Skiers triggered last week’s deadly avalanche at Silver Mountain despite the best efforts of the resort’s ski patrol to mitigate the danger, according to a preliminary report.
“They had done their control work just like they always do,” said Jeff Thompson, the director of the Idaho Panhandle Avalanche Center. “In fact, I think they did more than they normally do to open that slope.”
Three skiers died and two others were fully buried but were pulled alive from the snow on Jan. 7. Rescuers unburied the two men after 50 minutes. Both survived. The following morning it was announced a woman was still missing. Her body was found Thursday, Jan. 9.
According to a survivor of the avalanche, ski patrol allowed about a dozen skiers and snowboarders to pack a trail through fresh powder traversing beneath Wardner Peak.
Their movement along the flank of the 6,200-foot peak triggered the Size 3 avalanche (up to 1,000 tons of snow), Thompson said. Last week some rescuers erroneously described the avalanche as a Size 4 slide (up to 10,000 tons).
The details of how Silver Mountain’s ski patrol, volunteers and staff responded to the avalanche, in addition to the specific mitigation techniques used and decisions made, will be released by Silver Mountain some time in the “next couple weeks,” said Silver Mountain spokesman Gus Colburn.
That broader investigation is being conducted by Silver Mountain in conjunction with IPAC.
IPAC provides avalanche forecasts for 2.7 million acres in North Idaho, including the Selkirks, Cabinets, Purcells and the Coeur d’Alene/St. Joe Divide. The center is funded partially by the Forest Service and through donations from the Friends of the Idaho Panhandle Avalanche Center.
Thompson, who also works as a ski patroller at Schweitzer Mountain, raced to Silver Mountain on Tuesday with his avalanche dog to assist in the search. After he was no longer needed, he started to investigate the origin and cause of the avalanche.
According to his preliminary report, the 300-foot-wide avalanche released about 90 feet from the top of Wardner Peak, tumbling down the ski run called 16 to 1. Despite lots of snow in the previous days, the avalanche started deep within the snowpack on a buried layer of what’s called “surface hoar.”
Once the snow started sliding, the avalanche traveled more than 900 feet down the 35-degree slope.
“So, it wasn’t storm snow that was a problem,” Thompson said. “But it was the weight of the storm snow that put on that added stress.”
Surface hoar is essentially frost – like what you scrape off your car window on cold mornings. The crystals form during clear and calm conditions, according to avalanche.org. Once buried by snow, the crystals don’t adhere well to the snow above or below, creating a weak layer that can last for days, weeks or the entire winter. That layer can be difficult to detect.
The crown face, a sheared-off area where the avalanche originated, was about 2 to 3 feet deep, according to Thompson. It resembles dramatic sea cliffs, with the uphill layer of snow raised above the already avalanched downhill layers.
Although there was a second avalanche on the nearby run Morningstar, it didn’t fully bury anyone and appears to have been triggered by the avalanche on 16 to 1, Thompson said. A third avalanche roared down the backside of Wardner Peak, which is not in the ski resort boundary.
Snow science is notoriously complex, which made it possible for an avalanche to occur in an area that had been bombed and controlled by the resort, Thompson said.
“There are no fingers to be pointed from IPACs standpoint,” he said.
For example, near the top of Wardner Peak, wind had deposited additional snow, making the pack deeper and burying the unstable hoar layer lower down, he said. However, where the avalanche started, the snowpack was shallower – around 4 feet – meaning the instability of the hoar layer was more pronounced.
Thompson has worked with and around snow and avalanches for two decades, mostly in Colorado. In that time, he’s investigated several inbound avalanches, although none of those included fatalities.
“Sometimes explosives and mitigation practices aren’t enough,” he said. “Mother Nature has a mind of her own.”