The Coquihalla Highway is closed indefinitely while the provincial government launches major avalanche control work Friday, following a Class 4 avalanche the day before that buried a section of the highway at the Great Bear Snowshed.

A Class 4 avalanche is "highly destructive and could destroy a rail car and bury a house," according to the Canadian Avalanche Centre. Approximately 100 vehicles had to be helped off the highway following the slide. 

Transportation Minister Todd Stone says the avalanche control mission it hopes to undertake Friday — weather permitting — will be one of the largest avalanche control missions in the highway's history. It will involve up to three helicopters and take several hours to complete.

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Three helicopters stand by on the Coquihalla Highway Friday as provincial crews prepare to bombard nearby mountains with bags of explosives to dislodge snow in a major avalanche control project. (VSA Highway Maintenance)

The Coquihalla saw more than 30 centimetres of additional snow Thursday night, and has seen nearly three metres of new snow over the last 10 days.

"The huge volume of snow is sitting on a weak layer, making it very unstable," says Stone. 

"The result has been slides on paths that have never before reached the highway in the 27 years that the Coquihalla Highway has been open."

Once control activities are complete, the government says it will take several hours for maintenance crews to clear the highway before it is safe to reopen.

Detours are available through Highway 1 and Highway 8.

The Coquihalla was reopened briefly Wednesday after being closed Tuesday for more than a day by a smaller avalanche near the summit.

Special avalanche warning issued

The Canadian Avalanche Centre has issued a special avalanche alert for this weekend and into next week warning of a very high continuing avalanche risk in most of the mountain ranges in B.C. and Alberta.

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The Canadian Avalanche Centre is warning that conditions are dangerous in the mountains of B.C. and Alberta. (Adrian Wyld/Canadian Press) (Adrian Wyld/Canadian Press)

Canadian Avalanche Centre manager Karl Klassen says a prolonged drought has left the surface of the snowpack in very bad shape.

“Now the new snow is sitting on one of the worst weak layers we’ve seen in a few years. That weakness is currently anywhere between one and two metres deep so, when it’s triggered the resulting avalanches are very large.”

Last weekend, an Alberta snowmobiler was killed on Boulder Mountain near Revelstoke, B.C. Three skiers were rescued Saturday from an avalanche about a minute's helicopter flight from the Coquihalla the same day.

Last month, a snowmobiler died in an avalanche south of Valemount, B.C.

More snow expected

CBC meteorologist Johanna Wagstaffe says 200 centimetres of snow has fallen near the Coquihalla Highway over the past eight days, and it's the most dangerous kind.

"It is storm snow, meaning the winds have created these dangerous slab layers which are between 100 to 200 centimetres and it's that entire slab of fresh snow that can slip," said Wagstaffe.

Wagstaffe said there may be a bit of a break from the snow on Friday, before it begins to fall again on Saturday.

Weather permitting, crews will be conducting avalanche control on the Coquihalla Highway on Friday. DriveBC is advising drivers to take alternative routes on Highway 1 and Highway 8, or on Highway 3 to the Okanagan.

Map: Great Bear Snowshed Coquihalla Highway

with files from the CBC's Mike Clarke