Avalanche Problems that persist in old snow layers
The remaining avalanche problems persist in older snow layers. The avalanche forecast will likely have been talking about weak or problem layers that have been "persistent" or may refer to a weather event that could lead to a problematic "persistent" layer in the future. Familiar with avalanche problem types? Skip ahead to check your understanding.
Persistent Slab Avalanche
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near- surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky.
Deep Persistent Slab Avalanche
Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.
Glide Avalanche
Glide Avalanches are the release of the entire snow cover as a result of gliding over the ground. Glide avalanches can be composed of wet, moist, or almost entirely dry snow. They typically occur in very specific paths, where the slope is steep enough and the ground surface is relatively smooth. They are often preceded by full depth cracks (glide cracks), though the time between the appearance of a crack and an avalanche can vary between seconds and months. Glide avalanches are unlikely to be triggered by a person, are nearly impossible to forecast, and thus pose a hazard that is extremely difficult to manage.
For a quick reference summary of all the problem types bookmark this page: https://avalanche.org/avalanche-encyclopedia/#avalanche-character