Deep and old slab avalanches (ODS) are often
hard-to-forecast. The size and destructive potential of ODS avalanches can be
disturbing. As a starting point for a study of hard-to-forecast avalanches we
define ODS avalanches based on a large dataset spanning the three main mountain
ranges of western Canada: Coast, Columbia and Rocky Mountains. The definition
is based on extreme slab depth and extreme age of the snowpack weakness that failed
causing the avalanche. Our data did not reveal a difference in slope angle or
aspect between the deep-old-slab avalanches (ODS) and “Other” avalanches.
However, compared to Other avalanches, ODS avalanches were typically of greater
size, involved less human triggering, released more often on crusts and weak
layers of facets and tended to occur more often in early winter.
