Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 2
A persistent weak layer of faceted snow that formed during the December dry spell is our primary avalanche concern. This layer can be found about a foot below the surface on slopes facing W-N-E-SE. Near and above treeline, slabs of wind drifted snow have formed over top of this weak layer and this is where you are most likely to find trouble. Above treeline, however, northerly winds have created an erratic distribution of over-riding slabs, stripping and eroding snow off of many north and east aspects that are usually loaded. Cross-loading, however, is still a factor. This means you can be traveling through terrain that looks scoured and benign, and then suddenly find yourself on top of a hard slab with facets underneath. Suspect this scenario when the texture changes from rough and thin, to smooth, hard, and fat. Be alert to likely deposition zones such as gullies, depressions, and the leeward sides of terrain features. In these same areas, an outside chance lingers for avalanches to fail on weak facets near the ground. Avoiding extreme, rocky, slopes with a thin snowpack is the best way to mitigate this problem.
The profile below is from Colorado Bowl on Monday. Notice the very weak snow in the top of the pack. No slab is present in this profile, but on many slopes near treeline and above, slabs have formed on top of this very weak surface snow causing elevated avalanche conditions.