Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 1.5–2.5
What is it? 1.5-2.5 foot thick slabs overlie persistent weak layers (sugary facets, surface hoar, or crusts) on most aspects and elevations. Recently wind-loaded slopes with stiffer snow in the top couple of feet of the snowpack are most likely to produce large avalanches, but slopes sheltered from the wind are also suspect.
Where is it? The weak layer exists at all elevations, but slabs become thicker and stiffer as you climb in elevation. Shadier aspects contain the ugliest faceted layers. While the snowpack remains quite weak at lower elevations, some slopes are currently lacking the cohesive slab component of the avalanche equation (weak layer + slab + steep slope).
How can you recognize it? You're likely to experience snowpack collapsing or cracking while traveling. These red flags are obvious signs of instability. Wind-loaded areas feel stiffer under your skis or sled and are more supportable; these areas are especially dangerous. The weak layers plaguing us are easy to identify when you dig a quick hole in the snow.
How can you manage it? Sadly, avoiding slopes steep enough to slide is the only surefire way to manage this problem.Persistent slab avalanches are tricky. You could trigger them remotely (from gentler terrain above, below, and to the sides of steep slopes—explanatory video here) and they may break wider than expected or higher on the slope above you.
(12/7/2022) Skiers triggered this avalanche (on the far ridge, photo right) in the Lake Ck Drainage north of Ketchum. On descent near the older slide (foreground, photo left) they reported hearing/feeling a whumpf followed by the sound of the avalanche release.
Photo: A. Sevcik