"The weeks of warm weather may have caused the thin snow cover at high altitudes in the Lake Tahoe region to partially melt, creating an unstable base layer that was then covered by a dump of snow over the past few days, says climate scientist Daniel Swain of the California Institute for Water Resources. According to the National Weather Service, some parts of the area had three feet of snow in 24 hours, and it's estimated that more than five feet of snow has fallen there since Sunday."
"This combination of an unstable base below a thick layer of unconsolidated fresh snow is one of the most dangerous causes of snow avalanches in the mountains, Swain says. And it seems this led to the avalanche on Tuesday that trapped the skiers at Castle Peak, which is near the town of Truckee, Calif., and a few miles northwest of Lake Tahoe."
Most of the western U.S., including the Lake Tahoe region, experienced an exceptionally warm winter that broke records in many areas. Weeks of warm weather partially melted thin high-altitude snow cover, creating an unstable base layer. A sudden switch to heavy snowfall dropped three feet in 24 hours in some areas and more than five feet since Sunday, burying that weak layer under unconsolidated fresh snow. That unstable configuration likely triggered a catastrophic avalanche at Castle Peak near Truckee, Calif. Eight skiers' bodies have been recovered, a ninth remains missing and presumed dead, and six others were rescued by a snowcat.
Read at www.scientificamerican.com
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