Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Very Likely
- Size
- 3
Over the past week we have added several feet of snow (with nearly 4" of water) to our snowpack. In addition, several wind events with strong winds from the west/southwest have deposited dense slabs of wind-driven snow at the mid and upper elevations. This new snow and dense wind slabs have created dangerous avalanche conditions on mid and upper elevation slopes facing northwest through north and east where there is a persistent weak layer (PWL) of faceted snow down near the ground. Avalanches over the past few days have broken down to this weak layer, producing slides up to 4' deep and hundreds of feet wide.
This dangerous weakness may be disguised with none of the typical signs of instability such as cracking and collapsing. But if you do trigger an avalanche where this persistent weak layer (PWL) is present - perhaps from a thinner spot on the slope or on a steeper convex rollover - the avalanche will break down 4' deep (possibly deeper) and up to hundreds of feet wide. Unsurvivable.
We are moving in the right direction and ultimately the load of snow this past week should help settle the PWL problem, but for now your continued patience is required.
Mike Fogg's extended column test (ECT) with full propagation illustrates this weak layer and the slab on top:
