When do Persistent Slabs go away?

Steve Banks

Executive Director

CBAC

You may have noticed that the Crested Butte Avalanche Center’s daily bulletin has been talking about the persistent slab problem for most of the winter now. We have also had some other avalanche problems such as storm slabs, wind slabs and even wet loose avalanche problems. These problems seem to come and go, while persistent slabs stay in the bulletin day after day. So when will the persistent slabs go away?

Persistent slabs is the name we give to the avalanche problem when we have a weak layer that is made up of faceted snow grains, depth hoar or surface hoar with a thick denser layer above. These types of weak snow grains tend to change very slowly and often linger throughout the entire winter season. Really it is a persistent weak layer that is the problem, but since it is the slab that could potentially kill you, we call it a persistent slab avalanche problem. These particular weak layers need a lot of time to morph into a better type of snow grain, and often we get into full on spring corn cycles before these layers begin to look any better.

So it would seem that if persistent slabs are really a problem, we must be triggering a lot of avalanches, right? Well, not really. These weak layers can have a certain amount of strength to them. The trick is to be able to recognize where and when they are strong enough to support the overlying snow as well as the additional weight of a backcountry rider. This is why we perform snowpack test like compression test and extended column tests. We are trying to determine how strong the weak layers may or may not be. Given the idea that stronger layers over weaker layers in the snowpack create avalanches, merely looking at the structure of the snowpack would indicate dangerous conditions. However snowpack tests are beginning to show harder results leading us to believe that the snowpack is gaining strength.

Now comes the tricky part. If the weak layers creating the persistent slab avalanche problem are becoming stronger, how do we know where and when they are strong enough to trust? It is always difficult to assess where an avalanche will occur, but it is especially difficult during times of relatively benign weather. This is when forecasters will be talking about low likely hood and high consequences avalanches. While it is difficult to initiate the slide, the resulting failure could entrain a lot of snow and have a life threatening outcome. So where would you be more likely to trigger an avalanche? The best way to know the answer is to dig in to the snow to check out the layering and perform some tests to see just how strong the slab is and just how weak the weak layers are.  In general we have a weaker snowpack closer to the town of CB where the snowpack is shallower. Stronger snowpacks can be found deeper in the mountains where the snowpack is deeper. When in doubt, err on the side of caution, and always remember to check the current avalanche bulletin at www.cbavalanchecenter.org.