Heavy snowfall creates great powder plus avalanche hazard | TahoeDailyTribune.com

Heavy snowfall creates great powder plus avalanche hazard

Susan Wood

The “great white hope” for Lake Tahoe ski areas has also heightened awareness for the “white death,” with last weekend’s significant snowfall.

Although avalanche gurus claim the current situation could be much worse, every backcountry skier’s nightmare was rated Monday as “moderate” to “considerable” to “high,” according to Eastern Sierra avalanche bulletins from Sonora north to Yuba passes. The rating is contingent upon the elevation and the slope angles.

As a general rule, most avalanches – affected by weather, snowpack and terrain – occur on slopes between 30 to 45 degrees.

“On the steeps, the small stuff can grow to something bigger,” said Rand Carter of the Sierra Weather and Avalanche Center, a division of the Kings Beach-based Firnspiegel LLC. The atmospheric and snow research company provides research for NASA and the National Oceanic and Atmospheric Administration.

The first season for Carter’s 5-month-old division has turned out to be an interesting one.

Carter’s crew has kept an eye on the snowpack since its early beginnings in the fall because of dissimilar layers and a weak foundation referred to as “depth hoar” in avalanche talk.

Carter’s concern that the loading of last weekend’s quick and accumulating snowfall would trigger a lot of slides from this unstable layer, but the danger was soon minimized when he observed cannon blasts Monday morning at Alpine Meadows and Squaw Valley. Much of Squaw’s terrain was closed due to the avalanche hazard.

Apparently, the light, powdery snow – uncharacteristic of Tahoe’s “Sierra cement” – sloughed from the surface. The break in the storms weeks earlier also produced a level of some concern near the top layers.

“There are unstable layers way down deep, but they’re not letting go,” Carter said, relieved that “nothing deep” came crashing down. “Even though we have the great intensity (of snow) and wind, we usually have much more danger.”

Carter, a former heli-ski guide, ski patroller and National Avalanche School graduate in 1981, points to the 4 percent to 6 percent density in last weekend’s snowfall that’s often found in the snow in the Wasatch Mountains of Utah. Density equates to mass per volume that’s measured by dividing water content by the snow depth.

“It’s surprising that even with the shooting in areas that don’t get skier packing, (the backcountry testers) are not knocking down anything deep,” he said.

Bob Moore, who’s tracked backcountry avalanche hazards for the U.S. Forest Service for 20 years, said he believes the warm weather may have helped bond the snow layers. And the cohesion improves with time.

Bryan Carlson, local maintenance supervisor with the California Department of Transportation, said his crew fired off 11 cannons Sunday night at Echo Summit, one of a handful of avalanche hotspots.

Others areas include the Mount Rose Highway between the ski area and the summit, Highway 88 at Carson Spur, Kingsbury Grade and the Emerald Bay area. Caltrans was expected to open the scenic Highway 89 Tuesday afternoon, Carlson said.

Still, skiers and boarders are not out of the woods.

On north- and northeast-facing steep slopes, the danger is high above 10,000 feet and considerable above 7,000 feet. It falls to a moderate level below 6,000 feet. South-facing slopes are more dangerous in the spring.

The bulletin advises that backcountry travelers limit their terrain to sheltered areas in the trees on slopes less than 30 degrees. It’s also recommended to refrain from going alone and carry a transceiver, shovel and probe.

If they are not rescued within 30 minute, about half of all buried victims will die. Many are killed during or within minutes from suffocation.

There are two types of slides – loose snow and slab avalanches.

Loose snowslides begin from a single point and expand as they descend. The slide path looks much like an upside down V.

Slab avalanches represent the biggest threat to skiers and climbers. When the stress within the snow layer exceeds the strength of the snow, the slab releases much like a pane of glass when it shatters under its own weight.

Avalanches occur all over the world, including the Lake Tahoe Basin.

A 22-year-old snowboarder, who narrowly missed qualifying for the U.S. Olympic team, was killed in an avalanche near Donner Summit in February 1998. The cornice Jamil Khan was standing on gave way, triggering an avalanche that sent him 400-feet down the slide path.

A year later, a sledding expedition along the shore of Lake Mary near Norden turned deadly for Malcolm Hart, 21, of Dover, N.H., when he and three friends were caught in an avalanche. Hart’s friends were buried for six hours before they were rescued.

On the South Shore, an avalanche that started at the top of the face under the Gunbarrel chairlift at Heavenly a few days before Christmas in 1996 sent a ski instructor, Paula Ranne, careening down the face. She suffered a broken knee.

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