Vail, Summit regions prone to larger, more destructive avalanches as snowfall picks up (video) | VailDaily.com

Vail, Summit regions prone to larger, more destructive avalanches as snowfall picks up (video)

Jack Queen
Summit Daily News

Colorado is bouncing back from a slow start to the snow season, enjoying a healthy storm cycle over the past several weeks. But in the Vail and Summit County region, the new snow is ushering in one of the most dangerous backcountry snowpack regimes in several years, raising the prospect of unpredictable and devastatingly large avalanches.

The risk of deep persistent slab avalanches, which occur when snow loads heavily on top of old, weak layers from the early season, is now present on most slopes in Summit County, particular on northwest, north-northeast and east-facing slopes, according to the Colorado Avalanche Information Center. Areas near and above tree line are especially risky, as are bowls and other large, open terrain.

"We have very tricky conditions right now," CAIC director Ethan Greene said. "If you go to the wrong place, you can trigger a large and dangerous avalanche."

Deep persistent slabs tend to form during seasons where a lot of snow comes late, piling on top of old layers that have crystalized and weakened. When those deep layers break, they can trigger slides that send the entire season's snowpack roaring down the mountainside. The apparent strength of the thick layer of snowpack on top of that fragile layer can be deceptive.

"Because we have this really thick, hard upper layer, you can travel around it and it feels supportable — you can ski and snowboard and snowmobile across it without much of an issue," Greene said. "But if you hit just the right spot where you're able to impact the snow underneath, you start an avalanche. And because there's this thicker, harder layer on top, it ends up triggering much bigger and more destructive avalanches."

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The latest snowfall began Monday afternoon, Feb. 19, with Vail Mountain reporting 15 inches of snowfall in the past seven days as of Wednesday afternoon, and Beaver Creek reporting 22 inches of snow during the same time period.

"We are finally getting the storm cycle that we've been hoping for this season," meteorologist Joel Gratz wrote in a blog post on opensnow.com Monday morning. "We'll see additional storms during the next (week), and most of this time will be dominated by winds from the southwest to west-southwest to west. These directions favor the southern and central mountains (though the northern mountains will get some snow, as well)."

That additional snow was not expected to increase avalanche danger to "high," the level at which CAIC issues avalanche warnings. But each new inch of snow can exacerbate the deep persistent slab problem, even if it doesn't increase the short-term danger significantly.

"Most likely what will happen is when we have snowfall, it will rear its ugly head and things will get worse, and then if we have long periods without a lot of snow or without a lot of wind, it'll sort of subside and we may see a few avalanches but not that many," Greene explained. "And then when it snows again, the avalanches will be bigger and scarier."

The 2013-14 season was the last time deep persistent slab was a serious problem for an extended period of time, Greene said. That season, eight people were killed in avalanches in Colorado. Conditions were similar the previous winter, which saw 11 fatalities.

So far this season, there has been only one avalanche death in Colorado. Abel Palmer, 27, was killed in a slide near Silverton in southwest Colorado on Jan. 21 shortly after a storm there.

There have been two major slides reported near Eagle County in the past few weeks, one of which was a classic deep persistent slab. On Thursday, Feb. 8, a snowboarder triggered an avalanche on a north-facing, near-treeline slope near Vail Pass, causing a slide that grew to 400 feet wide and a maximum crown thickness of six feet, according to a CAIC report. It caught one rider, who was taken to the hospital with serious injuries.

"That's sort of a classic example in that they were riding an area, one person rode down before the second person came down and triggered the avalanche — and then that avalanche broke much wider than they expected," Greene said.

That slide and several others in the past few weeks prompted CAIC to start warning of deep persistent labs, which will likely pose a risk to backcountry travelers for the rest of the season.

On Sunday, Feb. 11, a skier triggered an avalanche in an area known as The Fingers near Berthoud Pass. The slide was not a deep persistent slab avalanche, and the skier was able to dig himself out after being carried to the bottom of the slide path.

What You Need to Know About These Avalanches

Deep persistent slab avalanches are destructive and deadly events that can release months after the weak layer was buried. They are scarce compared to storm or wind slab avalanches. Their cycles include fewer avalanches and occur over a larger region. You can trigger them from well down in the avalanche path and after dozens of tracks have crossed the slope. Avoid the terrain identified in the forecast and give yourself a wide safety buffer to address the uncertainty.

Source: Colorado Avalanche Information Center