Turnagain Pass

A very large avalanche was triggered by a skier remotely in the upper Winner Creek area yesterday; just northeast of Girdwood. Five skiers had successfully skied the slope prior. No one was caught. The slab was around 4′ thick and broke 6-10′ below the skier who triggered it. It ran around 1,000′. It is suspected that the weak layer is faceted snow that formed in early March.

Wide angle view of avalanche triggered yesterday. 3.28.23.

Closer up view of avalanche. Note the propagation in the slab, indicative of persistent weak layers like facets in this case. 3.28.23.

Over the past 4 days there have been three skier triggered very big avalanches and several deep slabs triggered by two groups of snowmachiners in the back bowls of Seattle Ridge last Saturday. The most recent occurred yesterday mentioned above. Due to the size and unsurvivability if someone was to be caught, we are keeping the danger elevated again today despite fairly quiet weather. Unfortunately, this probably is going to take some time to heal. As time goes on it will be harder to trigger these, but that chance will remain to some degree. We are moving into the realm of lower probability but deadly consequences. Not fun by any means.

The problem is a layer of weak faceted snow that sits under all the storm snow from mid March. This layer is buried around 4-6′ deep now because that storm snow has settled up to 2′ since the storms ended 6 days ago. The layer is too deep to use snowpack tests to assess it and no red flags are likely to be seen until it’s too late. Remember, just because a slope doesn’t slide it doesn’t mean it’s safe, it just means it didn’t slide and there is no way to be certain it could or not. In case you missed it yesterday, Andrew gets right to the point:

1. It’s big. These avalanches are huge. If you haven’t already, take a look at the reports and photos: Skookum Valley, Sunburst, Seattle Ridge, and Palmer Creek. These avalanches have been over 1000′ wide, running thousands of vertical feet, and failing 6′ deep or deeper in some cases. Multiple avalanches were triggered remotely from low angle terrain connected to steeper slopes. Luckily nobody has been caught in any of them.
2. It’s difficult to assess. At this point, the layer of concern is buried anywhere from 4-6′ deep or deeper on average. That makes it hard enough just to dig down and find it, and too deep to assess it. Given the widespread activity we have seen lately, it is best to assume the problem exists if you are traveling in our advisory area.
3. It will take time to heal. These persistent weak layers get their name because they take a long time to go away. For now, it is still clearly a problem that will be driving our terrain choices for the days to come.

Given the challenging snowpack we are dealing with, the only way to truly manage the problem is with cautious terrain choices. You can avoid the problem by avoiding traveling on or below steep terrain. Remember, we have seen multiple very large avalanches triggered from low-angle terrain connected to steeper slopes so be mindful of what is above you.

Snowpack profile in a thin scoured area on Sunburst from Sunday. Image shows the general setup that we are dealing with. Note that the slab on top of the faceted snow varies from a foot thick (thinner areas are trigger points) to over 12′ thick due to winds during the March storms. 3.26.23.

Ridgetop winds are slated to pick up from the east this afternoon into the 20mph range. This could be enough to start forming small wind slabs before sunset. Overnight, these winds look to increase slightly (20-30mph with stronger gusts) along with 2-4″ of snowfall. The cooler temperatures and winds will likely keep yesterday’s wet surface snow, that warmed in the lower and mid elevations, in the frozen form today.