With big snow comes avalanche danger | TahoeDailyTribune.com

With big snow comes avalanche danger

Not enough snow and then too much too soon triggered avalanches in the Sierra Nevada last week.

A powerful storm blasted the West Coast, causing flooding in California’s valleys and blizzard conditions in the mountains. Before the multi-day storm lost its punch Tuesday, more than 7 feet of snow fell on portions of the Sierra crest, calling out high avalanche warnings for the Northern Sierra by the United States Forest Service.

Forest Service Snow Ranger Bob Moore, of the Truckee Ranger District, said it was the shallow snowpack lying underneath the fresh powder that alarmed area snow analysts.

He said the shallow, early-season snow that barely covered the Sierra’s granite turned into sugary crystals – a layer that can prove unstable if heavy, moisture-rich snow falls on top of it.

“We were really concerned about that,” Moore said. “But it looks like the rain circulated through the pack and has stabilized things.”

The Northern Sierra has managed to avoid casualties. But in Colorado, avalanches have already claimed six lives this season.

“No huge (avalanches) have been reported to me, it’s been mostly smaller releases,” Moore said. Moore compiles avalanche data from snow pit analysis, weather reports, the Internet and backcountry skiers.

More than 170 avalanches of various magnitudes were recorded over Monday and Tuesday by the Alpine Meadows ski patrol. Some released naturally and others were started with the help of explosives.

Alpine Meadows patroller Kyle Collins said about 300 pounds of dynamite are tossed by hand or launched by artillery devices onto slopes during an average day of avalanche control work at the resort. The idea is to set off a slide before skiers or snowboarders rip through the powder and set one off on themselves.

One avalanche that happened during routine control work completely buried patroller Ken Bokelund in the resort’s Wolverine Bowl area. He was dug out within three minutes and had no injuries.

Collins said that amount of slide activity at Alpine Meadows is typical.

“That’s normal,” he said. “And that’s not including all of the natural activity that occurs during the night.”

The sugar snow, or depth hoar as snow scientists call it, actually helped Heavenly Ski patrollers in their avalanche control work this week.

Snow Safety Coordinator Joe Blanchard said many of the slide paths in Heavenly’s expert terrain areas slid about two weeks ago, allowing some of the runs to start with a clean slate during last week’s big dump.

“That got rid of some of the depth hoar,” he said. “But there’s still a few pockets out there that are scary.”

The good news is that time has helped the situation.

“Right now the snow has stabilized and our avalanche forecast is moderate,” patroller Collins said. “The temperature and density, along with a cold, clear night has consolidated the snow.”

Still, avalanche forecasters say to watch your step – especially on high-elevation, north-facing slopes.

Blanchard said an avalanche that ripped down a north-facing slope in Heavenly’s Mott Canyon Wednesday ranked at the highest magnitude an avalanche can receive – Class V.

The class ratings are in relation to the slope’s potential slide path. Class I is a slide less than 150 feet long. The ratings increase incrementally, with a top-out rating at Class V, which means the slide was major in relation to its path.

“Over the Falls went Class V because it went all the way down to the ground,” Blanchard said. “There was about 5 feet of snow there.”

After another round of tests, Mott Canyon was deemed safe by patrollers and opened Thursday to customers. Blanchard said skier compaction, when it is safe, on avalanche-prone slopes is the best measure for preventing avalanches.

Collins warned that conditions are still questionable in untouched areas.

“We don’t forecast for the backcountry,” he said. “But I would ski with caution on uncompacted slopes.”

The Forest Service downgraded its avalanche warning Thursday for the Northern Sierra above the 7,200-foot elevation between Sonora and Yuba passes to “considerable.”

“Considerable means that avalanches set off naturally are possible and human-caused avalanches are probable,” said Moore. “North slopes and cornices are still going to be a problem. (If heading to the backcountry,) be careful and watch what you’re doing.”

For current avalanche conditions in your backcountry area check out http://www.avalanche.org

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