-40% chance of precipitation across
higher terrain, it will also bring strong northwesterly flow
to the area. This will push a colder air mass into the area and
create gusty conditions near the surface. Temps are expected to
drop to near normal on Monday/Tuesday. As the cold front passes
on Monday, snow levels drop from 5-6 kft MSL that morning to
3-4 kft MSL that evening, with precipitation possible all day.
This would allow precip later in the day to fall as snow in
lower elevations. Gusts with this system will be highest Monday
at 15-25 mph, higher on ridges. The gusts and cold front should
help scour out any lingering inversion from the weekend.

As high pressure builds, later in the day Tuesday will still
be cool but drier. The ridge axis will be very broad, with
relatively low amplitude, which will work to keep the flow
aloft strong. That strong flow will pull in more Pacific
moisture, and the weak ridging will warm us up to 5 degrees
above normal Wednesday through Friday. Precipitation chances
are 20-30% in lower elevations and 40-50% in higher elevations
each day. Snow levels rise from 3-4 kft MSL to 5-6 kft MSL on
Wednesday and stay that way through the long term. This will
prevent much of the possible snow accumulation in mountain
towns, but flurries or mixed precipitation is likely. Late
Friday, models show a trough quickly moving into the area,
increasing precipitation potential. Supplemented by an
amplifying ridge in the Pacific, it looks like this system
could really cool us down past the long term.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Dense Fog Advisory until 10 AM MST Saturday IDZ012-014-016.
OR...None.

&&

$$

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DISCUSSION...SH
AVIATION.....ST
AIR STAGNATION...SH
SHORT TERM...LC
LONG TERM....JM