April 4, 2012 Posted by Emily in Montana, Outdoors
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Bridger Bowl avalanches a warning

Persistent weak layer could cause similar avalanches in other ranges

By Emily Stifler Explorebigsky.com Managing Editor

BRIDGER BOWL – It started as a small, loose soft slab avalanche triggered by a ski patrol explosive near the top of Bridger Gully. The moving snow picked up more mass as it went, and after a few hundred feet it had enough weight to affect a weak layer buried five feet down.

A jagged, 600-foot crown line zippered across the slope, tearing out the entire season’s snowpack, bulldozing mature timber, and running 1,800 vertical feet to a level almost adjacent to the bottom terminal of Bridger’s Alpine Lift.

The drainage where the avalanche ran was carved out smooth in some places, striated with dirty brown, white and red snow. Shredded trees were strewn about the 20-foot tall piles of debris.

It was March 27, and an eight-inch storm the previous night brought with it 1.5 inches of snow water equivalent (SWE). The two nights prior to the storm, the temperature had not dipped below freezing, causing the snowpack to be saturated all the way through.

As the morning’s work progressed, patrol triggered four other avalanches of the same stature. One—Boundary Chute—was off a ski cut. It was the same story: A small, oozing wet avalanche entrained more snow until it was large enough to trigger a massive, destructive wet slab avalanche.

The carnage in Slushman’s Ravine ran well past the bottom gate that allows skiers to cross from Southbound, off Pierre's Knob, to access the Slushman's lift.

“I want some of our locals to see this,” said patrol director Doug Richmond, looking at the gate. Across the way, cross-sections of trees were schmeared into the ice wall, a bizarrely beautiful and horrifying spectacle.

The ski area’s upper mountain terrain had been closed the previous day because of the warm temperatures and avalanche activity out of bounds.

The Bridgers had been in a major avalanche cycle for a month, with many natural and human triggered avalanches reported, including just outside the ski area boundaries. Like the Bridger Gully avalanche, most of the backcountry avalanches initiated at mid-elevations, and slid on a layer of facets left from October snow, said Pete Maleski, Bridger’s snow safety director.

With the ski areas closing in April, backcountry skiers and snowboarders will likely be dealing with wet avalanche danger. The persistent weak layer that was the culprit in the Bridgers exists in many places around the region.

“The Bridgers gave us a window into what can happen with these other ranges,” said Doug Chabot, director of the Gallatin National Forest Avalanche Center. Chabot pointed out concerns in the snowpack this spring:

- Depth hoar (like the October weak layer) can cause significant spring avalanche cycles.

- When that layer gets wet (from solar heating or rain), large wet slab avalanches can break easily on it.

- Once a wet slide occurs, the likelihood of many others occurring soon after increases.

But there’s also a bright side of snowpack assessment with wet slides, Chabot noted.

“As a skier, when it’s the most unstable out, it’s typically the worst conditions—when we’re breaking through, our skins are glopping up, it’s wet, and unpleasant.”

Safe skiing is all about timing, especially in spring, he said.

“Once the surface snow is frozen and supportable, we keep our fingers crossed for a bit of a corn cycle,” Chabot said. “In spring, a few hours makes all the difference. Is water moving through the snowpack? Is it getting weaker? Is it cloudy?”

Richmond’s lesson from the Bridger avalanches: sudden change should be a red flag, especially during a year with a weak layer like this year. That change could be a big snow or rain storm, wind loading near ridge tops, sluffs piling up in an apron, or big temperature changes.

“If you’re seeing natural wet slab activity, that’s a crucial piece of info for decision making. It’s telling us that things are ripe to avalanche that day.”

Here, Doug Richmond explains some of the factors in the avalanche event at Bridger Bowl: