Persistent slab avalanche problems occur when a cohesive slab forms over a buried weak layer such as surface hoar or facets. These layers are slow to bond with surrounding snow and can linger for weeks or months. Persistent slab avalanches can spread out far and wide, making them very hard to escape.
Persistent slab problems are tricky to identify and manage. If the weak layer is still relatively shallow in the snowpack, you may see signs of instability like shooting cracks and whumpfs. When it is buried deeper, those signs disappear. Pay careful attention to the forecast so you know where it exists and avoid slopes with a persistent slab problem. Be patient as the weak layer will eventually strengthen and the problem will go away.