SAN FRANCISCO (AP) — Crews pushed through mountainous wilderness near Lake Tahoe during a snowstorm to rescue six backcountry skiers who survived an avalanche but were trapped by its snow and ice. Nine others from their tour group remained missing.

Two of the six were taken to a hospital for treatment, said Ashley Quadros, a spokesperson for the Nevada County Sheriff’s Office.

The sheriff’s office said Tuesday night that there were 15 skiers on the trip — not 16 as initially believed.

FILE PHOTO: The snow-covered Sierra Nevada Mountains are seen from the air during a Pacific Gas and Electric snowpack surv...

Search and rescue crews were dispatched to Frog Lake in the Castle Peak area, northwest of Lake Tahoe, after a 911 call reporting an avalanche and people buried. A powerful winter storm was moving through California at the time.

Extreme conditions in the Northern California mountains slowed the rescue effort. It took crews several hours to reach the skiers and take them to safety, where they were evaluated by the Truckee Fire Department.

The sheriff’s office said it would provide another update on rescue efforts at a news conference Wednesday morning.

A three-day ski trip

The skiers were on the last day of a three-day backcountry skiing trek, said Steve Reynaud, a Tahoe National Forest avalanche forecaster with the Sierra Avalanche Center, which had contact with people on the ground in the area. He said the skiers spent two nights at huts on a trip that required navigating “rugged mountainous terrain” for up to 4 miles (6.4 kilometers) while bringing along all food and supplies.

Nevada County Sheriff Capt. Russell Greene said authorities were notified about the avalanche by the ski tour company that led the expedition, Blackbird Mountain Guides, and by emergency beacons the skiers were carrying. Rescuers made their way cautiously toward the scene of the avalanche because of the danger of more avalanches.

Blackbird Mountain Guides said in a statement on its website that it was coordinating with authorities on the rescue operation.

Dangerous backcountry conditions

California is being walloped this week by a powerful winter storm bringing treacherous thunderstorms, high winds and heavy snow in mountain areas.

“It’s particularly dangerous in the backcountry right now just because we’re at the height of the storm,” said Brandon Schwartz, Tahoe National Forest lead avalanche forecaster at the Sierra Avalanche Center, based in Truckee.

The center issued an avalanche warning for the area in the Central Sierra Nevada, including the Greater Lake Tahoe region, starting at 5 a.m. Tuesday with large slides expected into Wednesday.

The town of Soda Springs, near where the avalanche took place, recorded at least 30 inches (76 centimeters) of snow during a 24-hour period, according to the Soda Springs Mountain Resort.

The dangerous conditions were caused by rapidly accumulating snow piling on fragile snowpack layers coupled with gale-force winds.

The storm wreaked havoc on roads from the Sierra Nevada to Sonoma County. Traffic was halted temporarily in both directions on I-80 over and around Donner Summit due to spinouts and crashes, the authorities reported.

Several Tahoe ski resorts were fully or partially closed due to the weather. Resorts along highways have avalanche mitigation programs and were not expected to be at as high of a risk as the backcountry, where travel in, near or below avalanche terrain was strongly discouraged, the center said.

Area has dark history

Castle Peak, a 9,110-foot (2,777-meter) mountain north of Donner Summit, is a popular backcountry skiing destination. The summit, which can be perilous in snow, is named for the infamous Donner Party, a group of pioneers who resorted to cannibalism after getting trapped there in the winter of 1846-1847.

In January an avalanche in the region buried a snowmobiler in snow and killed him, authorities said. Each winter, 25 to 30 people die in avalanches in the U.S., according to the National Avalanche Center.

Training in avalanche assessment and rescue and safety equipment is highly recommended for backcountry skiing, also known as off-piste skiing, involves venturing deep into the wilderness far outside the confines of a resort. Backcountry skis are wider and heavier and have other features to handle going up and down ungroomed terrain, unlike cross-country skis, which are narrower and designed for flat, more groomed trails.