Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 2
Most of these avalanches we've seen since Thursday are soft slab avalanches failing on a persistent weak layer. This interface, now 12-20" down, is a facet or facet/crust combination that will remain problematic and reactive for some time, particularly with new "loading events". Expected loading events that will add additional stress to this weak interface include extensive wind drifting over the next couple of days along with the Tues/Wed storm of 1-2' of snow. As the overlying "slab" of snow becomes thicker and harder, avalanches will become much deeper and wider = more dangerous. What makes PWLs (persistent weak layers) so tricky is that you can trigger them from a distance, sometimes with no obvious signs of instability. The good news is that you can pull out your shovel to see if these weak sugary faceted grains exist in the terrain you're in. At this point, if the facets are present, it'll be key to stick to low angle slopes with nothing steep above. We've generally mapped these areas as west-northwest to north to easterly facing slopes above about 8500'.
In summary, avoid steep terrain that harbors this weak layering or choose the aspects that are free of this unstable layering. Forecaster Trent Meisenheimer explains below:
