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.

Six skiers, including one of the four guides leading the trip, survived. Two were treated at a hospital for injuries that weren’t considered life-threatening.

Key questions remain: Did the group of 15 skiers trigger the avalanche themselves? Or ...