581
FXUS65 KBOI 041617
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
917 AM MST Thu Dec 4 2025

.DISCUSSION...No updates to the forecast this morning. We are
tracking two primary rounds of precipitation in the short term:
one tonight and one tomorrow afternoon and evening. Tonight,
snow levels are still 3-5 kft MSL, so most mountain towns will
see snow. Below the snow level rainfall in lower elevations will
be around 0.1", and in higher elevations the rainfall/liquid
equivalent will be 0.1-0.3". This will support snowfall amounts
of 1-3 inches.

The second round Friday afternoon and evening will be much
wetter, but snow levels will be higher as a warmer airmass moves
in. Snow levels rise to 5-7 kft MSL. Generally snow levels will
be higher to the SW and lower to the NE. Rainfall in lower
elevations is forecast at 0.1-0.3" and rainfall/liquid
equivalent in higher elevations is 0.3-0.7". For areas that are
above the snow level, this will bring the bulk of the snow in
the forecast for another 3-5" in towns and up to 12" on the
highest summits.

The overall pattern is still dominated by the atmospheric river.
Though the flow directly into the forecast area is a little
weaker than some previous ARs, the moisture flux remains high
due to the tropical source of the warm and moist airmass. As we
have a warm airmass replacing our current cold continental
airmass, typical warm front concerns remain especially tonight
for the onset of the active weather. Some high resolution models
show areas of freezing rain, others keep it limited to just
snow/rain or a mix of the two. Forecast soundings show surface
wet bulbs right at 0 Celsius before the onset of the
precipitation, and quickly rising above that as the atmosphere
becomes saturated throughout. This should limit potential for
icy/freezing precip. The wet and unsettled weather continues
for about the next week, mainly in the north.

&&

.AVIATION...VFR this morning, except for patchy fog near KBNO,
mountain valleys, and near the Camas Prairie. Clouds increasing
today and gradually lowering, with widespread rain/snow arriving
from the northwest this afternoon and evening. MVFR/IFR in rain and
IFR/LIFR in snow. Snow levels 2.5-3.5 kft MSL today, rising to 3-5
kft MSL across SW ID and 7-8 kft MSL across SE OR late tonight.
Mountains becoming obscured. Surface winds: light and variable.
Areas of low-level wind shear developing tonight. Winds aloft at
10 kft MSL: NW 20-30 kt, increasing to 30-50 kt after Fri/00Z.

KBOI...VFR through early this afternoon. Increasing clouds with
lowering ceilings this afternoon, then rain developing around 22-01Z
late this afternoon and continuing tonight. Around a 30% chance of
rain mixing with snow to start, becoming predominantly rain later
tonight. MVFR/IFR conditions in precipitation. Surface winds: SE 4-7
kt this morning, becoming variable around 5 kt or less this
afternoon.

Weekend Outlook...Widespread rain and snow on Friday with snow
levels 6-8 kft MSL, except 4.5-6 kft across central Idaho. Lighter
precipitation is expected Saturday and Sunday with snow levels 4-6
kft MSL. MVFR to LIFR conditions in precipitation with mountains
obscured. Surface winds SW-NW 5-15 kt, except 10-25 kt with gusts 25-
40 kt late Friday into Saturday across higher terrain.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Saturday night...Light precipitation
will overspread the region this afternoon and evening as a plume
of deep moisture arrives in the northwest flow. Sheltered
valleys in northern Malheur and Baker counties could see a brief
period of light snow or rain/snow mix, but given the afternoon
arrival expect any impact to be minimal as temperatures will be
above freezing in most spots. In SW Idaho any early snowfall
would be limited to the lower Snake Plain this afternoon and
upper Weiser basin through early tonight. Similar to SE Oregon
any minor accumulation would be limited to colder surfaces. This
first round of precipitation will bring 1 to 3 inches of snow
above 4kft through Friday morning with liquid amounts of up to
0.15" in the valleys.

Friday will see a period of moderate to locally heavy
precipitation develop late morning-early afternoon. In the
mountains this will translate to snowfall rates of 0.5-1"/hour,
bringing 4-8" of additional snow to elevations above 6kft MSL.
While the heavier precipitation intensity will likely keep
mountain valleys in the snow through Friday afternoon,
accumulation will be fighting warmer air (temps in AOA freezing
by mid afternoon). The rain/snow elevation line continues to be
tricky. By Friday evening as the precipitation lessens expect a
broad range in snowfall totals across mtn valleys between
4-5kft MSL. Most mtn valley sites below 5kft MSL see totals
(today through Friday) of less than 2 inches, while areas from
McCall, New Meadows and Warm Lake to Yellow Pine are looking at
3 to 6 inches. Ensemble probabilities continue to show the tight
snowfall gradient in the Long Valley. The NBM5 puts a 60%
chance of 4+ inches at McCall with less than a 5% chance at
Cascade (HREF shows a similar gradient but is more bullish with
90% chance at McCall). For now will be holding off on any hazard
highlights as heavier snow looks localized in mtn valleys, and
impacts to road surfaces could be brief given the warming
temperatures on Friday. Sites above 6kft (7kft in the Steens and
Owyhees) will see 6-12 inches of wet snow through Friday
evening. Total liquid (liquid equivalent for the mtns) will see
ranges of 0.2-0.5" in the valleys to 0.5-1" in the mountains
with locally higher amounts (up to 1.5") along N/W aspects.

Periods of light precipitation will continue through Saturday
as the region remains locked under northwest flow. The focus of
precipitation will be in the mtns (60-80% chance) with lower
elevations seeing some drying as precipitation chances drop
below 30%. Snow levels will drop to 4500-5500 feet on Saturday
and mountains will pick up an additional 1 to 3 inches.
Northwest winds will become breezy Friday night into Saturday
across SE Oregon and higher terrain of SW Idaho (to include the
western Magic Valley) with gusts of 30 to 40 mph. Friday and
Saturday are mild with temperatures 5-10 degrees above normal.

LONG TERM...Sunday through Thursday...High confidence that an
active pattern will continue over the extended. A strong upper
level jet will ride over an upper level ridge and into the
Pacific Northwest. Pacific moisture and shortwave disturbances
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 on 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 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 to heavy 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 flow will also
bring breezy to windy conditions. Forecast confidence is lower
on Thursday, but strong, moist westerly flow will persist across
the Pacific Northwest, with the potential for additional
systems to bring more rain and snow Thursday and beyond.

&&

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

&&

$$

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DISCUSSION...JM
AVIATION.....CH
SHORT TERM...DG
LONG TERM....ST