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Storm snow avalanches

Storm snow avalanches are caused by a cohesive slab of new snow overloading the bond at an interface within the storm snow, or at the old surface if it forms an underlying weak layer. These slabs are often quite soft compared to other slab types and this can fool people into underestimating the potential for a slab avalanche. The moving snow often involves a significant powder cloud and the avalanche debris tends to be soft and powdery. Storm slab avalanches tend to release below the trigger, making it somewhat easier to escape if you trigger one. During and shortly after significant storms involving rapid accumulations of significant amounts of new snow, storm snow avalanches can be very large and destructive.

The relative weak layers and slabs required to produce storm snow avalanches are formed by fluctuations in the snow types falling in the storm. Storms associated with more violent weather having periods of high or fluctuating snowfall intensities, fluctuating temperatures, and fluctuating wind or winds that increase late in the storm are most likely to produce storm snow avalanches. Less violent storms with slow, steady snow accumulations, more constant temperatures, and little or no wind are less likely to produce storm snow instabilities. Warming temperatures and/or increasing winds during a storm can lead to ‘upside-down’ storm snow conditions (denser, more cohesive snow over lighter, less cohesive snow) that may produce storm snow avalanches.

Storm snow avalanche activity generally peaks during periods of intense snowfall and tend to stabilize quickly (within 24 to 36 hours) after the snowfall stops. Cold temperatures may cause storm snow instabilities to persist a little longer. Storm snow deposited on top of a persistent weak layer may develop into persistent slab instability instead of stabilizing quickly. Once a storm snow instability has stabilized, it is unlikely to become unstable again although additional loading on a partially stabilized storm snow instability may produce additional avalanches.

View larger image Fracture line and debris from a storm slab avalanche | Photo: S. Hebert

Widespread areas with a feeling of ‘upside-down powder’ (more cohesive storm snow above softer storm snow) can be an indicator of potential storm snow instability. In many cases, however, the surface snow will be good powder conditions with few signs of unstable snow!

Managing this kind of avalanches

For more information on how storm snow avalanches can hurt you and how you can manage the problem see: Good Travel Habits - Reducing the Risk in the Field

Wind slab avalanches