“A thoughtful and informative piece by Colin Mitchell, CAIC/CDOT avalanche forecaster on Highway 550” rŌbert
In my home state of Colorado, our winter snowpack tends to develop a dangerous type of hazard, the persistent slab avalanche. Persistent slab avalanches account for the vast majority of avalanche fatalities in the state. Persistent slabs are aptly named—days or even weeks after a storm event, when the danger seems to have passed, you can still trigger one of these avalanches. The problem demands a heightened awareness and a thoughtful approach to terrain selection and risk management. This article explores strategies and tactics you can use to help manage your risk when dealing with this challenging avalanche problem.
The term “persistent slab” refers to a cohesive slab that has formed above a persistent weak layer: facets, depth hoar, or surface hoar. Although these conditions are most common in interior ranges with a continental snowpack like Colorado, you can find the persistent slab avalanche problem in all snowpacks and mountain ranges. Avalanche experts consider persistent slab avalanches to be the most dangerous and unpredictable type of avalanche. There are several reasons why:
- Persistent weak layers can resist strengthening and remain reactive to triggers for weeks or even months. Avalanches formed by storm snow and wind are short-lived problems that cause a spike in hazard, then a rapid drop-off in danger. After a few days, avalanche danger decreases as the slabs bond with the underlying snowpack. With persistent slabs, the problem remains even as the danger rating drops over time. As the slab strengthens and the persistent weak layer is buried deeper in the snowpack, avalanches become harder to trigger but no less dangerous. You may be able to trigger an avalanche in areas of shallower snow, where the weak layer is easily impacted, and release adjacent, deeper areas, resulting in a large and destructive avalanche.
- Persistent weak layers can be widespread across the slope. They form avalanches that propagate long distances and break in unpredictable ways. They can cross multiple terrain features and aspects.
- Persistent slabs can be triggered from adjacent terrain. Another factor contributing to the unpredictability of persistent slabs is that they can be remotely triggered from outside the start zone. You can trigger these avalanches from above or to the side of the slope or from adjacent lower-angled but locally connected terrain. Unstable slabs have also been triggered from well below the start zone and even from flat ground.
- Most of the time, there are few visible clues on the snow surface to tell us where the persistent slab problem exists. Hazards created by falling or drifting snow, warming (sun or rain), or cornice failures all have visible clues recognizable by the backcountry tourer. The persistent slab avalanche problem provides few surface clues. Sometimes the weight of a rider can collapse the weak layer, and you may hear an audible whumph and see shooting cracks on the snow surface. These are compelling signs, and a warning of local instability. Otherwise, determining the distribution of a weak layer requires digging snow profiles, conducting snowpack tests, and investigating avalanches. This requires the skill and experience to conduct these observations safely. You must choose a representative observation site, identify grain types and layering, and interpret snowpack test results. For most recreationists, this means relying on the avalanche bulletin to help us identify and avoid terrain with the problem layer.
Managing the persistent slab avalanche problem starts during tour planning. Know the problem exists and plan to avoid it. First, look at the forecast and determine the aspects and elevations likely to have the problem. Persistent weak layers are formed by big-picture weather trends and are often distributed widely across the terrain. The persistent slab problem demands we think about the terrain on a large scale. If the problem exists on north and northeast aspects, for example, ruling out any slopes on the north-facing side of the drainage is a sensible approach.
But it is not always that simple. For example, in the photo below, the areas numbered 1 are north-northeast aspects. No. 2 is a south aspect. But what about No. 3? Technically, this is a southeast aspect. If the problem is isolated to north and northeast aspects, is this slope likely to have the problem as well? Quite possibly, yes. This slope is on the north side of the drainage, and despite its southerly tilt, it is shaded by nearby slopes most of the day. Its snowpack is probably more similar to that on the nearby surrounding terrain than to slopes on the south side of the drainage.
Sticking to lower-angled terrain is the best way to avoid any avalanche problem, but with a persistent slab problem, we need to pay attention to more than just the terrain we are riding. We have to take into account the terrain surrounding us. Is it connected to, or underneath one of the suspect aspects? Could we remotely trigger a slope that would overrun us? Slope No. 3, for example, is connected to and partially below steep, north-facing terrain, and both slopes run into a terrain trap. Consider your route carefully. Use maps and terrain imagery to help plan a safer tour. Give yourself a wide margin of safety from suspect terrain.