Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2–3
Several weak layers are buried in the upper 1-4' of the snowpack. Snowfall from yesterday and today is adding significant stress to these weak layers, and triggering a large slide involving these weak layers is likely. The nature of these weak layers varies by aspect and elevation, and the overlapping patterns are complex and hard to decipher. Crusts with facets, surface hoar, and near-surface facets—it's a safe bet that one or more of these layers exist on most slopes. Today is not a good day to try to thread the needle between them.
We are just over a week out from a spate of natural and human-triggered avalanches that failed on weak layers in our snowpack (see photo at right/above, avalanche database here). These layers are again being pushed toward their breaking points today. Avalanches are most likely in terrain where the wind is building slabs, but large slides are possible in sheltered terrain as well.
Persistent weak layers behave in ways that surprise even experienced backcountry travelers. Slides that fail on these layers may break much wider than you expect, wrapping around terrain features and producing large debris piles. Importantly, avalanches on these layers can be triggered from long distances away from steeper slopes. Pay attention to the terrain you are exposed to from above.
ADDITIONAL DISCUSSION: Weak snow makes up the base of the snowpack here. In most places, it is buried beneath a thick, dense slab of snow, and impacting it with your weight is unlikely. However, the two large avalanches triggered by skiers in this zone in the past 9 days are proof positive that this weak snow should remain on your radar. You are most likely to trigger an avalanche involving this snow in steep, rocky, wind-affected terrain where the slab over this weak snow is thinner and the weak layer is more "accessible". Smaller slides can break down into deeper weak layers, producing a much larger, more destructive avalanche.
(1/1/2023) Debris from a very large avalanche that was remotely triggered above Hyndman Creek in the Pioneer Mountains. This slide failed in a heavily wind loaded area and very likely involved a persistent weak layer. It covered the summer trail with a pile of debris that was 100-150' wide and 4' deep.
