SALT LAKE CITY (AP) - A 53-year-old man killed by an avalanche while snowmobiling underscores the dangers of Utah's rapidly building snowpack, experts said.

Dave Balls of Oakley was snowmobiling with his four sons on Christmas when a 400-foot-wide slide broke in the Thousand Peaks area of Summit County, about 35 miles east of Salt Lake City.

At nearly 10,000 feet in elevation, the steep bowl contained plenty of fresh snow, but underneath was a layer of "rotten" or sugary snow that set up in November, avalanche forecaster Craig Gordon said.

"I just don't trust it," Gordon reported on the center's Web site, adding the weak layers of snow take a long time to become stable. "Just when you think it's safe to start going after big terrain, these weaknesses come back to bite you."

The avalanche center has warned for days of the avalanche danger, which is expected to get worse with another storm moving across Utah on Wednesday.

Balls, who wasn't wearing an avalanche beacon, was caught up in a 4-foot-deep slide and buried for 90 minutes before his body was recovered, Gordon said.

He had six children, including two daughters adopted from Russia.

"He was up enjoying the day when the slide broke loose. He just wasn't able to outrun (it)," said Howard Sorensen, a local Mormon bishop.

It was Utah's second avalanche death this week. Jesse Williams, 30, of Grand Junction, Colo., was killed by a slide Sunday at The Canyons ski resort.

That avalanche also injured an 11-year-old boy, Max Zilvitis, but he is "doing incredibly well" at Primary Children's Medical Center, his parents said Wednesday.

"Max has been recovering steadily since he awoke Monday afternoon. We are forever grateful to the volunteers and ski patrol who helped recover Max," parents Brian and Samantha Zilvitis said in a statement.