Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1.5–2.5
What is it? Several weak layers are buried in the snowpack's upper 1-2 feet. On slopes that face away from the sun, you'll find some surface hoar and facets at these interfaces. On slopes that face the sun, you'll discover crusts with some small facets here. Further down in the snowpack, you'll find layers of weak snow near the ground. Triggering an avalanche on any of these layers is most likely in wind-affected terrain steeper than about 35 degrees.
Where is it? Expect to find at least one of these weak layers on most slopes in the mid and upper elevations. Most of them are more dangerous in areas with recent wind-affect. The surface hoar is most problematic in somewhat protected, middle-elevation terrain that faces NW-N-NE. Older weak layers (crusts and/or facets, occasionally surface hoar) can be found in rocky, wind-affected, alpine terrain where the snowpack is less than 3 feet thick (see Ethan's video at right/above).
How can you recognize it? Wind-loaded slopes have harder surfaces and a distinct appearance of pillows or lens-shaped slabs of snow. Cracks shooting out from your machine or skis clearly indicate an unstable slab. Even small slabs that knock you off your feet and send you downslope can carry high consequences in the wrong terrain (photo here).
How can you manage it? You can reduce your risk by avoiding very steep (greater than about 35 degrees), shady slopes (facing NW-N-NE) with hard or stiff wind-blown snow at the surface—especially those with ugly consequences if caught in an avalanche. Persistent slab avalanches may break wider than expected or higher on the slope above you.