What causes an avalanche? How common are they? What to know during winter storms

The winter storm bringing heavy snowfall and whistling winds to the Sierra Nevada peaks is sharply increasing the risk of dangerous avalanches, with the National Weather Service issuing a warning Tuesday.

According to the Sierra Avalanche Center, a nonprofit organization focused on education and safety for winter recreation, dozens of avalanches were reported in the days before and during the storm. The center issued a backcountry avalanche warning from Tuesday morning into Wednesday as a result.

The Sierra Avalanche Center forecast the avalanche risk for the central Sierra Nevada on Tuesday as “high,” which is a danger level 4 on the center’s five-point risk scale. Large and very large avalanches were “very likely,” according to the center’s forecast.

On Tuesday morning, a group of 16 skiers was reported missing in the mountain backcountry near Castle Peak after an avalanche. As of 4 p.m. Tuesday, 10 skiers were still missing while six were awaiting rescue.

The weather service issued a winter storm warning for the Lake Tahoe area, saying up to 4 feet of snow are expected through Wednesday.

As the winter storm pummels the Sierra Nevada, 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,” Steve Reynaud, a forecaster at the Sierra Avalanche Center, previously told The Sacramento Bee.

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.

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 a slight decrease in the danger level, from “high” to “considerable.”

The avalanche center forecasts the risk of danger from avalanches on a five-point scale, with one being “low” and five being “extreme.” The “considerable” danger level is in the middle, at three.

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, 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.

In 2025, two ski patrollers at Mammoth Mountain died while doing avalanche control work in separate incidents months apart. Cole Murphy and Claire Murphy — who were not related — were killed amid their avalanche mitigation efforts at the resort’s Lincoln Mountain.

Last month, an Oregon man died while snowmobiling when an avalanche swept him away in the same Castle Peak region as Tuesday morning’s avalanche.

This story was originally published February 17, 2026 at 4:34 PM with the headline "What causes an avalanche? How common are they? What to know during winter storms."