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.

Interpreting Snowpack Layers and Hardness

When the word avalanche is mentioned, images of curtains of snow cascading down the mountain come to mind. Specific conditions create avalanches. One of the basic elements, of course, is snow. The snowpack (the seasons snow on the ground) is a complex, but a key to understanding it is identifying layers and differences between them. Layers within the snowpack are a record of the winter’s weather. Like tree rings or strata of rock, layers can be traced to dates and conditions that formed them.

 One of the most important characteristics of a layer is its hardness. Harder snow is stronger and cohesive, while softer snow is weaker. For a slab avalanche we need a strong layer over a weak layer. We actually use a “Hand Hardness” scale by pushing our hand into each layer to determine the hardness. “Fist” hard snow is the softest, then “four finger”, then “one finger” snow being harder, and so on. No, there’s no “middle finger” on this scale. The greater the hardness difference of neighboring layers, the more likely we are to see avalanches at that interface.

The recent snowpack history from the Crested Butte backcountry is a great example of layer hardness relating to human triggered avalanches. Consider these three scenarios:

1) The last week in January brought up to 50” of snow over seven days. While this storm caused some natural avalanches, riders initially weren’t triggering many slab avalanches. Most of the snow came in light and soft. Digging in sheltered and shaded areas during the middle of the storm, you’d find very soft (fist hard) new snow on top of some slightly harder old snow.

2) The storm ended on Thursday January 31^st. By then the wind had formed stiff slabs at all elevations but especially near and above treeline. Digging on some leeward slopes near treeline you could find one finger hard wind-loaded snow sitting over four finger hard facets. Many reports of natural and skier triggered slides came in from wind-loaded areas.

3) By Monday, February 4th, the storm had ended four days prior. Mild daytime temperatures had created settlement in the height of the storm snow and an increase in its hardness. I was skiing on sheltered easterly aspects below treeline.  These slopes had seen very little wind in the past week. Throughout the day, we felt many rumbling collapses and we remotely triggered two sizable avalanches. On these sheltered slopes we found storm snow that had stiffened to a hardness of four finger sitting over fist hard facets.

This storm and avalanche cycle in the late January to early February shows how layer hardness often relates to skier triggering. The most consistent skier-triggering occurred where we found the most change in hardness in adjacent strong over weak layer. If this talk of layers seems daunting, remember that you don’t have to figure it out on your own. The daily avalanche forecast is best place for current conditions. You can check out the "Observations" page to read what other backcountry travelers are seeing. You could even look at the "Snow Profiles" page for technical graphs of recent snow pits. From there you can investigate the snow as much or as little as you want. But most of all have fun in the mountains, make sound decisions, and stay safe doing it. Check out the daily forecast at www.cbavalanchecenter.org

CBAC Forecaster Josh Hirshberg

Some Crusty Thoughts

South facing slopes were the go-to for many folks in town to find stable skiing and riding during our December storm cycle. The snow fell on dirt on many low elevation sunny slopes, rather than the problematic facets that were lurking on shaded slopes. Someone asked me recently, “When do south aspects become dangerous?” It depends, of course, but a lot of it has to do with crusts. With all the sun we’ve been getting in January, we’ve seen melt-freeze crusts form on most of our southerly slopes. So what does this mean for future stability and what happens when snow piles on above crusts?

There are a lot of variables at play, so I’ll illustrate with a few examples. The best case scenario is that we have a thick, stout crust that is still warm as a new storm arrives, and the new snow falls warm and wet. Instabilities at this crust/new snow interface will be short-lived because a good bond will form. This pattern is more often played out in the springtime, which is why we can sometimes find stable powder skiing after a spring storm drops snow on widespread crusty surfaces.

The worst case scenario is if we have facets form and stay preserved above a crust prior to a big snowfall. This can happen in a number of ways. Mid-winter, we will occasionally see crusts form on days when it is still pretty cold, but there is strong enough solar radiation to warm and melt the snow just below the surface. The actual surface of the snow stays cool because it radiates heat, so what we have is a thin crust forming below cold faceting snow at the surface. Suddenly we have a bed surface with a weak layer waiting for a slab to ruin someone’s day. A similar scenario is if we have a crust that has been forming for days or weeks, and then we get a dusting of snow on top followed by clear and cold weather. This gives us another scary bed surface with facets developing above it. Often these facets above crusts are difficult to spot with the naked eye after they are buried by a slab, but a stability test will usually give them away.

Now here’s the really tricky part. When a crust gets buried, it can have unusual behavior. Crusts act as vapor barriers to the normal, everyday movement of water vapor through the snowpack (Picture TSA security clogging the flow of people at an airport). This can lead to faceting above or below the crust, even if the bonds were originally strong. This type of problem is most common in cold, shallow snowpacks such as what we have in Crested Butte right now. We have already observed pronounced facets developing under our current crusts on south aspects. With a big enough load, that crust won’t be able to support the weight above it and the whole thing will come crashing down, failing on those facets below the crust.

So if I’ve lost you with all this technical jibber jabber, here’s the bottom line. Crusts have the potential to be a dangerous interface as more snow piles above them. Treat a buried crust as guilty until proven innocent. Watch for signs of instability and dig down to check how the snow is bonding to that crust before committing to steep terrain. And remember that south facing slopes that you were skiing or riding safely back in December may no longer be stable if we get another big storm cycle. For daily avalanche advisories and observations, visit cbavalanchecenter.org

CBAC Forecaster Zach Guy