211
FXUS65 KBOI 211634
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
934 AM MST Fri Nov 21 2025

.DISCUSSION...Dense Fog Advisory in the Treasure and Magic Valleys
will expire at 10AM MST, but lighter fog will continue until noon
to 2 PM. Latest models strongly recommend persistence forecast
through Sunday evening, before a cold front approaches from the
northwest. That means another two nights of valley fog, likely
requiring a Dense Fog Advisory again, but not re-issued at this
time. Current forecast is good through Saturday but we will
raise min temps for Saturday night and Sunday night (in the
afternoon package, in case any surprises appear in hi-res models
before that). Near the end of November, latest models show a
pattern change to northerly flow aloft and much colder air into
western North America. We`ll see if that continues to hold true
in subsequent model runs.

&&

.AVIATION...Fog and low stratus continue across the area, though
a broad shield of lower ceilings seems to be clearing in SE OR
and far SW ID. LIFR-IFR fog/stratus will begin to dissipate
around Fri/17z. Conditions improve this afternoon and evening
before redevelopment of fog/stratus tonight. Most confidence
for LIFR is in the Snake Plain and mtn valleys. Surface winds:
light and variable. Winds aloft at 10kft MSL: variable less
than 10 kt.

KBOI...LIFR fog will begin to lift around 17Z, lingering near
the foothills until 20Z. VFR through the rest of the day with
another 50% chance of fog tonight. 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 support stable
conditions through Sunday night, with daytime mixing heights
peaking near or slightly above 2000 feet AGL. Light winds and
strengthening near-surface inversions will result in relatively
poor ventilation in the valleys. A cold front will arrive
Monday morning and lead to improved mixing and ventilation.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Sunday night...An upper ridge will
remain over the region through much of the weekend, resulting
in dry weather under mostly clear skies. This setup, along with
the recent rainfall, has allowed for widespread fog and stratus
to develop across much of the Snake Plain and many sheltered
mountain valleys this morning. The worst conditions are from
Meridian through the Western Magic Valley, with visibilities
below a quarter mile. Thus, a Dense Fog Advisory will remain
in place.

Fog will likely lift by early afternoon with stratus lingering
in the Snake River Plain into the evening, especially for the
Lower Treasure and Western Magic Valleys. This could keep
afternoon highs a few degrees cooler.

Stratus and fog will redevelop again this evening, likely
shifted further west, with the Treasure Valley more likely
(60 percent chance) to see dense fog. Areas east of Boise
should see more low stratus. Either way, low temperatures in
the valley will be far warmer than suggested by most guidance,
as fog and stratus will keep temperature in the valley well
above freezing.

Much of the fog and stratus will dissipate Saturday afternoon,
though the Lower Treasure Valley will remain in the stratus the
longest. It will likely see more stratus again Saturday night
into Sunday, mainly due to the light southeasterly surface winds
expected ahead of the next cold frontal passage early Monday.
The front does look to begin light precipitation Sunday night
in Baker County and the West Central Idaho mountains, with snow
levels between 5500 and 6500 feet. However, the front does not
look strong enough to completely break the valley inversion.

LONG TERM...Monday through Friday...An upper Alaskan low is
expected to steer eastward across the Pacific Northwest and
Northern Rockies Monday, bringing a 20-40% chance of
precipitation mainly over higher terrain. Northwest flow from
this low passage is expected to drop snow levels down to
2000-3000 ft, favoring the potential for snowfall. Valley
daytime temperatures are expected to drop into the mid to upper
40s Monday through Wednesday. Long-range ridging is expected to
build behind this low, but there is slight guidance uncertainty
over the steering of an oncoming moisture band off the Pacific
NW coast; precipitation remains possible for higher terrain on
Tuesday. Wednesday through Thursday will also see chances of
precipitation throughout the area, with snow remaining possible
over higher elevations, as snow levels are expected to trend up
into the 4000-5000 ft range and into the 6000 ft range for
lower elevations. Valley daytime temperatures are expected to
take on a slight upward trend into the upper 40s and lower 50s
Thursday through Friday. Late Thursday into Friday we see
considerable guidance uncertainty over the arrival of either a
long wave trough off the NW or a closed upper low digging into
our area.

&&

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

&&

$$

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DISCUSSION...JDS
AVIATION.....JM
AIR STAGNATION...LC
SHORT TERM...JDS
LONG TERM....JY/LC