Unstable snowpack causes heightened avalanche risk in Turnagain Pass…

A human-triggered avalanche last weekend caught two skiers in Turnagain Pass, a popular backcountry spot for recreationists.

Both skiers were okay, though one ended up with broken ribs. It’s the latest in a winter that’s seen a handful of large human-triggered avalanches due to an unstable snowpack — though forecasters say the danger is slowing down.

John Sykes, a forecaster with the Chugach National Forest Avalanche Information Center, said the reason for the uptick in human-triggered avalanches this winter has been the presence of what he calls “persistent weak layers” in the snowpack.

“Which means some kind of a layer that either forms on the surface and then gets buried, or sometimes they form within the snowpack as the snow grains change shape over the course of the season,” he said.

One of this season’s persistent weak layers is from a crust of ice that dates back to early winter and was covered with weak snow. The other was a layer of buried surface hoar from mid January. Both were tied to a series of human-triggered avalanches in the pass.

As the name implies, persistent weak layers can last a long time. And they’re unpredictable. The majority of the time, they’re just sitting in the snowpack.

But there’s always the possibility that something will get set off. That means just a few bad apples can ruin the whole bunch.

“The presence of those weak layers makes it really hard to make good decisions,” Sykes said. “They basically require a lot of patience.”

He said this season is somewhat unique.

“It’s more typical for us to get a few layers that would last maybe a few weeks to a month,” he said. “But having these layers that are on our radar for the whole season is a little more of an outlier.”

He said as weak layers get pushed deeper by more snow, that risk starts to go away because the weight of a human can’t trigger an avalanche quite as easily — though he said where the snowpack is thinner, like in the Summit Lake area, those layers could still pose more of a danger. As a result, he said the risk is improving as the season progresses, especially along more moderate slopes.

But human-triggered avalanches are still possible — like the avalanche Sunday.

Sykes said that avalanche was triggered on a weak layer closer to the surface, not one of the deeper layers from earlier in the season. The avalanche happened in an area called Tincan Library, where terrain is steep.

Another variable that comes into play at this point in the season is sun.

“In the early season, we treat everything the same,” Sykes said. “But from now all the way through the end of season in May, we have to pay really close attention to differences between aspects that are getting sun and aspects that aren’t getting sun. And it kind of adds a whole other layer to the complexity of trying to break down conditions and figure out where different avalanche problems are happening.”

You can check out the avalanche forecast for Turnagain Pass and recent avalanche observations at cnfaic.org.