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Persistent deep slab avalanches

Persistent deep slab avalanches are caused by a thick and hard cohesive slab of snow losing its bond to an underlying weak layer that is very deeply buried in the snowpack, often on or near the ground. For a deep slab problem to develop, weak layers must be deeply buried by further snowfalls and they must be layers that are highly resistant to settlement and bonding. The persistent weak layers that created persistent deep slab problems generally form when an early season snowpack is exposed to periods of clear, dry weather.  The persistence and propagation potential of the instability is greatly increased if the weak layer forms in conjunction with a rain crust.

Persistent deep slab avalanches are most common above treeline where the snowpack is deeper. In continental climates, they are most likely to be found in leeward areas of high alpine terrain. Persistent deep slabs are common on all aspects and show little preference for specific terrain features. However, avalanche activity is often variable in distribution over terrain. That is, a particular mountain range, drainage, mountain, slope on a mountain, a given elevation, or a particular aspect may have a persistent deep slab problem while adjacent ranges, drainage, mountains, slopes, elevations, or aspects do not.

Where the snowpack is deep and evenly distributed and when the weather is benign, deep persistent slabs generally require heavy triggers such as large explosive charges, large cornices, or icefall. Step-down triggering is also common, that is: a smaller or shallower avalanche on the slope in question or falling from above in turn triggers a deep slab. Remote triggering from shallow or weak spots in the snowpack is common.

View larger image Deep slab avalanche running on a basal weakness | Photo: C. Dietzfelbinger

Persistent deep slab avalanches are highly unpredictable and destructive; essentially not survivable. It’s common for persistent deep slab avalanches to become dormant for extended periods of time then “wake-up” again, weeks or sometimes months later,  when the weather changes or when warm spring weather arrives.  When active, they often occur without warning or with light triggers. In dormant phases, they may require heavy triggers such as snowmobiles, cornice falls or smaller avalanches from above or on the slope which step down to deeper layers!

Managing this kind of avalanche

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

Cornice falls