Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1.5–2.5
You are most likely to trigger a large 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. 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.
On most slopes, hefty 2-5 foot thick slabs rest above various weak layers (facets, surface hoar, crusts, and combinations). You're most likely to impact these layers where the slab is thinner. Thin spots are common at the edge of a slab, near ridgelines, or along rocky, midslope ribs. Remotely triggering an avalanche from flat terrain near steep slopes is a possibility.
Unfortunately, we can't expect obvious signs of instability like snowpack cracking and collapses or whumpfs to highlight the danger. However, this lack of feedback does not mean a slope is safe. Multiple people could ski or ride a slope before triggering an avalanche. Persistent slab problems are tricky and aren't effectively managed with micro-scale route finding. The best way to manage your risk is to choose lower-angled slopes and steer clear of wind-loaded terrain peppered with rocks.
Additional Discussion: You may be able to trigger fresh, thin wind slab avalanches that formed near ridgelines and in cross-loaded areas on the sides of gullies in alpine terrain. Look for recent cornice growth, dunes, or ripples in the snow surface. These are signs the wind has been at work transporting snow. These avalanches could step down to trigger the much larger avalanches described above.
(1/4/2025) Snowmobilers remotely triggered this very large avalanche in the Salmon River headwaters. One rider was caught, fully buried, and successfully rescued. The crown is on a NE aspect near 9300'. Photo: Sawtooth Avalanche Center