Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Unlikely
- Size
- 2
photo of skier catch and carry off the Flagstaff Ridge into Toledo on Monday
A thin layer of weak faceted snow above an ice crust that formed about a week ago has been the culprit in a number of natural and human triggered avalanches over the past several days. These soft slab avalanches have resulted in at least a couple close calls with one skier caught and swept into trees in the Toledo area of LCC. These slides are roughly 1-2' deep and average 200' wide. Collapsing and cracking sometimes accompany this unstable layering.
WHAT IS UNUSUAL is that this PWL problem is on the sunny slopes. We tend to see tricky PWL (persistent weak layers) on northerly slopes. Now we are dealing with them, although momentarily, and in spotty fashion, on west to south to southeast facing slopes.
Mark Staples describes how this facet-crust layer forms in this video.
Here's the good news - This layer mostly exists on slopes that should have an ice crust on the surface this morning, and those are ones you will likely avoid anyway. Northerly facing slopes seem to lack this layer and will have better snow.
Nikki went to check out the avalanche in Mule Hollow of BCC yesterday. You can see the thin layer of soft faceted grains above a crust near her snow crystal card.