Keystone – As spring sunshine brings welcomed warmer temperatures to the mountains, officials warn that the threat of deadly wet-slab snowslides has increased.
The Colorado Avalanche Information Center this past week took the unusual step of issuing an avalanche watch as temperatures rise rapidly but the mountain snow remains deep, luring late-season skiers to the backcountry and the one ski area that’s still open.
“At high elevations, we have basically a late-winter snowpack that’s undergoing a very rapid warm-up,” said Ethan Greene, director of the avalanche center. “That combination in the past has produced significant wet-slab cycles.”
A year ago this weekend, skier David Conway of Boulder was killed in a rare in-bounds avalanche at Arapahoe Basin ski area, drawing attention to the general inability to anticipate and mitigate late-spring wet-slab snowslides – when entire faces of snow break away due to weakening caused by meltwater.
“It’s a really difficult thing to predict, both because it’s a very rare occurrence and because we don’t understand the mechanisms that lead to that kind of thing,” Greene said.
The May 20, 2005, avalanche, on the popular First Alley run, dispelled the notion that heavily skied terrain cannot slide.
A big avalanche last weekend on Torrey’s Peak that swept a climber 1,000 feet down indicates conditions are ripe again, Greene said.
Officials at Arapahoe Basin, citing concerns for a potential lawsuit stemming from Conway’s death, declined to comment on what, if any, changes they have made in determining whether to close runs because of wet-slab avalanche danger.
A review by the U.S. Forest Service indicated that Arapahoe Basin – which will remain open until June 4 – did not specifically address wet-slab avalanches in its snow-safety procedures.
But Molly Cuffe, spokeswoman for Colorado Ski Country USA, said resorts took steps in the wake of the accident to consider the possibility of wet-slab avalanches when determining whether to keep slopes open – although she could not provide specific examples.
Meanwhile, the avalanche center has begun its own study of the conditions leading to wet-slab avalanches, Greene said.
“What we’d like to do is come up with an approach for forecasting these avalanches, but the first thing we need to do is kind of understand when they break,” he said, noting the organization is taking temperature and snowpack readings at A-Basin as part of its work.
Wet-slab avalanches most commonly occur on slopes in direct sunlight during the heat of the day, when overnight temperatures don’t drop low enough for a solid freeze, and they are considered particularly dangerous because of the cement-like consistency of the snow once it settles.
Four people were killed in three avalanches in Colorado this winter, a number that Greene hopes won’t increase. On average, the center reports that seven people are killed by avalanches each year in the state.
“The avalanche danger will be gone only when the snow is gone,” Greene said. “In general, now through the next month or so, the snow is pretty solid as long as you follow pretty common-sense rules as far as going early and getting off early before the snow becomes too wet.”
Staff writer Steve Lipsher can be reached at 970-513-9495 or slipsher@denverpost.com.
