Turnagain Pass

The Solstice wind event, though pretty short lived, created a natural wind slab avalanche cycle early yesterday morning. These northwest winds peaked just over 24 hours ago and have diminished since then. Crowns of several small to large wind slabs were visible, especially on the southern end of the forecast zone and in Summit Lake. Some of these wind slabs stepped down to the weak layers buried in the snowpack, causing a much larger avalanche than your typical wind slab.

We also had a report of one human triggered avalanche in the Girdwood Valley, west of the Crow Pass trailhead. This was a remotely triggered wind slab on a steeper rollover.

Natural avalanche triggered by the strong Solstice winds early on Tuesday 12.21.21, photo taken later that day. The wide propagation of this avalanche is indicative of buried weak layers. 

Two natural slabs triggered by the Solstice wind event on easterly facing slopes in Lynx Creek as well as on Silvertip. 12.21.21.

Natural wind slab avalanches that stepped down to buried weak layers in the Summit Lake area. These are on the east facing gullies of Summit Peak. Photo: Paul Wunnicke, 12.21.21.

Large natural wind slab from the Solstice wind event, NE shoulder of Summit Peak in the Summit Lake region. Photo: Paul Wunnicke, 12.21.21.

Skier remotely triggered this wind slab on a steeper rollover in the upper Girdwood Valley. Photo: Ryan Mclaughlin, 12.21.21.

Triggering a wind slab formed by yesterday’s winds will be the main concern today. These new wind slabs are likely to be quite hard and up to 2′ thick. They could lure us out on the slab before releasing and alternatively, be triggered remotely (from the side, below, or the top of a slope). Although the northwest winds are still breezy (~10mph), they are not forecast to be strong enough to move much snow until late tonight and into tomorrow when they are expected to ramp back up.

Reports from yesterday are the winds made it to most open areas at and above the trees. That means we could find wind slabs lower on slopes than we may expect. Keeping a close eye out for any signs of wind loading will be key. A thin wind crust can transition into a wind slab quickly, especially near the tops of ridges and on rollovers. Watching for changes in the snow surface, a stiffer layer of snow over softer snow and cracks that shoot out from you, are great ways to assess a wind slab.

Keep in mind that even a small wind slab that is triggered could ‘step down’ and trigger a larger slab that breaks in buried weak layers. This was the case with many natural avalanches yesterday in areas with a shallower snowpack (south end of Turnagain Pass and Summit Lake).

Wind transport along the Magnum Ridge yesterday. This was after the brunt of the winds moved through. Photo: Scott Johnson, 12.21.21.

Winds were able to blow in tracks through the day on the Southern end of Turnagain Pass. This is from Pete’s North yesterday, 12.21.21. 

With the evidence from yesterday’s wind event that overloaded buried weak layers, we definitely have to remember that triggering a deeper slab is still a concern. These faceted weak layers are from early November and are slowly getting squished under a harder slab of snow anywhere from 2 to 5+ feet thick. The likelihood for one of us to trigger a deep slab (2 to 5ish feet deep) is becoming less as the days go by, but still something we all should keep in mind. The most concerning areas are those with a more shallow snowpack. This would be areas on the south end of Turnagain Pass, Johnson Pass, and further inland on the Kenai (i.e., Summit Lake).

It is also good to remember that unlike wind slabs, deep persistent slabs rarely give us any clues they are close to avalanching until they do. Snow pit tests are unreliable and signs of instability are often nonexistent. The snowpack can ‘feel fine’ but it might not be. Now that the winds have redistributed the surface snow to some degree, there could be more thin spots and trigger points as well. All reasons to keep our guard up coming into the Holiday weekend!