Weeks in the making: How a brittle snowpack primed the Sierra for disaster

"The conditions that led to California's deadliest avalanche in modern history began forming weeks ago, when the high Sierra's snowpack thawed and froze again and again into glass-like shards of ice. That fragile layer sat exposed for much of January, warming during the day and hardening at night. Then this week's blizzard buried it beneath feet of heavy powder a classic setup for a catastrophic slide."

"Three days after at least eight backcountry skiers died in the wilderness near Donner Pass, it remains unclear what triggered the avalanche. Another person is missing and presumed dead. As of Friday morning, the bodies remained on the mountain slope because extreme winter weather and continued avalanche risk did not permit first responders to safely access the area. Investigators also have been unable to reach the site to determine the precise cause of the slide, said Wendy Antibus, a spokesperson for the Sierra Avalanche Center."

"While officials await safe access to the site, avalanche scientists say many of the warning signs were embedded in the snowpack. It's quite possible that they triggered an avalanche, said Daniel Swain, a UCLA climate scientist who focuses on extreme weather events. Although under those conditions, it's very easy to trigger one. Which is why backcountry travel is so strongly advised against under those conditions."

A fragile layer of refrozen, glass-like ice formed after repeated thawing and freezing in the high Sierra and sat exposed through January. A recent blizzard buried the weak layer under several feet of heavy powder, creating a classic setup for a catastrophic slide. At least eight backcountry skiers died near Donner Pass and another person is missing and presumed dead. Extreme weather prevented rescuers from safely reaching the slope, leaving bodies on the mountain. Six skiers survived; two had non-life-threatening injuries. Investigators could not determine the trigger, though many warning signs were embedded and the weak layer likely failed under new snowfall.

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