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