Turnagain Pass

Girdwood Valley, under Notch Mtn (1600-1700′ elevation, WNW facing):  A group of skiers knocked a small cornice chunk onto a small steep slope and triggered this persistent slab avalanche yesterday. It was up to 2′ deep at its max and 100′ wide. This avalanche failed in a weak layer of facets on top of a 3-4″ thick buried melt/freeze crust.

Photo of the persistent slab avalanche described above. 2.18.21.

It is a day to pay close attention to what the winds are doing and avoid slopes with fresh wind deposited snow. As of 6am this morning, the weather stations are still reporting light to moderate NW winds, however, that could all change quickly through the course of the day. How much snow is available for transport is somewhat variable, anywhere from 4-12″ (plus or minus) depending on your location.

If these winds do verify, we can expect to see plumes off the peaks and the potential for natural wind slabs and cornice falls, especially in areas such as Crow Pass and Portage Valley. Additionally, this will be most likely in the Alpine, above the trees, yet watching for wind loading at the mid elevations will be key as well. New wind slabs could be up to a foot thick and possibly thicker. If found, they are likely to be easy to trigger and could step down to old buried weak layers below. In this case, a larger avalanche is possible. Along with watching for active wind loading, feeling for stiff snow over softer snow and watching for cracks that shoot out from you will be signs you’ve found a wind slab.

This flow direction is tricky for Turnagain Pass. It can funnel through the Pass from the south and load north aspects on the non-motorized side, while at the same time load the SE face of Seattle Ridge. It can also split around the Pass and not affect much of the terrain in the heart of Turnagain at all. You might find you are in an area that is seeing no wind, or not enough to move the surface snow into wind slabs. If this is the case, see below as there are some buried weak layers still causing concern.

View from Tincan yesterday that looks SE over 4-6″ of low density snow from the past couple days. 2.18.21.

Buried in the top 2′ of the snowpack, likely deeper in areas near Portage and Placer Valley, are a couple of weak layers (facets and buried surface hoar). At the lower elevations, below 1,600′ at Turnagain Pass and 2000′ in Girdwood and Placer Valley, a crust sits just under one of these faceted layers. This was the setup in the avalanche triggered under Notch Mtn yesterday. Crusts can accentuate this persistent slab problem by providing a slick bed surface, encourage wider propagation if an avalanche is triggered and inhibit the weak layer from gaining strength.

In short, the reactivity of these weak layers continues to be quite variable around the region; meaning whether they are able to cause an avalanche or not. They may be sitting under old wind slabs and hard to identify. Snowpack tests are pointing to both good and not so good stability, making it difficult to truly assess how stable steep slopes are. As Aleph said yesterday, this uncertainty makes the whole situation tricky and the snowpack not that trustworthy… Simply knowing these weak layers exist, watching for signs of instability and following safe travel protocol can help hedge our bets if choosing to push into steeper terrain. Add to that, digging a representative snowpit and assessing the pack yourself is a great way to get more information for the slope in question.