Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 2–3
What and where is it: You are most likely to trigger a 2-4 foot thick avalanche on wind-exposed slopes facing the northern 2/3 of the compass. Triggering an avalanche is less likely—but still possible—on steep slopes that appear to be completely sheltered from the wind. The slabs rest above a variety of ugly, widespread weak layers (facets, surface hoar, crusts, and combinations).
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.
How can you manage it: Before triggering an avalanche, you may or may not experience obvious signs of instability like snowpack cracking and collapsing or whumpfing. Avoid obviously wind-loaded slopes. 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.
Additional Discussion: You may be able to trigger thinner avalanches involving only the wind drifted snow from the tail end of the storm. Watch for them near ridgelines and in cross-loaded areas on the sides of gullies. The biggest hazards these soft and hard slabs pose are 1) carrying you where you don't want to go and 2) triggering the much larger Persistent Slabs discussed above as the slides move downslope. Avoid large, consequential avalanche paths where the wind formed stiff drifts or slabs.