The conditions that led to California’s deadliest avalanche in modern history began to form weeks ago, when the Sierra’s ice pack melted and refrozen into glass-like ice shards.
That fragile layer remained exposed for most of January, warming during the day and thickening at night. This week’s snowstorm then buried it under feet of heavy powder – a classic setup for a disastrous slide.
Three days after at least eight backcountry skiers died in the wilderness near Donner Pass, it remains unclear what caused the massacre. Another person is missing and presumed dead. As of Friday morning, bodies remained on the mountainside because extreme winter weather and the constant avalanche danger did not allow first responders to reach the area safely. Investigators were also unable to reach the area to find the cause of the slide, said Wendy Antibus, spokesperson for the Sierra Avalanche Center.
Six skiers, including one of the four guides leading the trip, survived. Two were treated at the hospital for non-life threatening injuries.
The key questions remain: Did the group of 15 skiers cause the snow themselves? Or was the fatal slide released naturally under the weight of the storm?
While officials are still waiting for safe access to the area, avalanche scientists say several warning signs were posted in the snowpack.
“They could have caused an avalanche,” said Daniel Swain, a climate scientist at UCLA who specializes in extreme weather events. “Even under those conditions, it’s very easy to start. That’s why walking backwards is strongly advised under those conditions.”
“I don’t know what the directors were doing,” said Craig Clements, a professor of meteorology and climate science at San Jose State University. “But we don’t know the story yet.”
When the storm hit the central Sierra on Tuesday, the old layer of ice crystals likely couldn’t support the weight of several feet of snowfall, Clements said. That fragile foundation layer used to crack and send the ice flowing backwards without human intervention, he said.
Even the group could have caused an avalanche as they tried to return in hot weather from the villages at the end of the planned three-day trip. The trip was led by Truckee-based Blackbird Mountain Guides, which is responsible for the law enforcement investigation after the disaster.
Regardless of the exact cause, experts say these mountains were a dangerous place. The Sierra Avalanche Center issued a warning early Tuesday advising against travel “on, near, or below” avalanche-prone slopes.
“A widespread natural avalanche cycle is expected in the next 24 hours,” the warning read, warning that large avalanches could also pass through forested areas that are generally considered safe areas.
The first report of this center describes the snow that attacked the group as it has the power to break trees.
Clements, a figure skater, said he himself would not have gone to the backcountry that day. He also said that it would be unusual for most of the members of the group to be revealed at the same time; Rural hikers tend to spread out, some waiting in safe places such as bowls or thick trees while others cross more exposed slopes, reducing collective risk.
However, investigators have not clarified the details of how the group was traveling or where they were located when the avalanche was released.
The lead company did not respond to requests for comment. The company faced heavy criticism on social media for going ahead with the trip despite warnings of an outbreak of rain. Nevada County Sheriff Shannan Moon said Wednesday there were “major questions” about the directors’ decisions but added that the company was cooperating with first responders.
Swain, of UCLA, noted during the live stream that the disaster occurred during the warmest winter on record for the California mountains — a season marked by intermittent snow followed by long dry spells.
After a few storms over the holidays, the central Sierra was dry in January. The warmth of the afternoon softened the snowpack; the cold of the night intensified again. That cycle created so-called sections in the ice — “like sheets of glass” with “no cohesion or strength,” said Robert Rice, a UC Merced teaching professor who previously predicted floods for Utah’s transportation agency.
In technical terms, this process creates a “solid continuous layer,” Rice said — a fragile base that can’t support heavy new snow.
Rice said he expected there to be a flood under those conditions but was surprised that such a disaster would strike a team led by experienced directors. He said he thought they would go to safer places, like lakes or forested areas. Because investigators have not yet reached the site, it is unclear where the group was when the slide was removed. The first report places skiers at the base of Perry’s Peak, a popular 8,000-foot peak near Frog Lake.
“It was probably too much snow,” Rice said. He added that high ice levels alone may not be enough to naturally release more of the group.
Conditions are similar to those common in the Rocky Mountains, Rice said, where persistently weak layers often contribute to deadly flash floods. Between 1950 and the 2024-25 ski season, 328 people died on Colorado’s slopes, compared to 74 in California.
In January, an experienced snowmobiler died in a dangerous area near Castle Peak, not far from this week’s tragedy.
Although the avalanche warning in the central Sierra was downgraded on Friday, the risk remains “visible”.
That didn’t deter Marshall Yanzick, 36, of Portland, and three friends from the Bay Area from venturing into the backcountry Friday morning to ski the south side of the mountain that was closed for renovations.
He said they were monitoring a “solid, bullet-proof layer” under several meters of snow and planned to stick to forest openings.
“Making sure you pick the right location is important,” Yanzick said.
Experts say the weak bottom layer will eventually compress and stabilize under the weight of the new snow. Until that process continues, however, the ice pack can remain unpredictable — and the risk of another slide continues.
