The pleasures of prime ski season in the Sierra have arrived with a dark side -- the peril of avalanches.

Tahoe area officials are warning skiers and back-country recreationists of "considerable" avalanche danger this week after three incidents caused several injuries and killed two people -- including a ski patrol member doing avalanche control work.

Sierra Avalanche Center officials say heavy snowfall in recent days, along with fluctuating temperatures and wind, have left the Sierra snowpack unstable.

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"All these things create the perfect storm for avalanche conditions," said Jenny Hatch, center director.

This week's two deaths matched last year's avalanche fatality total for the Tahoe area. There have been 19 avalanche fatalities in California in the last decade.

Cal-OSHA on Wednesday launched an investigation into the death of Bill Foster, 53, a ski patrol veteran at Alpine Meadows Resort, who was buried Monday in an avalanche he and other patrol members had triggered as part of the resort's avalanche-safety program.

Cal-OSHA spokesman Greg Siggins said investigators will review Alpine Meadows' procedures, safety protocols, training and equipment. "They'll look at details of the (avalanche prevention) plan, talk to colleagues ... make sure employees have been trained properly."

Foster was part of a five-person team that was throwing explosives into a closed section of Sherwood Bowl, Alpine Meadows President Andy Worth said. He was stationed at a spot that was believed to be safe, but an explosive tossed by a team member caused an avalanche to initiate higher and wider on the slope than expected, carrying Foster with it.

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Fellow ski patrollers found him within a minute and uncovered him within eight minutes, Worth said.

Foster was pronounced dead at Renown Regional Medical Center in Reno, where he had been airlifted.

Also on Monday, snowboarder Steven Mark Anderson, 49, of Truckee, was killed when buried by an avalanche at Donner Ski Ranch.

The day before, snowboarders at Squaw Valley Ski Resort touched off an avalanche, causing minor injuries to two skiers. Neither skier, a 39-year-old woman who was treated at the resort's clinic and a 16-year-old boy who went to a Truckee hospital with a shoulder injury, was buried in snow.

Avalanches often occur when a heavy layer of newer snow fails to adhere well with the existing layer of snow from a previous storm. The danger is heightened this week, U.S. Forest Service officials said, because the previous snowfall was mixed with rain, creating a frozen surface.

Officials said humans can trigger a slippage between two snow slabs with only a small amount of downward pressure at the wrong spot on the snow surface.

"We have some weak layers right on the edge of failure just sitting there," said Forest Service avalanche forecaster Andy Anderson. "It doesn't take much extra force. Once you break one spot it is like dominoes across the rest of the slope."

Joanne Roubique, district ranger for the Truckee District of the Forest Service, issued a special warning Wednesday to snowmobilers and others who venture into remote areas outside ski area boundaries, where avalanche possibilities are higher and where rescue is more difficult. "For backcountry skiing, snowshoeing and snowmobiling, we encourage everybody to get the best education they can (first)," Roubique said.

Snow safety classes are offered by colleges, ski associations, sports shops and guide companies.

Roubique said recreationists also should check the avalanche center's website -- www.sierraavalanchecenter.-org -- for daily updates on avalanche conditions before venturing beyond groomed and populated areas.

The nonprofit avalanche center, in conjunction with the Forest Service, employs researchers who check snow conditions daily and post their findings on the center website and on Facebook, and broadcast them on Twitter.

Forest Service officials say anyone traveling outside of groomed ski areas should carry safety devices, including a tracking beacon, whistles, shovels and poles that act as snow probes in case someone gets buried.

"Everybody in your party should have a beacon and know how to use it," Roubique said.

Ski areas -- where slope grooming and avalanche control occur -- are generally much safer than the back country, experts say, thanks in large part to the avalanche control work done by ski patrols. But they are not immune to problems.

Worth, president and CEO of the Alpine and Squaw Valley ski areas, said skiers and snowboarders should ski in pairs for safety.

"Snow conditions are variable right now, and we need people to use real caution," Worth said. "Ski and ride with a partner."

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Alpine Meadows posted an homage to Foster on its Facebook site Wednesday. Worth described Foster as a man with a passion for the mountains and said he was a member of a professional team that does a dangerous job.

"Our ski patrols are among the best in North America," Worth said. "They are experts with a great deal of training."

Mark Fisher, a longtime Alpine skier and publisher of the blog, www.unofficial alpine.com, said Foster's death cast a pall among workers at the resort on Christmas day, even as skiers enjoyed deep snow.

"On the surface, it was just another sweet powder day at Alpine Meadows," Fisher wrote. "But underneath the surface, employees were reeling from the death of patroller Bill Foster. ... It was easy to notice the Alpine employees and other friends of Bill, in hushed conversations, often with tear-filled eyes."

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