498
FXUS65 KBOI 050346
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
846 PM MST Thu Dec 4 2025

.DISCUSSION...Precipitation will continue overnight, although
it will generally be lighter as the associated shortwave has
exited the area to the southeast. Overall, precipitation and
snowfall totals thus far have been close to forecast. A mix of
rain and snow was reported in Ontario this evening, but the area
from Caldwell through Boise remained as rain, with amounts
ranging between 0.05 and 0.10 inches. McCall and Bogus reported
approximately 3 inches of wet snow as of 8 PM, with web cameras
showing roads largely covered in wet slush. Consequently,
travelers heading north on Highway 55 and Highway 21 should
expect winter driving conditions tonight through Friday. An
additional 2 to 4 inches of snow is possible in the Long Valley
through Friday morning as showers persist across the area.

Snow levels will continue to rise overnight, reaching 8,000 feet
across southeast Oregon and 5,500 feet across the West Central
and Boise Mountains by Friday afternoon. This rise is associated
with the arrival of the next, stronger, and more moist system.
The higher snow levels will limit significant snowfall below
5,500 feet, leading to wet, slushy conditions on Highway 55.
However, areas above 6,000 feet, such as Banner Summit, are
expected to remain as snow through Saturday morning, with an
additional 4 to 6 inches possible. For the Treasure Valley,
expect steady rain, moderate intensity at times, from noon
Friday through at least midnight, with temperatures remaining in
the low 40s.

&&

.AVIATION...Widespread MVFR/IFR ceilings and fog expected overnight
and Friday morning. Intermittent rain showers expected overnight
through Friday morning/afternoon, and snowfall in the W-Central ID
mountains (KMYL). MVFR/IFR conditions in rain and IFR/LIFR in snow.
Snow levels 3-5 kft MSL across SW ID and 7-8 kft MSL across SE OR
overnight. Widespread mountain obscurations. Surface winds: light
and variable below 10 kts, except KTWF/KJER gusting near 25 kts
overnight and Friday morning/early afternoon. Areas of low-level W-
NW wind shear developing tonight. Winds at 10 kft MSL: W-NW 30-40 kt
overnight.

KBOI...Rain showers and MVFR ceilings tonight through Friday
morning/early afternoon, with lowering OVC ceilings. MVFR/IFR
conditions in precipitation. Surface winds: variable around 5 kt or
less.

Weekend Outlook...Lighter precipitation expected Saturday and Sunday
with snow levels 4-6 kft MSL. Rain or rain/snow mix over most areas,
with snow mainly over the West-Central ID mountains. MVFR to LIFR
conditions in precipitation with mountains obscured. Patchy fog
expected in mountain valleys overnight/morning hours on
Saturday/Sunday. Surface winds SW-NW 5-15 kt, except 15-20 kt with
gusts 25-40 kt late Friday into Saturday across higher terrain.

&&

.PREV DISCUSSION...
SHORT TERM...Tonight through Saturday night...An atmospheric
river in the Pacific is streaming a relatively warm moisture
plume up around a Pacific high, and into our area from the NW.
The flow aloft steadily increases through the short term,
mixing the warm air down to the surface by Friday and mixing
strong winds and gusts down to the surface Friday night. There
will be two major pockets of moisture that impact the area, the
first is occurring now through Friday morning, and the second
warmer, but more moist will move through Friday afternoon
through Friday night.

The first round of moisture occurring this evening will feature
an increase in PW up to the 85th to 90th percentile, and the
replacing of the cold airmass with the warmer AR airmass tonight
through Friday morning. As with most ARs the precipitation will
be closely tied to terrain due to orography. Rainfall/Liquid
equivalent during this time will be up to 0.2" in lower
elevations and up to 0.4" in higher elevations. With snow levels
at 3-5 kft MSL, snowfall will be limited to the mountains and
highlands, which could see 1-4 inches. Additionally, the timing
of the warmer airmass moving into SE Oregon overnight means a
localized freezing/icy precipitation threat will be present,
especially this evening through midnight.

Precipitation weakens briefly but continues Friday morning,
before the second round arrives that afternoon through the
evening. While we are now within the warmer airmass, PWs rise to
near the 95th percentile for this time of year. Rainfall/Liquid
equivalents are higher, with 0.1-0.4" in lower elevations and
0.4-0.8" in higher elevations. With snow levels now at 5-7 kft
MSL, increasing to the SW, snow totals rise but are limited to
high mountain towns, passes, and ridges. An extra 2-6" of snow
are possible around 5-6 kft, but up to 9" above 6 kft.

Both rounds of precipitation occur in a 30 hour window, and
given the very moist system with impacts to travel in and out of
mountain towns a Winter Weather Advisory has been issued. Storm
total rainfall/liquid equivalent is 0.1-0.5" in lower
elevations, and 0.6-1.2" in higher elevations. Similarly
snowfall below 4 kft MSL will be less than an inch to flurries
with mixed precip. Above 4kft to 6kft storm total snow of 4-7"
is expected, and above 6kft it could be as high as 8-12". Behind
the moisture, wind gusts Saturday morning through the day remain
25-35 mph. Winds highest in SE Oregon, SW Idaho highlands, and
ridgetops. The Weiser basin and Treasure Valley may end up being
just sheltered enough from the wind to see too many impacts.

LONG TERM...Sunday through Thursday...A strong upper-level jet will
ride overtop a strong upper-level ridge, existing in the Eastern
Pacific, and into the Pacific Northwest. Pacific moisture and
embedded upper-level shortwave disturbances amid the northwesterly
flow will bring periodic rain and mountain snow. Our area will be on
the edge of the storm track for much of the period, and a strong
temperature gradient will exist over the area. This increases
uncertainty in precipitation amounts and temperatures. However,
overall the pattern strongly leans mild and wet with temperatures
well above normal and periodic chances of precipitation.

A disturbance afternoon into evening Sunday will bring a 20-50%
chance of valley rain and a 50-80% chance of mountain rain and snow.
Snow levels will be around 4000-6000 feet, lowest in the morning
across the north. Precipitation amounts should be light to moderate.
Precipitation chances will dip briefly on Monday with the upper-
level ridge axis building overhead before increasing once again
Tuesday and Wednesday as an atmospheric river moves into the Pacific
Northwest. Our area will be on the southern edge of the moisture,
resulting in some uncertainty in precipitation amounts. However,
moderate precipitation totals are possible, especially in the
mountains, and the air mass is expected to be very mild with snow
levels pushing up to 6500-8000 feet. By Wednesday, high temperatures
are forecast to reach 15-20 degrees above normal. Strong
westerly/northwesterly flow will also bring breezy to windy
conditions. Forecast confidence is lower on Thursday, but strong,
moist westerly/northwesterly flow will persist across the Pacific
Northwest. Currently this forecast pattern looks to exist beyond the
extended as well.

&&

.BOI WATCHES/WARNINGS/ADVISORIES...
ID...Winter Weather Advisory until 11 PM MST Friday IDZ011-013.
OR...None.

&&

$$

www.weather.gov/Boise

Interact with us via social media:
www.facebook.com/NWSBoise
www.x.com/NWSBoise

DISCUSSION...JDS
AVIATION.....JY
SHORT TERM...JM
LONG TERM....CH