This was a classic overrunning event with an arctic air mass in place which lasted several days. A push of subtropical moisture associated with an atmospheric river arrived early Friday morning of the 29th and caused a prolonged period of moderate to heavy snow across portions of northwest and west-central Montana and central Idaho. Freezing rain was reported as far north as Polson, east of Thompson Falls, the Bitterroot and Missoula Valleys and in the Seeley/Swan Valley for a period.  Between Missoula and Stevensville the precipitation type changed back and forth between snow and freezing rain Friday evening and night. Severe driving conditions were reported all across western Montana on the 29th due to icy roads and blowing and drifting snow.

Winter Storm in WEST GLACIER REGION, MONTANA
From 28-DEC-17 05:00:00 to 30-DEC-17 17:00:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

Montana Department of Transportation reported severe driving conditions on Highway 2 near Marias Pass Thursday night the 28th. Conditions deteriorated Friday morning the 29th as the arctic winds funneled through Badrock Canyon, and severe driving conditions were declared from West Glacier to Marias Pass. Periods of severe driving conditions continued through Saturday evening the 30th on US-2. A weather spotter in Essex measured a storm total snowfall of 28 inches over 72 hours. Other snow totals include: 3 feet Swan Lake, between 2 and 3 feet in Nyack, 30 inches Polebridge, 24 inches at Devil's Creek between Essex and Marias Pass and 18 inches at the Hungry Horse Dam. Dangerously cold wind chills dropped down between -35 and -40 degrees Fahrenheit between Essex and Marias Pass during the morning of the 30th.

Winter Storm in LOWER CLARK FORK REGION, MONTANA
From 28-DEC-17 05:00:00 to 30-DEC-17 10:00:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

Montana Department of Transportation declared severe driving conditions between Trout Creek and Heron due to heavy snow. The forest service in Trout Creek reported 21 inches of snow with one inch per hour rates during the evening of the 29th. Lookout Pass reported 17 inches of snow in 24 hours. Total 3 day snowfall amounts included: 35 inches 7 miles southeast of Noxon, 31 inches Lookout Pass, 28.9 inches 2 miles northwest of Heron, 23 inches Haugan, and 10 to 12 inches at Plains, St. Regis, Nine Mile and Alberton.

Winter Storm in KOOTENAI/CABINET REGION, MONTANA
From 29-DEC-17 16:00:00 to 30-DEC-17 11:00:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

On the evening of the 29th, the Montana Department of Transportation reported severe driving conditions due to reduced visibility from blowing and drifting snow between Noxon and Bull lake and between Whitefish and Stryker. Libby and Troy reported 6 to 8 inches of snow in 24 hours and 26 inches of snow was reported near Rexford over 3 days. Three feet of snow fell at Bull Lake while two feet was reported at Happy's Inn.

Winter Storm in FLATHEAD/MISSION VALLEYS, MONTANA
From 29-DEC-17 05:00:00 to 30-DEC-17 11:00:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: County Official

On Friday afternoon the 29th at 3 pm, Lake county officials declared emergency travel only due to snow drifts which made roads impassable. Also at 4 pm on the 29th, the Flathead County Road Department reported that east and west oriented roads located east and north of Glacier Park International Airport were drifting closed faster than road crews could plow. The Montana Department of Transportation declared severe driving conditions between Whitefish and Kalispell on US-93, Whitefish to Columbia Falls, Polson and St. Ignatius, between Hot Springs and Whitefish from the afternoon into the evening hours on the 29th. Total storm snow amounts between 12 and 18 inches with a few isolated amounts near 2 feet were reported across the Flathead and Mission valleys. Freezing rain caused icy roads on Friday morning in the Mission Valley.

Winter Storm in POTOMAC / SEELEY LAKE REGION, MONTANA
From 29-DEC-17 11:00:00 to 30-DEC-17 10:00:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

Montana Department of Transportation declared severe driving conditions between the Clearwater Junction and Helmville and also on I-90 between Bearmouth and Drummond. The combination of antecedent wet snow and wet conditions with cold arctic air moving in during the day on Friday the 29th, triggered a flash freeze which caused area roads to become a sheet of ice. Also heavy snow combined with strong gusty winds created blizzard conditions between Potomac and Clearwater on Highway 200 and also along Interstate 90 between Clinton and Drummond. Further north between Condon and Seeley Lake, between 2 and 3 feet of snow fell over the course of 3 days.

Winter Storm in BITTERROOT / SAPPHIRE MOUNTAINS, MONTANA
From 28-DEC-17 16:00:00 to 30-DEC-17 10:00:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Public

Lost Trail pass received 34 inches in 72 hours and Lolo pass saw 22 inches of snow. Freezing rain also developed at Lolo pass on Thursday afternoon.

Avalanche in BUTTE / BLACKFOOT REGION, MONTANA
From 29-DEC-17 22:00:00 to 29-DEC-17 22:30:00 (MST-7)
Property damage: 5.00K
Crop damage: 0.00K
Reported by: Department of Highways

The avalanche moved across Highway 200 for approximately 40 yards in length near mile-marker 55.9 and was 15 to 20 feet deep and 55 yards wide. The cab of a semi-trailer truck was partially buried. Highway 200 was closed until 530 am MST the next morning. Most likely heavy snow and strong easterly winds created severe wind-loading on Mineral Hill contributing to the conditions of the avalanche. According to the Montana Department of Transportation an avalanche of this magnitude had not occurred in this location in over 20 years.

Winter Storm in MISSOULA / BITTERROOT VALLEYS, MONTANA
From 28-DEC-17 21:00:00 to 30-DEC-17 11:00:00 (MST-7)
Property damage: 20.00K
Crop damage: 0.00K
Reported by: Department of Highways

Between the afternoon and evening of the 28th, snow changed over to freezing rain. Severe driving conditions were declared by the Montana Department of Transportation early Friday morning the 29th between Florence and Stevensville due to black ice and freezing rain. By 9:30 am MST, the Missoula County Sheriff's office issued emergency travel only for Missoula County which was extended through 7 pm that evening. The arctic front brought very strong winds to Hellgate Canyon and surrounding areas. Numerous trees fell down in the Rattlesnake Valley and vicinity of Missoula which was likely a result of the combination of extremely wet snow and strong winds. One tree fell on top of a house and caused considerable roof damage. The ASOS sensor at the Missoula International Airport recorded wind gusts to 37 mph which meant that winds were likely much stronger towards Hellgate Canyon. A NWS employee located northeast of Stevensville measured a total of 0.15 inches of glaze that didn't melt for at least a week afterwards.

Winter Storm in BUTTE / BLACKFOOT REGION, MONTANA
From 29-DEC-17 08:00:00 to 30-DEC-17 11:00:00 (MST-7)
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Department of Highways

Montana Department of Transportation(MDT) reported 12 inches of snow in Drummond with blowing and drifting snow. MDT issued severe driving conditions between Bearmouth and Garrison junction around 2 pm Friday. A spotter 10 miles ENE of Ovando reported 18 inches of snow in 24 hours from this event. Total snowfall amounts included: near 2 feet in the Ovando, Helmville and Elliston areas, 9 inches Hall and 3 inches in the Anaconda area. A weather spotter located 10 miles east-northeast of Ovando reported 3 inches of heavy snow that fell in one hour ending 9:13 am on the 30th due to a transient snow band that set up over northern Powell County.