Do you remember where you were on November 19? That date is less than three weeks ago and then, we were talking about thin and discontinuous snow cover and barely enough snow to slide on. Turn the page to December 10 and the mountains look a whole lot different and have a much more dynamic snowpack.
There are two snowpack concerns to look out for. The first lies near the surface and the second is at the bottom. Several observations throughout the San Juan Mountains highlight one if not two surface hoar events that are now buried under a cohesive slab. This persistent weak layer will lay silent below a slab until a rider, machine, or a new load collapse these delicate feathers and the snow above comes crashing down. Observations highlight reactive surface hoar near Wolf Creek Pass and in the Telluride area but digging into the snow is the only way to determine its presence. Northerly, wind-sheltered terrain is where this weak layer stayed preserved.
The larger and more dangerous avalanche issue is the October snow that survived at the bottom of the snowpack. This snow metamorphosized into facets and depth hoar and has been reactive with each loading event. It’s now buried under a three to four-foot-thick slab. The stiffness of the overlying slab makes it more difficult for a rider or machine to impact this weak cohesionless layer and trigger an avalanche. If you do find the not so sweet spot, it may propagate wider across the slope than expected and lead to an avalanche that would be difficult to survive. Most likely trigger locations are near the edge of the slab or near rock outcrops where the snowpack in shallow.
Winds shifted from the southwest to northwest overnight. This may increase the distribution of wind-drifted slabs in the alpine. These should be generally small and easy to avoid. You might encounter new drifts on southeast and south-facing slopes forming above a firm melt-freeze crust. These drifts may not bond well to the old snow surface and could move quickly down slope with ski pressure.
The bottom line is that our snowpack has some complexity to it. Different weak layers and variable snow depths make for tricky conditions. In the deepest areas the snowpack is showing some signs of promise and strengthening. If it continues to snow, we may eventually have less concerns with basal weak layers. However, it’s still too early to determine and caution is advised. Without a careful snowpack evaluation, paying attention to general snow depth, weak layer type and reactivity, you should consider simply avoiding steep slopes that face a northerly or east aspect.