Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 2–3
You could trigger a 2-4' thick persistent slab avalanche on buried weak layers (sugary facets or crust + facet combinations). This problem is worst on middle and upper elevation slopes facing W-NW-N-NE-E-SE.
Unfortunately, unstable slopes probably won't provide obvious signs of instability, like snowpack collapsing or cracks shooting out in front of you. Snowpack stability tests are likely to be inconclusive and are of limited use. Tracks on a slope don't mean it's safe. Avalanches can be triggered by the second, third, or fourth rider on a slope. In low probability/high consequence situations like this, there's a good chance the first sign of instability you'll see is the avalanche you trigger.
The likelihood of triggering a slide is decreasing, but the thick, dense slab makes the consequences severe. Persistent weak layers can fail unpredictably and break wider and higher on slopes than you might expect. Avoiding avalanche terrain is the only surefire way to manage this problem. Short of that, you can reduce your risk by avoiding large avalanche paths and those with ugly consequences if you are caught in a slide.