Avalanche runout

The runout path of Thursday's avalanche stretched 800 feet down the side of Mount Jumbo but stopped short of the valley floor. The slab origin at upper right was nearly three feet thick and 75 feet wide.

While the overall avalanche danger is low in west central Montana’s backcountry, the hazard remains high on Mount Jumbo.

Missoula area officials are warning people at the base of Mount Jumbo, especially from Missoula Avenue to Elm Street, to continue to stay out of their back yards and for all area residents to stay off of the slopes. They’re also asking people to call 911 to report any sloughs of snow or debris that slides into their yards, and to report any trespassers on the mountain.

“If you’re in your backyard and it feels like you’re going up a set of stairs, don’t do it,” Travis Craft, an avalanche specialist and director of the West Central Montana Avalanche Center said on Tuesday. “Stay on flat ground.”

The backcountry has small wind slabs and weak layers of snow, but overall the conditions are pretty stable, Craft said. They haven't had any reports of avalanches during the past week, and the primary backcountry concerns are loose snow in sheltered areas.

However, the “Hellgate winds” combined with heavy snows last week to create conditions that are prime to slide on Mount Jumbo.

Last Thursday, a slide there stretched down 800 feet but stopped short of the valley floor. The slab origin was almost three feet thick and 75 feet wide.

Avalanche pits that Craft dug on Jumbo show a wind slab layer on top of a weak snow pack on top of a wind slab on top of a rotten snow base. Craft said that's mainly in the ravines, where they’re finding 3-to-5-feet of snow. Some of that blew in from other areas of Mount Jumbo, scouring those slopes nearly snow-free.

“We’ve done three different trips to Mount Jumbo to assess the snowpack,” Craft said. “Saturday was the last one. Every time we found dangerous avalanche conditions.”

Their primary concern is artificial or natural “triggers” that set off slides. While they can’t protect against natural ones, closing Mount Jumbo to the public lessens the chance of artificial triggers. Five years ago, a snowboarder triggered an avalanche that buried four people, killing one of them, and destroyed a home.

“All three times we were there we found ski tracks as well as people and dog tracks,” Craft said. “The main way we are mitigating the hazards is not putting triggers on them.”

Generally, the elk roaming Mount Jumbo’s slopes actually help to stabilize the snowpack because their hooves break up the slabs, Craft said. However, the second time they went up the mountain — trying to avoid trigger zones — he saw an elk start a small slide, which didn’t reach the valley floor.

Patrick Black, executive director of the nonprofit West Central Montana Avalanche Foundation, said they’re partnering with federal, state, and in this case, the city of Missoula to continue with the forecasts. They’re looking for warm days and cool nights to help the snow base settle; what they don’t want is rain on snow or a large amount of snow with strong east winds.

“Lifting the avalanche warning will be dictated by the weather,” Craft said.

While the focus right now is on Mount Jumbo, Craft said portions of Mount Sentinel also are steep enough for avalanches to occur. But that’s less of a hazard due to where people have built residences.

Missoula is one of only four cities with urban avalanche threats. The others include Sun Valley and Ketchum in Idaho, and Juneau, Alaska. 

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