What causes an avalanche? How common are they? What to know after Palisades Tahoe death

Experts say the avalanche that killed one person and injured another at Palisades Tahoe Wednesday morning isn’t the first to occur in the Tahoe region this season and it won’t be the last.

According to the Sierra Avalanche Center, a nonprofit organization focused on education and safety for winter recreation, avalanches had been occurring throughout the Tahoe region in recent days.

“We had avalanches occur just this weekend,” Steve Reynaud, a forecaster at the Sierra Avalanche Center, said. “A few days ago, we had a pretty active storm pattern that caused avalanches during the weekend and on Monday.”

While those avalanches were reported and documented by the avalanche center, Reynaud said there were no people involved in the incidents during that three-day cycle.

The avalanche at the popular ski resort began around 9:30 a.m. Wednesday, above the GS gully area of KT-22, according to a social media post by Palisades Tahoe.

As the winter season continues, here’s how avalanches can happen and how often they occur across California:

What causes an avalanche?

An avalanche is an increased amount of snow sliding or tumbling down a slope.

Avalanches can reach speeds of up to 100 mph and can vary in their power to kill and destroy, according to the American Avalanche Association.

“Avalanches are based on the storms that come into the area and the existing snowpack structure,” Reynaud said.

As snow continues to fall, Reynaud said the winter storms can cause additional weight and load to the snowpack.

“With avalanches being able to be triggered by human interactions, an avalanche is also possible with skiers or snowmobilers,” Reynaud said.

The heavy amounts of snow can then roll down a nearby slope, such as a hill or mountain, Reynaud said.

“Avalanches can also be caused from existing snow, the snow that comes in, or avalanches that occur down deeper in the snow,” Reynaud said.

With much of Northern California under winter storm warnings, Reynaud said the weather system could have played a role in the avalanche at Palisades Tahoe.

However, Reynaud said each ski resort has its own avalanche predictions.

It most common for an avalanche to be triggered by nature, such as a storm dropping a large amount of precipitation or wind drifts that move a lot of snow into an area, said Ethan Greene, director of the Colorado Avalanche Information Center. It’s less common for skiers, people on snow mobiles or others on a slope to trigger an avalanche, he said, but it can happen.

“Anytime we have snow on steep slopes, you have a chance of an avalanche,” Greene said.

How common are avalanches in Tahoe?

The Sierra Avalanche Center releases avalanche forecasts on a daily basis.

“Avalanches occur in every steep, snowy and mountain environment,” Reynaud said. “We get a lot of avalanches in the Tahoe area.”

For Wednesday’s forecast, the center predicted an increase in possible avalanche danger.

“A strong winter storm will enter our area today bringing high intensity snowfall and gale force winds,” the report, written by Reynaud, states.

According to the report, the avalanche danger was expected to quickly increase in a variety of areas by the afternoon and throughout the early evening hours.

The avalanche dangers for the Tahoe region were “considerably high” on Wednesday, Reynaud said. Out of five-point scale, Wednesday’s avalanche danger was at a three.

As the winter storm progresses, Reynaud said avalanche forecasters predict avalanche dangers until tomorrow evening.

How often do avalanches occur across California?

Though not every avalanche is reported and documented, Reynaud said the Sierra Avalanche Center, located in the Tahoe area, calculates about 15 to 40 avalanches for its coverage area during the winter season.

To prevent being caught in an avalanche, California residents can contact their local avalanche center to view where possible dangers exist.

Work done to reduce risk from major avalanche

Greene, with the Colorado Avalanche Information Center, said work is done well in advance — especially at a ski resort — to reduce risk of a major avalanche occurring, “as soon as the first snow of the season starts falling.”

Authorities monitor the snow pack throughout the season, constantly checking its layers and terrain for characteristics for a potential avalanche. Greene said ski resort personnel move in and break up these layers in the snow pack to reduce the chances of a large avalanche.

Members of ski patrols might ski through the slope to break up the layers or use other equipment to change the snowpack. Greene said they sometimes might use explosives to trigger a smaller event.

“So they never build that pressure (in the snowpack) that could trigger a big avalanche,” Greene said. “We’re dealing with a natural hazard in mountain conditions. You can never completely eliminate the hazard, but you can reduce the hazard.”

He said he’s collaborated before with avalanche specialists at ski resorts throughout the country, including Palisades Tahoe, and they have extensive training and experience in avalanche mitigation.

How common are avalanche deaths?

Greene said it’s not rare for an avalanche to cause a fatality; an average of 26 people die in the United States annually. But an avalanche death at an operated ski area is unusual, he said, with the most recent death in January 2020 at Lake Tahoe’s Alpine Meadows Ski Resort. One man was killed and another was seriously injured after the avalanche swept up the men on the ski run known as Subway Cirque.

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