Turnagain Pass

Triggering a deep slab avalanche, a deep persistent slab avalanche to be correct, is our main concern.

With a couple days of quiet weather behind us and a mostly sunny day on tap, the snowpack is finally getting a chance to adjust to the loading from the past few weeks. Unfortunately for us, we have those buried weak layers to deal with and they don’t heal up in a few days. In fact, it can take weeks or more. These are the same buried weak layers of facets and buried surface hoar that have been responsible for almost all the avalanche activity over the last month, including the several close calls on Dec 2nd and 3rd when two skiers were caught and one snowmachiner was buried. This last storm (12-26″, 6 days ago) also overloaded these layers, producing large natural avalanches.

How reactive are the layers now? There has not been a lot of traffic to test them, but avalanche science says, with time and being buried over 3′ deep, they should be more difficult to trigger. This is the scary MODERATE scenario… slabs are becoming harder to trigger, but can be large and have high consequences.

Here are the main characteristics of a deep persistent slab:

• Difficult to trigger – stubborn
• Capable of producing very large avalanches that could propagate across terrain features
• No signs of instability before a slope releases
• Can be triggered after many people have been on a slope
• Can be triggered remotely (from the side, top of below)
• The slab itself is more than 3′ deep and is composed of hard snow

The weak layers are roughly 3-5′ deep in Girdwood and Turnagain Pass, but are much shallower to the south of the pass and in Summit Lake (south of the forecast area). They are most concerning in the 1,500 – 3,000′ elevation band where the Halloween crust sits under them and can exacerbate the issue. Hence, open steep slopes in the trees can be just as dangerous, or more so. It’s a tricky situation and many of us are simply avoiding the issue all together by sticking to slopes 30 degrees or less.

If you are considering stepping into larger/steeper terrain, be mindful of this potential. It’s also good to know slopes that have seen little to no traffic this season are more suspect. Watching your partners closely, only exposing one person at a time, and having an escape route planned are all ways to help hedge your bets. Also, thinking about the consequences if the slope does slide; will the debris pile up deeply in a terrain trap (bad) or spread out (also bad, but better chance for rescue)?

Schematic of a deep persistent slab. (avalanche.org)

Wind Slabs:  Lingering wind slabs from Sunday’s winds are likely to be found in exposed terrain and especially in areas on the periphery of the forecast zone – south end Settle Ridge, Summit Lake, etc. Watch for the classic signs of wind loading, cracking in the snow around you and stiff snow over softer snow. Wind slabs could trigger a deeper slab, creating a much more dangerous situation.

Photo from Eddie’s looking toward Kickstep by Andrew Schauer. Andrew’s comment: “Daydreaming of better stability. Getting big ‘You can look but you can’t touch’ vibes right now.” 12.13.2021