Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 3
The mid-November drought created a very weak snow surface composed of facets and patches of surface hoar on all aspects and elevations throughout the Wasatch Range. Avalanches have been triggered at this weak interface nearly every day in the Central Wasatch since November 29th..with an extensive avalanche cycle occurring in Provo on this layering late weekend. North and East facing aspects harbored some of the weaker faceted snow and have also seen the most wind loading (primarily last Wednesday and Thursday). Not surprisingly, the bulk of the natural and human triggered avalanches have occurred on these aspects.
These soft and hard slab avalanches are failing on this weak PWL (persistent weak layer) 1-4' deep and up to 400' wide (across the Wasatch). These avalanches are sometimes accompanied by audible whumphs or collapses...with many of them triggered at a distance. It should be noted that collapsing and cracking exist in the low elevations as well, although there is not as much of a well developed slab there as in higher terrain.
If you dig down into the snow, you can easily see the poor snow structure that is causing avalanches, particularly on wind loaded slopes.(photo Nichole Dye, Powder Mountain snow safety)
November's low sun angle and colder than normal temperatures led to a poor snowpack structure on south and west facing aspects and backcountry observers have experienced cracking and collapsing in this terrain. This is unusual. Although these "off aspects" have a more complicated snow structure and have experienced less loading (in fact some scouring away of snow), I am not ready to trust these aspects just yet.
TREND: STEADY