Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2
On north facing slopes above 8000 ft, old snow became weak and faceted in some places and will produce a soft slab avalanche. This persistent weak layer does not exist everywhere, and we're still trying to determine where it does and doesn't exist. In many places the old snow from October and early November is capped by a hard ice/rain/rime crust and December snow is well bonded to it. Other slopes have a thin layer of small facets above this crust and the December snow is not well bonded to it.
Friday on Red Pine Ridge and near Logan Peak, we were hunting for this weak layer. Even though we could only find it in a few places, for now the only safe option is to assume it exists until proven otherwise, and avoid avalanche terrain on north facing slopes.
- People are triggering audible collapses or "whumpfs" in the backcountry. These heart-stopping booming noises indicate unstable snow and potential for dangerous avalanches failing on a persistent weak layer.
- Collapses are often accompanied by shooting cracks in the snow surface. Recently, we've seen some of these extend hundreds of feet away from where we trigger the collapse.
