FXUS65 KBOI 040401
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
901 PM MST Tue Dec 3 2024

.DISCUSSION...Forecast was updated and the Dense Fog Advisory
was expanded to include the Upper Treasure Valley, including the
Boise metro area. The foggy conditions are expected to persist
through Monday morning, with some improvement likely in the
Upper Treasure Valley while continuing in the Lower Treasure
Valley. Otherwise, expect clear skies and light winds.
Temperatures remain inverted with the lower valley highs
expected to only reach into the upper 20s to mid 30s while
higher elevations attain the 40s to mid 50s.

&&

.AVIATION...Areas of LIFR fog and low stratus in the lower 
valleys of SE Oregon and SW Idaho, including near KONO, KEUL, 
and KMAN has expanded eastward to KBOI and may reach KMUO 
overnight. Patchy mountain valley fog is also expected to 
develop, including KMYL. Elsewhere, it will remain VFR under 
clear skies. Surface winds: variable less than 5 kt. Winds aloft
at 10kft MSL: variable around 5 kt.

KBOI...LIFR in fog/low stratus overnight with improvement 
likely after Wed/18Z. Winds light and variable less than 5 kt.

&&

.AIR STAGNATION...No significant improvement through Saturday
so we have extended the Air Stagnation Advisory 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 Thursday night...Webcams in the Lower
Treasure Valley have shown no decrease in the dense fog and models
indicate little change through Friday, so we have extended the 
Dense Fog Advisory through 2 PM MST Thursday and will re-evaluate
to see if it will continue longer. Forecasts of other elements 
(temperature, sky cover, wind) should also repeat each day through
at least Friday.

LONG TERM...Friday through Tuesday...An upper level ridge will 
remain overhead through early Saturday, keeping the inversion in 
place. However, a cold upper level trough and cold front will move 
through the area Saturday night and Sunday. This system will be 
accompanied by cold air aloft and breezy surface winds, which should 
scour out the inversion. The mountains will see 10-20 degrees of 
cooling, but lower valleys that had cold air trapped at the surface 
will actually warm by 5-15 degrees on Sunday. A chance of 
precipitation will accompany the cold front Saturday night, with 
chances around 30-50% at lower elevations and 50-80% in the 
mountains. Snow levels around 4500-6000 feet will result in light 
snow accumulations in the mountains and light rain at lower 
elevations. The main trough axis will swing through on Sunday, 
keeping a slightly lower chance of rain and snow going across the 
area. Snow levels will be close to valley floors on Sunday, but 
accumulations in lower valleys are not expected at this time. An 
upper level ridge will amplify just off the coast Monday and 
Tuesday, placing our area in cool and drier northerly flow aloft. 
This will keep temperatures near to a couple degrees below
normal.

&&

.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 Thursday IDZ012.
     Dense Fog Advisory until noon MST Wednesday IDZ014.
OR...Air Stagnation Advisory until 5 PM MST /4 PM PST/ Saturday 
     ORZ061>064.
     Dense Fog Advisory until 2 PM MST Thursday ORZ064.

&&

$$

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