The big arctic storm that arrived over the weekend has left Northwest Montana and the Flathead Valley covered with deep snow and increased the backcountry avalanche hazards.

More than 18 inches of new snow had piled up on the summit of Big Mountain in Whitefish by 9 a.m. Sunday morning, with 11 inches atop Blacktail Mountain in Lakeside.

Columbia Falls saw 14 inches of powder, Bigfork had 12 inches, and Glacier Park International Airport reported 6.1 inches.

The Flathead Avalanche Center advises all backcountry skiers, snowmobilers and other outdoor enthusiasts to use extreme caution.

“Substantial new snowfall and wind-drifted snow in the past 24 hours added a considerable stress to a previously weak snow pack,” the center’s advisory says. “Conservative decision making and terrain selection are essential today.”

Any ground above 6,000 feet in elevation, or on wind-loaded slopes, is considered high risk, and travel in those areas is not recommended. In those high-risk areas, human-triggered avalanches are considered very likely.

The avalanche advisory is in place for much of Flathead County, but in particular everything to the west of Kalispell. All of Flathead, Lake and parts of Sanders counties are dealing with a Winter Storm Warning. Unnecessary travel is not advised.

Weather for Kalispell is forecasted to be bitterly cold for most of the week, with highs in single digits and lows dipping as far as 7 below zero.