photo credit: Tahoe National Forest
The Castle Peak area has been closed since Tuesday.

The avalanche struck a group of 15 backcountry skiers near Castle Peak during a winter storm Tuesday morning.

Officials say the slide was about the size of a football field.

Bruce Tremper is a former director of the Utah Avalanche Center and has 47 years of avalanche experience. He says thin snowpacks like the one in the Sierra this year can be deceptively dangerous.

"We've been in a snow drought, as everybody knows," Tremper said. "When it's a drought like that, the snowpack actually gets weaker and weaker. When it's a thin snowpack, it's a weak snowpack."

That weak layer acts like a foundation of sand. When heavy new snow piles on top, it can collapse.

"It's kind of like putting a brick on top of a pile of potato chips, said Tremper. "So it just can't hold up the weight."

Tremper says about 93 percent of deadly avalanches are triggered by people, often the victims themselves or someone in their group. But he cautions against blaming those caught in Tuesday's slide.

We all want to point our fingers after these accidents, which is a mistake," Tremper said. "It's not helpful, and second of all, you're usually wrong. Even when I do it. So it's better just to wait for the facts to come in."

The U.S. Forest Service has closed the Castle Peak area through mid-March to allow for recovery operations. Officials say the snowpack remains unstable.