This avalanche failed on the persistent slab problem - unlikely to find, but possible. Fortunately, the skier and dog were able to get out uninjured. This avalanche was most likely triggered on the skin up, in a shallow spot. This is common with this type of problem and with such a large pre-existing condition as the buried near surface facets and depth hoar. With slabs this deep, you might get out to the middle of the slab before it breaks above you. Hard slabs are hard to predict. Be careful.