Persistent Slabs
- Alta quota
- Limite del bosco
- Sotto il limite del bosco
- Probabilità
- Possible
- Dimensione
- 2
The primary avalanche concern is a weak layer of faceted snow that formed during the December dry spell and was buried on 12-25. This layer can be found about a foot below the surface on slopes facing W-N-E-SE. This weak layer is widespread, but the overlying slab is not, and it takes both to make an avalanche. The bullseye location for this instability is near treeline northerlies. You can encounter this weak structure above treeline, however, strong 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 at the top of the pack. No slab is present in this profile, but on some slopes near treeline and above, slabs have formed on top of this very weak surface snow causing elevated avalanche conditions.