A plume of Pacific moisture interacted with an arctic air mass leading to widespread heavy snow across western Montana.
Every event with a place to put it; 8 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.
Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS MSO wrote it up.
A plume of Pacific moisture interacted with an arctic air mass leading to widespread heavy snow across western Montana.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Missoula, MT wrote its last area forecast discussion before the first event at Dec 18, 3:03 AM MST, 1 hour 57 minutes earlier.
Area Forecast Discussion
National Weather Service Missoula MT
303 AM MST Sat Dec 18 2021
...Potent Winter Storm to Impact Northwest Montana and North-
Central Idaho Saturday through Monday...
.DISCUSSION...The Northern Rockies is still on track for a long
duration and potent winter weather system. Currently, there is a
cold Canadian airmass over Western Montana. A strong Pacific low
pressure system will cut off from the flow and sit west of Oregon
through Monday. This in turn will force moist westerly flow into
the Northern Rockies beginning this evening. Sunday a reinforcing
push of cold air will move into northwest Montana. This boundary
is expected to interact with the moist flow aloft and enhance
valley snowfall at the point of interaction.
Saturday will start very cold with many locations along the
Continental Divide below zero, most locations in W. Montana will
be low single digits except highway 200 and 2 corridors west of
the Flathead Valley; where clouds have slowed the cooling process.
Light snow will develop around daybreak at Lookout Pass, along
the Idaho border, and the northern Clearwater Mountains. Expect
moderate snow at Lookout Pass by noon and Marias Pass by midday,
with deteriorating road conditions in both locations. Initially,
Lincoln and Sanders Counties will be the focus of precipitation.
The moisture will begin to sag south after midnight, as
precipitation lightens up Sunday morning for the Highway 200 & 2
corridors, freezing rain or drizzle may develop producing a light
glaze of ice.
Sunday morning the Arctic front will arrive at Marias Pass, expect
heavy snow and winds shifting from the east. Winds could gust 40
to 50 mph. The Arctic boundary will continue to progress south
through the day. Snowfall will be enhanced as the boundary comes
in contact with the moist westerly flow. The Arctic airmass should
push as far south as the I-90 corridor, where it is expected to
stall.
On Monday, the Arctic boundary is forecast to retreat north with
the moisture. So locations in Lincoln and Sanders Counties may
see a brief return to snow.
Total snowfall by Monday morning moderate confidence:
Lookout Pass 16-24"
Marias Pass 6-12"
Valleys Sanders and Lincoln Counties 5-10"
Flathead Valley 3-6"
Swan Valley 6-10"
Clearwater Mountains of Idaho 10-18"
US-12 west of Lolo Pass, ID 6-12"
Lolo Pass 12-18"
West-central Montana and the I-90 corridor there is less confidence.
This is due to downsloping considerations and the exact locations
of the cold air and surface low pressure. Since the potential
snowfall will have a higher impact on the community than the
current forecast, both will be discussed.
I-90 west of Missoula up to St. Regis 2-5" moderate probability,
5-10" low probability.
Missoula Valley 2-3" moderate probability, 4-8" low probability.
I-90 east of Missoula 2-4" moderate probability, 3-6" low
probability.
This active pattern is expected to continue through at least the
end of year. The Climate Prediction Center has forecast below
normal temperatures and above normal precipitation through the
31st.
&&
.AVIATION...
Clouds are increasing across Northern Rockies air space, ahead of
a long-lasting and complex winter storm event (through Monday).
The first impact to flying weather will be west winds gusting
over 50 kts over the terrain on Saturday, particularly near the
Continental Divide. Also, southerly winds gusting to 30 kts will
develop near KGIC Saturday and Sunday. Areas of passing light snow
will move across the air space beginning 18/1200Z, further
lowering ceilings, occasionally lowering visibility, and obscuring
terrain. Areas west of KGPI will see an increase in snow and
freezing rain after 18/1800Z, which will eventually spread to all
locations north of KBTM on Sunday. An Arctic airmass will push
south across the region on Sunday to around the I-90 (KMSO)
corridor. Expect winds to shift from southerly to east
northeasterly and become gusty, potentially lowering visibility
with blowing snow.
&&
.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning from 11 AM this morning to 5 AM MST Monday
Lower Clark Fork Region.
Winter Storm Warning from 11 AM this morning to 5 AM MST Monday
West Glacier Region.
Winter Storm Warning from 11 AM this morning to 5 AM MST Monday
Kootenai/Cabinet Region.
ID...Winter Storm Watch from Saturday evening through late Sunday
night Northern Clearwater Mountains.
&&The national radar mosaic for the moment the first event began.
4 heavy snow, 4 winter weather. Local standard time, as filed.
Heavy snow, followed by a period of freezing rain was observed along the Highway 200 corridor near the Idaho border. Valley locations near the Idaho border received five to eight inches of snow, with 15 inches at Lookout Pass.
Six to twelve inches of wet heavy snow was observed across valley areas, with one to two feet of snow impacting mountain areas above 5,000 feet.
One to three inches of snow was measured across the Flathead Valley, with little to any accumulation across the Mission Valley. Snow changed to freezing rain by the early morning hours of December 19th. Trained spotters reported a light glaze of ice on area roadways.
Periods of moderate snow led to travel delays along the I-90 corridor. Two to five inches of snow was observed across valley areas.
Snowfall rates around one inch per hour led the Montana Department of Transportation to declare severe driving conditions on US Highway 12 from mile markers 0.0 to 4.0 over Lolo Pass.
Periods of moderate snow led to travel impacts. Two to five inches of snow was observed across valley areas. Finally, a light glaze of ice was observed along the I-90 corridor near Clinton.
A wintry mix of snow and freezing rain was observed across the Missoula and Bitterroot valleys. One to three inches of snow along with a light glaze of ice made for slick travel conditions.
Heavy snow during the morning commute led to numerous accidents, especially across the Missoula Valley. Dangerous travel conditions led to a two hour delay for Ravalli County school districts.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Montana in 2021Every state in 2021This day in weather historyWarnings in Montana, December 2021
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 997575, 997573, 997574, 997586, 997583, 997585, 997581, 997582.