Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2
A dense cohesive slab now exists over top of a pre-existing layer of weak, faceted snow that has been sitting around since October. This layer exists at all elevations on slopes facing W-N-E, and on SE aspects near and above treeline that still held snow prior to the storm. Collapsing of the snowpack and reactive snow pit tests indicate serious instability in these areas. This problem will be with us for awhile, and slopes steeper than 30 degrees are to be avoided. You can also trigger avalanches from lower angle terrain beneath slide paths. Give them a wide berth.
In my travels yesterday I dug on a lower elevation W facing slope and found very poor structure. An extended column test produce results of ECTP 11. See photos below:
The snowpit diagram below illustrates the current structure of strong snow over weak snow. This was dug on Nov 27 on a N aspect at 11,200'. On top you can see the dense slab created by the recent heavy snow. Underneath is the weak, faceted snow, with the red line indicating where we got failure in an extended column test (ECTP 16). A layer of deteriorating basal ice makes up the bottom 5 cms.