New Delhi, Feb. 20 -- A weekslong "snow drought" in Northern California's Sierra Nevada helped set the stage for Tuesday's deadly avalanche, after several feet of new snow fell on an earlier layer that had hardened, making it unstable and easily triggered, experts said.

The new snow did not have time to bond to the earlier layer before the avalanche near Lake Tahoe killed at least eight backcountry skiers, said Craig Clements, a meteorology professor at San Jose State University, who has conducted avalanche research. Six skiers survived and rescuers were still searching for another one who was still missing on Wednesday.

The group was on a three-day backcountry trek in the Sierra Nevada on Tuesday morning when they were trapped by the a...