Blue Sky Syndrome
"We call it blue sky syndrome," said Bill Seline, a mountain guide and avalanche safety instructor with the California-based Alpine Skills International company. "It's the human factor at work. The sun comes out the day after a storm, and everything is beautiful and conditions seem perfect. What could happen on such a great day?"

Avalanches could happen.



"Even people experienced in the backcountry could get caught in avalanches," Seline said. "So many things can happen in a day, so many changes in the snow conditions."

Recent snowstorms, high winds and warming weather are creating dangerous avalanche conditions in the Sierra, U.S. Forest Service officials said.

An advisory was posted late last week but probably will continue as wintry storms and spring weather alternate in the mountains. The Forest Service's avalanche advisory covers the central Sierra Nevada between Yuba Pass on the north and Sonora Pass on the south. It applies to backcountry areas outside of developed ski areas, officials said.

"All our advisories are predicated on weather," said Bob Moore, avalanche specialist with the Truckee Ranger District. "The sun warms the snow and puts some weight on top, which opens us up to some wet slides. We're likely to see slab avalanches on southwest slopes. The more heat on the slopes, the more avalanches can be expected. "The danger will remain as long as we've got snow on the ground, and we've got a heck of a lot of snow up high."

So far this year, avalanches have caused 21 deaths in the United States and six in Canada, according to Forest Service reports. Within the last week, avalanches claimed one person in Utah and two snowmobilers in Idaho. In February, an off-duty Mammoth Mountain ski patroller died in an avalanche while skiing the backcountry near Bridgeport, Calif.

Mt. Rose and Tahoe/Donner Area
On Thursday, officials said, sunny skies created areas of unstable snow along the Sierra crest and in the Mount Rose areas. That day, the Forest Service reported a large avalanche on the east side of Pyramid Peak in Desolation Valley and another near Donner Pass.

Avalanche Mechanics: Temperature, Slope, Wind
Studies have found that the avalanche risk increases with the difference in density between layers of snow. The steeper the slope, the greater the risk that a slide will occur.

Wind also is an architect of avalanches, experts said. High winds blast snow over ridge tops and help build up weight on cornices, which are snow ledges on ridges. Those cornices become increasingly sensitive to collapse, either from human intervention like a passing snowmobile or from solar warming. Car- to bus-size pieces of collapsed cornice fall and smash into the slopes below, causing large avalanches.

Officials warn that any backcountry area wide enough to ski, snowboard or snowmobile through is especially susceptible to avalanches.

Skiers, snowmobile drivers and snowboarders who travel in the backcountry out-of-bounds of ski areas where avalanche dangers are controlled are aware that they can be swept away in great slabs of snow, but they may not take all the precautions necessary, experts said. "It's easy to get complacent," Seline said.

Precautions
He had some advice for backcountry travelers:
--Read the Forest Service avalanche reports and warnings.
"A lot of science goes into those," he said. "They (people on the staff) check the snowpack and do things like throw two-pound bombs to see if it triggers an avalanche on a slope. There are a lot of standardized tests involved."

If a group of skiers sees an avalanche, they need to exit the area. "People are tempted to say, 'well we saw an avalanche over there, but it still looks stable over here or on the side of that slope,'" Seline said. "But seeing one is the best indication you are in avalanche-prone territory."

--Learn from the danger reports, advisories, and experience.
"Even if you don't take an avalanche course, you can learn a lot from paying attention to Forest Service advisories," Seline said.

--Know when avalanches are most likely.
"Most avalanches happen during a storm event or 24 hours after," he said.

--Be prepared. Tell people where your group is going (in the backcountry) and take survival equipment such as a first-aid kit, shovel, snow probes, avalanche transceivers that send and receive radio signals, and an Avalung, a device that allows people to draw oxygen from the snow pack.

By FRANK X. MULLEN of Reno Gazette Journal, 4/10/2006. Photos by Mark McLaughlin