Turnagain Pass

With a quiet day of weather on tap for today, the main concern will be that deeply buried layer of weak, faceted snow that plagues us throughout the advisory area. This layer is buried 3-4′ deep on average towards the north end of Turnagain Pass and in the Girdwood area, just barely 3′ deep on the south end of the pass, and even shallower in Summit Lake. The most likely place to trigger a deep slab avalanche is where the snowpack is the thinnest, so given that spatial trend it is important to be aware of potentially more dangerous conditions as you move further south in the advisory area.

In addition to changes in reactivity with snow depth, there are a few other things to key in on with this tricky layer. First, deeply buried weak layers tend to stop showing any of the classic warning signs even when they are still reactive, including the way the react in stability tests (see video below- or linked here). That makes it hard to recognize potentially dangerous conditions. All you can rely on is recognizing poor structure (i.e. a stiff slab on top of weak snow), which we know we have throughout the advisory area. The other dangerous characteristic of this deep slab problem is how wide it can propagate. We have seen this snowpack lead to very large avalanches, with crowns wrapping around large terrain features and sometimes wrapping around corners connecting different aspects. The deep and stiff slab can also propagate up into low-angle terrain above steeper slopes, and it can be triggered from low slope angles below steeper slopes. All of these challenges can be managed by smart terrain use. Hedge your bets by avoiding the big, steep slopes that can produce unsurvivable avalanches for now. Problems like this do improve with time, but it takes some patience. Luckily we still have plenty of winter left.

Large natural avalanche on the west face of Comet Peak, likely from the 12/9-12/10 natural cycle. This shows what our current snowpack is capable of. Avalanches like these are becoming more difficult to trigger, but the possibility remains. Photo courtesy of Chugach Powder Guides. 12.15.2021

With the exception of a few zones outside of our forecast area (Portage, Seward), we are not expecting strong enough winds to build fresh slabs today. But we did see some slopes growing surface hoar prior to the last wind event, which will make it possible that some lingering wind slabs can still be reactive to human triggers today. Be on the lookout for stiff, punchy, or hollow snow at upper elevations near ridgelines, in gullies, or on the downslope side of convexities. Unlike our deep slab problem, these older wind slabs will likely give you warning signs like shooting cracks or collapsing. A relatively small wind slab avalanche could turn into a serious problem if it steps down to the deeper weak layer mentioned in problem 1 above.

(If the video below doesn’t load you can find it on our YouTube channel or this link)