A 15-year-old Wyoming boy was killed Monday while snowmobiling in the Uinta Mountains backcountry.

Hayden Ellingford of Evanston, Wyo., was on a family trip with his father and two brothers. The four were riding their snowmobiles through the backcountry near the locally known areas Yamaha Hill and Humpy Peak when the massive avalanche began.

Shortly before 4:30 p.m., the 500- to 600-foot wide slide, with 4- to 6-foot crown, careened down a steep slope in the Windy Ridge/Moffit Basin area of Summit County, said Summit County Sgt. Nick Wilkinson. The slide sent snow sliding about 1,000 feet down the northeast face of the mountain and into a grove of trees.

Ellingford was caught in the avalanche, about 8 miles west of the Bear River Service on State Road 150.

The boy's family, as well as Bear River Service, called for emergency help. Both the Uintah County Sheriff's Office and Summit County Search and Rescue crews responded to the scene.

But Ellingford was buried in six to seven feet of snow without any locating equipment, such as a beacon. Crews found one of the boy's boots sticking out of the snow and began combing that area, but it took nearly one-and-a-half hours to find the 15-year-old boy.

Rescue workers found the teen at 5:54 p.m., and medical crews tried for 20 minutes to stabilize him before pronouncing him dead, Wilkinson said.

The boy was being sent Tuesday to the Utah State Office of the Medical Examiner to determine the cause of death.

An investigation into what triggered the avalanche is under way.

The slide took place on a 40-degree slope in an area deemed treacherous by forecasters at the Utah Avalanche Center, which put a special avalanche advisory in effect for the western Uintas on Sunday.

"A high avalanche danger exists on all steep mid- and upper-elevation slopes. Human triggered avalanches are certain, natural avalanches are possible. Backcountry travel is not recommended," avalanche center forecaster Craig Gordon wrote.

Gordon said in an interview on Monday evening that the center has been aggressively trying to spread the word about this season's unusual avalanche patterns and urging people in the Uintas to stay away from slopes that are 30 degrees steep.

A rain crust that developed in snow packs over Thanksgiving is contributing to layering that is atypical for the Uintas, Gordon said.

"The bottom line is that we have a very unusual snow pack that we're dealing with right now. Avalanches are occurring in terrain that people have never seen avalanches occur in," Gordon said. "I think it's a time when recreation users really need to think about their terrain choices and need to think about the consequences of their terrain decisions."

Utah Avalanche Center forecaster Bruce Tremper said "weak sugary snow" in the Uintas compounded by new, heavy snow is a recipe for dangerous avalanches.

He encouraged people who plan to travel in the backcountry to carry basic rescue gear, like beacons, shovels and probes. He also recommended checking the avalanche report at http://utahavalanchecenter.org or 888-999-4019 before heading outdoors.

The 15-year-old's death on Monday marks the fourth avalanche death this month, following incidents at Snowbird and Logan Peak.

Monday's avalanche took place in the same region as a fatal 2007 Christmas Day avalanche.

Last year 53-year-old David Balls, of Oakley, was snowmobiling with his four sons when a 400-foot-wide avalanche swept over him and killed him near the Thousand Peaks recreation area in Summit County.

Balls was overcome by a sheet of snow 4 to 5 feet deep that plunged about 500 vertical feet in the Super Bowl area of Whitney Basin, said Craig Gordon of the Utah Avalanche Center.

mrogers@sltrib.com