Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 1–2
You're likely to trigger a persistent slab avalanche large enough to bury you. A layer of weak snow underlies a 1-2 foot thick slab on many upper elevation slopes and middle elevation slopes that face away from the sun. Recent snowfall and drifting snow added stress to this weak layer in both wind-loaded and sheltered terrain.
Direct feedback, like cracking or collapsing of the snowpack, indicates that conditions are dangerous. You may even trigger avalanches remotely, from gentle, low-angle terrain as you ascend a safe ridge (video). Without these obvious clues of instability, you can quickly and easily identify this layer by digging a quick hole. The weak facets will appear grey and feel loose and crumbly. On middle elevation slopes that face away from the sun, they may rest atop a stout ice crust, a particularly alarming and weak structure. The only surefire way to avoid this problem is to steer clear of steep slopes where these old facets exist.