038
FXUS65 KBOI 091657
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
957 AM MST Tue Dec 9 2025

.DISCUSSION...Continued warm and moist conditions are
dominating the region this week due to an Atmospheric River (AR)
event impacting much of the Pacific Northwest and the Northern
Rocky Mountains. This system will produce moderate to locally
heavy rain across the northern third of our County Warning Area
(CWA), specifically Baker County, Oregon, and Idaho north of the
Snake Basin. Between 0.50 and 0.75 of liquid precipitation is
expected from now through Wednesday across the above mentioned
areas. The snow level is expected to remain high, staying above
8,000 feet Mean Sea Level (MSL). Given the current limited
snowpack, widespread river flooding is not anticipated; however,
conditions are favorable for rock slides, and a rock slide has
already been reported, closing Highway 21 near Mile Post 87.
Temperatures will remain well above normal today into Wednesday,
with lower valleys reaching the mid-50s today and possibly the
low 60s on Wednesday. Looking ahead, a strengthening valley
inversion is expected, particularly as a ridge builds over the
area on Thursday, which could limit daytime warming and allow
for the development of stratus and fog.

&&

.AVIATION...Precip continuing across E-Central Oregon and much
of SW- Idaho today. VFR conditions in light precip and MVFR/LIFR
ceilings in heavier rain and high-elevation snow. LLWS across
most sites through the morning. Snow Levels: 6.5-7.5 kft MSL
north of the Snake Plain, 8-9.5 kft MSL in/south of the Snake
Plain. Surface winds: SW- SE 5-15 kt with gusts to 20-30 kt in
Magic Valley and S`rn ridges/mountains. Winds aloft at 10kft
MSL: W-NW 40-55 kt.

KBOI...Generally VFR with light rain continuing through this
afternoon/evening, with periods of MVFR ceilings this afternoon.
Threat of LLWS through the morning until early afternoon.
Surface winds: SE 6-10 kt this morning, becoming N-NE up to 10
kt this afternoon.

&&

.AIR STAGNATION...An upper-level ridge is expected to
strengthen over the area this week, potentially setting up a
prolonged air stagnation event later in the period. Conditions
remain favorable for good mixing and precipitation through at
least Wednesday. However, the ridge will begin to expand across
the Intermountain West on Thursday, leading to mixing heights
around 2,000 feet. The ridge is forecast to reach its peak
amplitude this weekend, which will cause mixing heights to drop
further, between 1,200 and 1,700 feet. Due to this sustained
poor ventilation, an Air Stagnation Advisory may be needed late
this week, specifically covering the period from Friday through
Sunday.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Thursday night...Warm, moist
air mass, with an atmospheric river extending across the
eastern Pacific along 45N then E-SE-ward through our CWA,
will continue through Wednesday. The atmospheric river
will produce moderate to locally heavy rain in the northern
third of our CWA, i.e., Baker County/OR, and Idaho north of
the Snake Basin, with snow level staying above 8000 feet MSL.
Northern rivers will run higher but should stay safely below
flood stage, with most cresting Friday. The atmospheric river
and steady rain will shift slowly north Wednesday and Thursday
allowing our CWA to dry out. High temps today will be several
degrees warmer than yesterday as yet another warm front passes
through, and Wednesday looks warmer still, reaching 60 degrees
in many southern valleys. Only slight (1-3 degree) cooling
Thursday. On the other hand, partial clearing will allow
colder nighttime lows and patchy fog in wind-sheltered
southern valleys. Winds through Thursday will be light to
moderate southeast through southwest, strongest in south-
central Idaho this afternoon with gusts 30 to 35 mph.

LONG TERM...Friday through Tuesday...Come Friday, a building
ridge will have pushed the jet stream and Pacific moisture
plume north of our area. With high pressure setting in beneath
the ridge, we will be looking at dry conditions with partly
cloudy skies through Sunday. While the ridge will keep our
area dry, it will introduce its own subset of hazards. Model
soundings show valley inversions developing under the
subsidence aloft, This is also marked by mixing heights below
2000 ft AGL. With the added surface moisture, thanks to the AR
event in the short-term, fog and low stratus will likely be in
play Friday- Sunday. Another consequence of the inverted
conditions is increased uncertainty with regard to temperatures.
While a warmer airmass will be in place, colder air could get
trapped in the valleys. For now, am maintaining temperatures
10-15 degrees above normal (at the higher end of that range on
Friday, gradually decreasing toward the lower end by Sunday).
The added caveat would be in the inversion influence, by both
the cooler air trapping and fog/low stratus. This could tamper
down temps below the inversion... and un-luckily for us, are
not captured well by the guidance, especially this far out.

Beyond Sunday, unsettled conditions will make their way back
into the forecast. A shortwave and its associated cold front
is set to flatten down the ridge early this coming week.
However, the timing of the first system is still tricky to pin
down. While the GFS and Euro ensemble suites agree on a system
early next week, they diverge on the timing. The Euro suite
brings the system in earlier Monday, whereas the GFS suite is
lagging a little behind and holding off until late Monday/early
Tuesday. In either case, this supports increasing precipitation
chances come Monday, increasing further through Tuesday.

&&

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

&&

$$

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DISCUSSION...JDS
AVIATION.....JY
AIR STAGNATION...JDS
SHORT TERM...LC
LONG TERM....NF