Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1.5–3
What and where is it: You are most likely to trigger a large, thick, 2-5 foot thick avalanche in terrain where the wind frequently blows, and the snowpack depth is variable. These wind-affected slopes are mostly found at upper elevations near ridgelines and prominent terrain features exposed to the wind. Triggering an avalanche is much less likely—but not out of the question—on steep slopes that appear to be completely sheltered from the wind. The slabs rest above a variety of weak layers (facets, surface hoar, crusts, and combinations) on most slopes.
How can you recognize it: Watch for cornices, snow pillows, dunes, and ripples (photo). Feel for stiff snow under your feet or sled. These are signs the wind has been at work transporting snow. The slab above buried weak layers is not uniformly thick, and you may find a shallow spot, making it easier for you to impact them. Rocks sticking out of the snowpack are clues to indicate a relatively shallow snowpack.
How can you manage it: Before triggering an avalanche, you may or may not experience obvious signs of instability like snowpack cracking (photo) and collapsing or whumpfing. Avoid obvious wind-loaded slopes and terrain that is peppered with rocks sticking out of the snow. Persistent Slabs can release in surprising ways, break wider than expected, and release above or hundreds of feet away from where they are triggered. We can't assume tracks on a slope mean it is safe; several people could descend a slope before triggering an avalanche. Remotely triggering an avalanche from flat terrain near steep slopes is a possibility. Pay attention to how steep the terrain above you is.
(12/31/2024) Explosives-triggered slab avalanche on a NE-ESE facing bowl @ 8,400'. The avalanche broke ~300' wide and 2-3' deep. Shoulder of Peak 2 in the Soldier Mtns.
Photo: Sawtooth Avalanche Center