Deadly Tahoe-area avalanche was probably triggered either by the ski party, or by nature, avalanche expert says

What triggered the avalanche last month near Lake Tahoe that killed nine skiers amid a fierce blizzard may forever remain a mystery, but the weight of accumulating snow or the group of skiers themselves are the likeliest causes, a Sierra Avalanche Center expert said.

“We really don’t know how it happened,” said Sierra Avalanche Center forecaster Steve Reynaud.

By the time the center’s staff were able to reach the site of the Feb. 17 slide, three days had passed and several feet of snow had blanketed the area, Reynaud said. Helicopter crews that dumped water and dragged a large, heavy bucket through the snow to prevent body-recovery teams from getting hit by additional avalanches had further obscured evidence of the deadly slide, Reynaud said.

“Normally we show up to an avalanche and we can do a bit of an investigation, we can see where it started,” Reynaud said. “You can get a pretty good idea of where the avalanche started, where it ran, where it stopped. You can find what it failed on. With these conditions none of that is really possible.”

The avalanche hit all but three people in a 15-member group on a guided backcountry skiing tour near Truckee, as they were on their way out of the wilderness after three days.

A dozen skiers were buried. One freed himself, according to the New York Times, while the other two — Capitola resident Jim Hamilton and a guide — escaped. Hamilton, the guide and a third man dug out three of their buried companions.

Despite investigators’ inability to pinpoint what triggered the slide, the two most probable explanations are that it started naturally, from the accumulating weight of falling and blowing snow, or that one or more of the ski party set it off by their weight and movements, Reynaud said.

“Historically, 90% of people that get killed in avalanche, themselves or someone in their group triggers it,” Reynaud said.

Within the snowpack at the time of the avalanche was a weak layer of crystalline snow that developed during a dry spell in January and early February before getting covered by several feet of snow from a storm that hit while the ski group were on their three-day trip to Frog Lake Huts, Reynaud said.

“That weak layer potentially could’ve been triggered by the group down low, and propagated up the slope and remotely triggered the avalanche,” Reynaud said.

Alternatively, Reynaud said, “It could very well just be a natural avalanche that came down the mountain where they just happened to be at the bottom of the slide path.”

The victims were found near the end of the slide, in a 20-foot-by-20-foot area, “in a small depression in the terrain, bounded by trees and boulders,” the center said.

“It’s a little bit of a bowl feature that allowed a lot of that avalanche debris to come down and pile up deeper in that one spot than in other spots,” Reynaud said. “We call these terrain traps — places that can increase the consequences of being in an avalanche.”

Six close friends were killed: Carrie Atkin, 46, of Soda Springs; Danielle Keatley, 44, of Soda Springs and Larkspur; Kate Morse, 45, of Soda Springs and Tiburon; Caroline Sekar, 45, of Soda Springs and San Francisco; Katherine Vitt, 43, of Greenbrae; and Lizbeth Clabaugh, 52, of Boise, Idaho. Also dead in the slide were three of the four guides on the trip, from Truckee-based Blackbird Mountain Guides: Nicole Choo, 42, of South Lake Tahoe; Andrew Alissandratos, 34, of Verdi, Nevada; and Michael Henry, 30, of Tampa, Florida.

According to a recent update from the avalanche center, the 400-foot-long avalanche buried the nine dead skiers between five and eight feet deep.

Slides like February’s incident that involve slabs of sliding snow typically happen on slopes with an angle from 30 degrees to 50 degrees, a similar incline to a ski run rated black or double black diamond, the U.S. National Avalanche Center said.

The Sierra Avalanche Center reported that the deadly avalanche last month appears to have started on a slope above the skiers with a steepness of 36 degrees to 40 degrees.