Winter Storm and Heavy Snow in Montana, December 11 to 12, 2021

Name: Winter Storm and Heavy Snow in Montana, December 11 to 12, 2021
Start: 2021-12-11T01:00
End: 2021-12-12T20:00
Location: Montana

December 11 to 12, 2021 · NWS MSO · 4 events in 4 counties and zones

An atmospheric river brought heavy snowfall across Northwest Montana and the Bitterroot Mountains.

Where it happened

Every event with a place to put it; 4 filed by county or forecast zone are drawn at its center, in a lighter ring. Tap one for its entry below.

Tornadoes
Tropical storms and surge
Floods
Wind
Hail
Winter weather
Heat and drought
Cold
Fire, smoke and dust
Marine and coastal
Other

Open the live radar over this ground

The toll

Deaths: 0
Injuries: 0
Property damage: None recorded
Crop damage: None recorded
Events: 4

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.

What happened

The episode as NWS MSO wrote it up.

An atmospheric river brought heavy snowfall across Northwest Montana and the Bitterroot Mountains.

What forecasters said that day

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 10, 2:36 PM MST, 10 hours 24 minutes earlier.

Area forecast discussion, Dec 10, 2:36 PM MST, 10 hours 24 minutes before

Area Forecast Discussion
National Weather Service Missoula MT
236 PM MST Fri Dec 10 2021

...HEAVY SNOW AND GUSTY WINDS OVER NORTHERN ROCKIES MOUNTAIN PASSES
TONIGHT THROUGH WEEKEND...

.DISCUSSION...

A surge of moisture will combine with a powerful jet stream over the
Pacific Ocean today and move inland over the Northern Rockies
tonight through the weekend. Considering the cold air already in-
place across much of the region, the environment ahead of the storm
is primed for efficient snowfall production (particularly across
high terrain of northwest Montana and north central Idaho). The
strong winds aloft will not only enhance snow rates (1-2 inches per
hour at times over area passes), but also combine with the snow to
decrease visibility and drift snow over pass roadways in a short
amount of time. Lookout, Lolo, and Marias Pass will experience high
snow rates, however Marias Pass will be particularly impacted with
that blowing/drifting snow and high snow rates combination. Thus,
travel may become quite hazardous over these passes over this
weekend, especially during the evening through morning hours. If you
do travel over passes this weekend, remember to pack emergency food,
water, and a blanket in case your route of choice suddenly becomes
impassible.

The strong winds aloft this weekend will also translate to low
elevation population centers of the Northern Rockies on Saturday.
The Camas Prairie of Idaho, including Grangeville, will see a
combination of south winds gusting to 50 mph and brief bouts of snow
creating episodic hazard winter travel conditions. Lesser wind
gusts, but also variable snow intensity, will also be present for
much of the lower elevation population centers of northwest and west
central Montana. Currently, the wider valleys (e.g., Flathead,
Mission, Missoula, Bitteroot) will have episodic snow and wind on
Saturday, however the overall winter weather impact timing is still
fluid through Sunday. Also, there is a bit of uncertainty with
respect to snow/wind impacts for the Butte/Blackfoot Region and
Lemhi County Idaho (including mountain passes along the Continental
Divide, such as Bannock, Lemhi, MacDonald, and Homestake Passes).

The pattern stays active next week as a large scale trough
gradually works onto the West Coast. Southwest flow ahead of the
trough will warm up the lower and mid levels of the atmosphere
which could allow for a light wintry mix in valleys as cold air
remains settled in some sheltered valleys. On Tuesday, there is
decent agreement on a cold front passage with a round of minor
pass level snow, potentially a light dusting in the valleys. A
general northwest flow is favored for late next week with
seasonably cold air settling in and highs only in the 20s to near
30F. Subtle shortwaves will leave open the opportunity for
occasional snow showers late next week as well.

&&

.AVIATION...Strong west winds aloft will move into the Northern
Rockies air space after 11/0600Z. Expect gusts to 60 knots above
high terrain, as well as strong low level shear within area
valleys. Terrain obscurations will become commonplace by 11/1200Z.
Heavy snow will be locked onto the high terrain thereafter,
particularly true for air space north and west of KMSO (including
KGPI, KMLP). Snow will intermittently fall into low elevations
and valleys, as the winds will have a localized downslope (drying)
effect at times. Much of the terrain obscurations will extend
into Sunday, as will the snowfall. However, winds will begin to
diminish after 12/0000Z, except for air space along the
Continental Divide.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Warning from 2 AM Saturday to 11 AM MST Sunday West
     Glacier Region.

     Winter Storm Warning from midnight tonight to 5 AM MST Sunday
     Kootenai/Cabinet Region.

     Winter Weather Advisory from 8 AM Saturday to 11 AM MST Sunday
     Bitterroot/Sapphire Mountains.

     Winter Weather Advisory from 8 AM Saturday to 11 AM MST Sunday
     Potomac/Seeley Lake Region.

     Winter Storm Warning from midnight tonight to 5 AM MST Sunday
     Lower Clark Fork Region.

ID...Winter Weather Advisory from 4 AM to 7 PM PST Saturday
     Orofino/Grangeville Region.

     Winter Storm Warning from 4 AM to 4 PM PST Saturday Northern
     Clearwater Mountains.

     Winter Weather Advisory from 4 AM Saturday to 10 AM PST Sunday
     Southern Clearwater Mountains.

     Winter Weather Advisory from 4 AM to 7 PM PST Saturday Lower
     Hells Canyon/Salmon River Region.

&&

The radar

The national radar mosaic for the moment the first event began.

Open the radar archive at this moment

Event by event

2 heavy snow, 1 winter storm, 1 winter weather. Local standard time, as filed.

Heavy Snow, Lower Clark Fork Region

When: December 11, 1:00 AM MST to December 12, 1:00 PM MST
Where: Lower Clark Fork Region forecast zone
Reported by: Department of Highways
Coordinates: 47.286, -114.685 (the zone’s center; the record gives no point)

Heavy snow led Montana Department of Transportation officials to declare severe driving conditions on I-90 over Lookout Pass from milepost 0.0 to 5.0 during the morning hours of December 12th. Numerous accidents occurred, leading to travel delays across Lookout Pass. The Mullan Pass ASOS measured 2.59 inches of precipitation during this event; suggesting at least 18 inches of snow fell for nearby Lookout Pass.

Winter Storm, West Glacier Region

When: December 11, 4:00 AM MST to December 12, 10:00 AM MST
Where: West Glacier Region forecast zone
Reported by: Department of Highways
Coordinates: 47.971, -114.069 (the zone’s center; the record gives no point)

Montana Department of Transportation officials reported eight to ten inches of snow impacted US Highway 2 from Essex to Devils Creek on Saturday, with another eight to twelve inches falling by Sunday morning. Mesonet sites from Essex to Marias Pass measured an eight to twelve inch snow depth increase. Southwest wind gusts up to 33 mph were observed at Marias Pass.

Winter Weather, Kootenai/Cabinet Region

When: December 11, 7:00 AM MST to December 12, 8:00 AM MST
Where: Kootenai/Cabinet Region forecast zone
Reported by: Department of Highways
Coordinates: 48.171, -114.863 (the zone’s center; the record gives no point)

A mix of rain and wet snow was observed across valley locations, with generally three to six inches of snow being reported. Mountain area SNOTEL sites measured around one foot of new snow.

Heavy Snow, Bitterroot / Sapphire Mountains

When: December 12, 9:35 AM MST to 8:00 PM MST
Where: Bitterroot / Sapphire Mountains forecast zone
Reported by: Department of Highways
Coordinates: 46.508, -113.828 (the zone’s center; the record gives no point)

Heavy snow led Montana Department of Transportation officials to declare severe driving conditions on US Highway 12 over Lolo Pass from milepost 0.0 to 4.0 during the morning hours of December 12th. The Lolo Pass Snotel measured 0.40 inches of precipitation between 800 am and noon, suggesting snowfall rates of one inch per hour.

What these are

Heavy Snow: Snowfall reaching the local warning threshold without the other hazards of a winter storm.
Winter Storm: A storm that brought snow, sleet or freezing rain at warning levels, usually over several hours or days.
Winter Weather: Snow, sleet or freezing rain below warning levels that still caused disruption, damage or casualties.

After NWS Directive 10-1605, the rules offices file Storm Data by.

The same day elsewhere

Every storm in Montana in 2021 Every state in 2021 This day in weather history Warnings in Montana, December 2021

Sources

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: 997570, 997244, 997243, 997571.