The avalanche that rumbled down Mount Jumbo late Friday afternoon was set up by bands of heavy snow, followed by warm temperatures, more snow, and strong winds.

All it needed to come roaring down the hill was a trigger, a snowboarder.

West Central Avalanche Center Director Steve Karkanen said on Sunday, March 3, that the slide on Friday was the first of its kind to impact a residential neighborhood in known history.

"On some big snow years, we do see some of these wide open slopes get enough snowfall that there's the potential to produce an avalanche," Karkanen said. "Certainly in the past 200 years or so there's been no recorded evidence of a slide coming down the mountain to the valley floor to take out houses."

Karkanen described how the avalanche set up.

"Thursday, it started snowing heavily and the winds really picked up on Friday," Karkanen said. "By Friday afternoon all of that upper terrain, those catchman basins where the snow was being stripped from the east side of Mount Jumbo, that snow was being redeposited onto the upper reaches of those steep gullies. They loaded up on top of that ice crust, which was the perfect recipe for an avalanche to happen, and all we needed was a trigger."

Karkanen said the trigger was a lone snowboarder who happened to choose just that moment to travel down the slope.

"These things don't happen randomly, they need a trigger to come down off the hillside, and unfortunately a person got onto one, and it started an avalanche," Karkanen said. "There was just one snowboarder who was on that slope, and he was lucky to be able to self arrest (stop on his own). He was carried down the slope for a number of feet and then he continued to walk down the path and help out when he got to the bottom."

Karkanen said no charges have yet been filed against the unidentified snowboarder, but an investigation is ongoing by the Missoula Police Department.

Karkanen said the weather in the coming week may create a risk of even more avalanches.

"Anytime we have a rapid change in temperatures, especially rain on top of a relatively cold storm event, we could have trouble, Karkanen said. "Two of our avalanche specialists were out on Mount Jumbo this afternoon (Sunday). We got permission from the city to go up and check  the snowpack stability and do some documentation of that avalanche path. What we're concerned about is that if we do get a bunch more snow with this next storm Sunday night and Monday, given the character of that current snowpack with that ice crust, that could produce more avalanche activity on its own. We're watching it very closely, and we're in communication with the city of Missoula and the folks who need to know."

West Central Avalanche Center Director Steve Karkanen

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