422
FXUS65 KBOI 081700
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
1000 AM MST Mon Dec 8 2025

.DISCUSSION...An impressive atmospheric river is lined up across
the Pacific Ocean and is beginning to impact the Pacific NW this
morning. A warm front arrives this evening, with widespread
precipitation north of a line from Burns OR to Jerome ID through
Tuesday night. Warm and moist zonal flow continues over the
next several days as a significant atmospheric river impacts the
Pacific NW across WA, northern ID and western MT. The central
ID mountains will see significant precipitation, but far less
amounts compared to northern ID. Snow will be confined to the
highest peaks above 7000-8000 feet. Current forecast on track
with no updates.

&&

.AVIATION...Generally VFR, patchy valley fog this morning.
Precip increasing from the northwest this afternoon and evening,
heaviest north of Snake Plain and expected to stick around
through the period. Periods of MVFR-LIFR in moderate/heavy
rain/snow. Snow levels today increasing to 7-9 kft MSL as the
precip arrives. Areas of LLWS as precip moves in. Surface winds:
SW-SE 5-15 kt, with gusts to 20-30 kt in E Oregon and mtns of
SW Idaho. Winds aloft at 10kft MSL: W 25-40 kt.

KBOI...VFR under increasing clouds. Light rain returning after
Tue/00Z through the period with low VFR ceilings. Surface winds: SE
4-10 kt.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Wednesday night...Clouds are on the
increase today as the leading edge of an atmospheric river
arrives on the Pac NW coast. Precipitation will develop across
Baker County and w-central Idaho this afternoon, spreading
southeast into the lower Snake Plain and Boise Mtns by early
evening. Once it starts, mtns will see nearly continuous light
to at times moderate and heavy precipitation through Wednesday
night, while lower elevations from Baker County through the
Snake Plain see periods of light precipitation. In terms of
measuring these atmospheric rivers, this one is on the upper
bounds with moisture, winds (and thus moisture transport), and
temperatures aloft ranking in the 99th percentile across our
mountains on Tuesday and Wednesday. As such snow levels will be
quick to rise tonight reaching 7-8.5kft across e-central Oregon
and w-central Idaho mtns by Tuesday morning. They relax
slightly during the day, dropping to 6.5-7.5kft before
rebounding to 8.5-9.5kft Tuesday night and Wednesday. This will
limit most snow accumulation to between Monday afternoon and
Tuesday evening with all but the highest peaks turning to rain
by Tuesday night. Snow that does accumulate will be quite wet
with snow ratios around 8:1. Total rain/snow-water equivalent
across the mtns will be a widespread 1-2 inches with local 2-3
inch amounts and potential for 3+ inch amounts through Wednesday
night. Higher terrain across the region will tap into the
strong winds aloft so that north and west facing slopes see
gusts of 30 to 45 mph through Wednesday. River flows will
certainly rise through the period in response to the rain and
some snowmelt (whatever little there is so far this season).
Rivers are forecast to stay below flood stages, but will
certainly need to keep an eye on all streams/rivers during and
after this event.

As mentioned, lower elevations of Baker County through the lower
Snake Plain will see periods of light rain during this time
with precipitation totals up to a quarter inch. The western
Magic Valley and much of SE Oregon and SW Idaho south of the
Snake River will see little if any measurable precipitation. In
line with the rising snow levels, temperatures peak on
Wednesday as many lower elevation sites reach 60 degrees. Sites
with older periods of climate records will likely not reach the
records set on that date in 1939 (for example the record high of
70 in Boise on that date), but there are a few that are in
reach.

LONG TERM...Thursday through Monday...Some reprieve in the wet
and unsettled weather late this week. The ridge that has been
shuttling moisture into our area throughout the short-term will
begin building in over our area come Thursday. This will push
the stream of Pacific moisture north and allowing our area to
dry out. Elevated precipitation (20-70%) chances will remain
mainly in our West Central and Boise mountain zones Thursday
morning, tapering off entirely by Friday morning. Thursday will
continue to feature strong flow aloft, 30-50kt at 700mb,
favoring higher terrain for any additional accumulations.
Throughout the day Thursday in Friday morning, additional liquid
accumulations of 0.1-0.3 inches are expected across the higher
peaks of Valley County, and generally under 0.1 inches in the
lower elevations of Valley County and in the far northeast
portion of the Boise Mountain zone. With a warm airmass in
place, snow levels will be up at 7-8 kft MSL; so any additional
precipitation will be predominately rain, with a rain-snow mix
above 7500 feet. The highest peaks may see a dusting up to an
inch of additional snow through Friday morning. River swells
will start to recede after Thursday. The strong winds aloft on
Thursday will translate to breezy conditions at the surface, but
not as strong as those felt in the short-term with gusts of
15-30 mph across open terrain and ridges.

Friday and Saturday are looking to be dry and less breezy as high
pressure sets in over the area. This high pressure and subsidence
aloft could bring inverted conditions to valleys. With the soil
moistures elevated from all the fun the past week, fog and low
stratus would be in play. Barring any inversions, which could trap
colder air near the surface in valleys, temperatures will be 10-20
degrees above normal throughout the long-term (warmest on Thursday
and Friday). Early next week, guidance is hinting at a trough and
associated cold front moving through, but aren`t aligned on the
timing and strength. This signal is leading to increasing
precipitation chances late Sunday into Monday.

&&

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

&&

$$

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DISCUSSION...KA
AVIATION.....JM
SHORT TERM...DG
LONG TERM....NF