226
FXUS65 KBOI 212247 CCA
AFDBOI

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

.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.

&&

.AVIATION...Low clouds remain, though only MVFR-VFR. A band
of IFR stratus is generally lifting near the Magic Valley this
afternoon. Conditions won`t deteriorate again until midnight,
where fog returns across the area, most likely in mtn valleys
and the Snake Plain. LIFR-IFR fog/stratus will last through
the morning. Surface winds: light and variable. Winds aloft
at 10kft MSL: variable less than 10 kt.

KBOI...VFR with low ceilings this afternoon generally clearing
to partly cloudy. LIFR-IFR conditions expected to return
tonight in fog/low stratus. The chance of LIFR is around 40%,
with the best estimate of formation time being Sat/06Z. Surface
winds remain light and variable.

Weekend Outlook...Areas of valley fog/stratus Saturday/Sunday
mornings with improvement each afternoon. Snow levels 5500-6500
ft MSL. Surface winds: SE and variable under 10 kt Saturday and
early Sunday, and SW 5-15 kt Sunday night ahead of a cold front
Monday.

&&

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

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Dense Fog Advisory from 11 PM this evening to 10 AM MST
     Saturday IDZ012-014-016.
OR...None.

&&

$$

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SHORT TERM...LC
LONG TERM....JM
AVIATION.....JM
AIR STAGNATION...LC