401
FXUS65 KBOI 050415
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
915 PM MST Wed Dec 4 2024

.DISCUSSION...Little change to the pattern and subsequent low
clouds and fog through Thursday. The lowest valleys of SE Oregon
and SW Idaho, including the Treasure Valley (mainly west of
Mountain Home) will see dense fog (1/4 mile or less) overnight,
with only minimal improvement likely during the afternoon. There
will also be flurries at times that may accumulate a light dusting
in some locations. No updates.

&&

.AVIATION...Areas of LIFR fog and low stratus in the lower
valleys will expand into the Snake Basin and mountain valleys
overnight, with surface visibility 1/4SM or less at times.
Elsewhere, skies will remain clear with VFR conditions. Surface
winds: variable less than 5 kt. Winds aloft at 10kft MSL: W-NW
5-15 kt.

KBOI...LIFR fog/low stratus will continue overnight with
surface visibility 1/4SM or less expected at times. Winds light
and variable less than 5 kt.

&&

.AIR STAGNATION...No significant improvement is forecast
through Saturday, so the Air Stagnation Advisory remain in
effect until 5 PM MST Saturday. A surface cold front late
Saturday night will mix the air at lower levels, and a cold
upper trough Sunday will produce deeper mixing and ventilation,
ending the stagnation.

&&

.PREV DISCUSSION...
SHORT TERM...Tonight through Friday night...A strong
temperature inversion will persist through at least Friday,
keeping widespread stratus clouds and fog over the lower valleys
of Malheur County and much of the Treasure Valley. This
afternoon, the stratus layer has retreated slightly west of
Boise but is expected to return shortly before sunset. There is
a 50% chance fog will extend to areas like Mountain Home and
Glenns Ferry by Thursday morning.

Over the next two days, the stratus and fog are expected to
spread eastward, covering much of the Upper Treasure Valley by
Friday. As a result, the Boise Metro area will likely remain
under stratus and fog conditions from Thursday through Friday.
Cold surfaces, such as grass and trees, may develop rime ice,
with patchy icy spots and occasional flurries possible on Friday
as a weak trough moves through the area. Commuters should plan
for slower travel due to these conditions.

Air quality will remain poor because of the stagnant weather
pattern. Forecast adjustments have been made to account for the
typical inversion-related temperature discrepancies, which often
result in models being 510 degrees too warm for the valleys and
too cold in the mountains. No updates to current weather
advisories are necessary at this time.

LONG TERM...Saturday through Wednesday...An upper level trough
and cold front will approach the area on Saturday. The front
will move through Saturday night bringing much colder air aloft.
This should mix out the temperature inversion by Sunday, with
breezy surface winds also helping to scour out the inversion.
The front will bring precipitation to the area, with
precipitation chances increasing to 20-40% in the valleys and
50-80% in the mountains Saturday night. These chances will drop
slightly on Sunday as the trough axis crosses the area.
Precipitation is mainly expected to fall as rain the lower
valleys. However, models are struggling to capture the strength
of the inversion, and precipitation could begin as freezing rain
or even snow in the colder valleys before the inversion mixes
out. Precipitation amounts overall are expected to be light, but
several inches of snow are possible in the mountains above
4000-5000 feet.

An upper level ridge will amplify off the West Coast late Sunday
through Tuesday, placing our area in northerly flow aloft and
bringing drier conditions. Temperatures will be near to slightly
below normal. By Wednesday, forecast confidence decreases as models
disagree about the strength of the ridge as it moves inland. While
most ensembles favor a stronger ridge and drier conditions, enough
bring moisture over the ridge for a 10-30% chance of precipitation.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Air Stagnation Advisory until 5 PM MST Saturday
     IDZ011>014-016-028-033.
     Dense Fog Advisory until 2 PM MST Friday IDZ012-014.
OR...Air Stagnation Advisory until 5 PM MST /4 PM PST/ Saturday
     ORZ061>064.
     Dense Fog Advisory until 2 PM MST Friday ORZ064.

&&

$$

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DISCUSSION...BW
AVIATION.....BW
AIR STAGNATION...JDS
SHORT TERM...JDS
LONG TERM....ST