As the winter storm pummels the Sierra Nevada, here’s how avalanches can happen and how often they occur across California: 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.

Several skiers were reportedly in the area of an avalanche near Castle Peak.

The Nevada County Sheriff's office reported that a group of back country skiers was involved in an avalanche incident, which occurred Tuesday. Several members of the party are still missing at this time.

A British man is among three skiers who have been killed in an avalanche in the French Alps. A group of five people were skiing off-piste in the mountains near the resort town of La Grave when the avalanche struck on Tuesday.

Authorities say six backcountry skiers were found alive and 10 others were still missing Tuesday in an avalanche in Northern California during a powerful winter storm moving through the state.