Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 2
A cold dry period for two and half weeks in mid November resulted in a very weak snowpack. A persistent weak layer of faceted snow developed at all elevations on aspects facing W-N-E. This weak foundation of snow was buried by seven inches of new snow on 11/29. Warm temperatures over the past several days have caused the surface snow to form a cohesive slab on top of the buried weak faceted snow. Snowfall starting this morning and continuing into Wednesday will begin to add stress to our fragile snowpack. Red flags will not be in your face this morning. Audible collapses and shooting cracks are signs that this layer is being overloaded and failing. Look for increasing danger as the snow piles up and adds weight to this weak foundation. The danger for triggering an avalanche on this layer will rise as the storm goes on. With an inch of water and a foot of snow forecasted to accumulate by Thursday morning we should start to see some avalanche activity.
If you see the surface snow cracking out like this, you have found the slab. Notice the cohesive nature of the surface snow in this photo.
Strong snow over weak snow. The weak layer of facets below the slab is obvious in this photo.