Nine skiers are missing after an avalanche struck the Castle Peak area of the Sierra Nevada mountains, California. The avalanche happened about 10 miles north of Lake Tahoe around 11:30 am on Tuesday. It buried a group of backcountry skiers - four guides and 11 clients. Six stranded skiers were rescued after taking shelter in a tarpaulin-made shelter. They communicated with rescuers via radio beacon and text. "Due to extreme weather conditions, it took several hours for rescue personnel to safely reach the skiers and transport them to safety where they were medically evaluated by Truckee Fire," the Nevada county sheriff’s office said. Two of the rescued skiers were taken to hospital for treatment. The search for the missing nine is ongoing but hindered by bad weather. The sheriff’s office updated the group size from 16 to 15 people. If the missing skiers do not survive, this avalanche could become one of the deadliest single avalanches in US history. Heavy snow, strong winds, and low visibility have created very dangerous conditions. Ski rescue teams from Boreal Mountain and Tahoe Donner’s Alder Creek center joined the search. A total of 46 emergency responders are part of the operation. This season, six avalanche deaths have already been recorded in the US; the average is 27 deaths yearly. A winter storm warning was in effect in northern California during the avalanche. Soda Springs, near the site, received 30 inches of snow in 24 hours. The Sierra avalanche center issued a 'high avalanche danger' alert before the incident. Avalanche forecasters called the backcountry very risky due to the ongoing storm. The missing skiers were completing a three-day ski trip over rugged terrain, carrying all food and supplies. Authorities were notified by the ski tour company. Nevada county sheriff captain Russell Greene expressed concern, saying, "I don’t think it was a wise choice," but admitted details are still unclear. The area is a popular backcountry skiing spot around Castle Peak, a 9,110-foot mountain near Donner Summit, known historically for the Donner Party tragedy. California Governor Gavin Newsom has been briefed and state teams are coordinating with local emergency crews for the rescue effort.