539
FXUS65 KBOI 210400
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
900 PM MST Thu Nov 20 2025

.DISCUSSION...Precipitation has tapered off this evening,
leaving behind a moist atmosphere across east Oregon and
southwest Idaho. Mid to low clouds continue to engulf much of
eastern Oregon, the terrain of central/west-central Idaho, and
near the ID-NV border, with satellite imagery already showing
some expansion from nighttime radiational cooling. Elsewhere,
clearing did occur across much of the western Snake Plain around
sunset, which will now pave the way for areas of fog to form
where winds remain light. The exception will be around
Huntington, Ontario, and Weiser where northwest winds at or
above 10 mph will persist into the early morning hours. Current
hi-res models are favoring the development of fog (60-90%
chance) from the Boise metro to the Magic Valley overnight, as
well as in portions of eastern Oregon where cloud decks are
expected to lower to the surface. Fog should lift and improve
throughout the day Friday due to daytime heating, but light
winds with moist, inverted conditions could limit the amount of
stratus erosion across the valleys and basins.

&&

.AVIATION...Low to mid level VFR/MVFR stratus will continue to
decrease overnight. This will allow areas of IFR/LIFR low stratus
and fog to develop across valleys of SE Oregon and SW Idaho. Fog
most likely in sheltered valleys where winds are light. Conditions
improving Friday afternoon, but may redevelop Friday night. Surface
winds: light and variable, except locally up to 10 kt from the N-NW
overnight. Winds aloft at 10kft MSL: variable less than 10 kt.

KBOI...Mid level stratus has cleared and light winds are expected
overnight. This should allow LIFR fog or low stratus to develop (60%
chance through the night) and persist into Friday morning. Surface
winds light and variable.

Weekend Outlook...Areas of valley fog/stratus each morning with
improvement each afternoon. On Sunday evening a cold front will move
into SE Oregon. This will bring light precipitation mainly to
northern areas and mountain obscuration late Sunday night into
Monday. Snow levels 5500-7000 ft MSL. Surface winds: variable under
10 kt, except SW-NW 5-15 kt late Sunday night into Monday as the
cold front moves through.

&&

.AIR STAGNATION...High pressure 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 late Sunday into
Monday and lead to improved mixing and ventilation.

&&

.PREV DISCUSSION...
SHORT TERM...Tonight through Saturday night...At 1 PM MST radar
still showed light rain echoes from Baker County through Malheur
County into Owyhee County but slowly decreasing. Expect most of
it to end by sunset. A few more weak echoes in the Boise Mountains
and Upper Weiser River Valley should also end by sunset. Today`s
rain has added moisture to the lowest atmospheric levels which
should lead to more fog than previously forecast. Difficult to
determine where fog may become dense so no advisory will be
issued with this package. Expect little day-to-day wx change
through the short term, with a weak upper high creating an
inversion in the valleys below, with the usual fog and low
clouds. Except for a slight chance (15-20%) of rain near the
ID/NV border tonight there should be no pcpn through Saturday
night. Light winds through the period.

LONG TERM...Sunday through Thursday...High pressure and weak
flow aloft will keep Sunday mostly dry with temps remaining
above normal. However, that evening and early Monday a broad
Alaskan low will swing mostly north of the area. High terrain
in SW Idaho and SE Oregon will see a 20-40% chance of
precipitation. The strong northwesterly flow behind the low
will lower snow levels to 3.5 to 5 kft MSL, lower to the north.
This means most mountain towns could see snow with minimal
accumulation Monday morning. Temps also drop behind the low
on Tuesday, down to normal for this time of year. Weak high
pressure will again build into the area, but flow will remain
strong and moist enough for another shot at precipitation
Wednesday. Precipitation is likely to be very orographic as
only a few model members show a shortwave providing dynamics.
Precipitation will last through much of Thanksgiving day, but
that night another brief dry spell will set in before what is
shaping up to be a much colder weekend.

&&

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

&&

$$

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