Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Unlikely
- Size
- 2
The January/February drought resulted in widespread weak, faceted snow across the northern end of the compass at mid and upper elevations. These weak faceted grains are now covered by roughly 8-16" of new snow, shallow wind drifts, and some variable crusts from the past week.
Over the past few days, we have been seeing avalanches failing on this persistent weak layer as soft slab avalanches roughly 8-16" deep and up 80' wide. Generally, these avalanches have been confined to isolated terrain features where the slab is slightly stiffer and more connected. The warm temperatures are changing the game, and I would now approach steep northerly facing terrain at mid and upper elevations with caution.
The challenging part right now is figuring out where a slab sits atop the weak faceted grains. In areas that have not received enough wind, or enough snow there still may not be a slab formed. It also seems the most suspect terrain is the mid to upper elevation sheltered slopes that did not see any high winds or sun during our January/February dry spell, making the surface facets that much weaker.
The avalanche danger on this layer will be increasing rapidly over the weekend. With increased snowfall, comes increased avalanche danger. As we continue to add more snow, the slab will only become more connected and widespread increasing the likelihood of triggering a large avalanche. Pay attention to changing weather, as the danger will spike during any periods of elevated precipitation. Cracking and collapsing are red flags that unstable snow exists and should not be ignored.
If the clouds break, and the sun comes out, look for any signs that the snow surface is becoming damp and unstable. Rollerballs/pinwheels are generally the first sign of unstable wet snow.