Key takeaways
- A persistent slab is a cohesive slab resting on a persistent weak layer (facets, surface hoar, depth hoar).
- The weak layer resists healing and lingers for days, weeks, or months.
- It's among the most dangerous, unpredictable avalanche problems.
- It can be triggered remotely or by a later party, producing large, deadly slides well after a storm.
This is general educational information, not avalanche-safety training. Avalanches are deadly — get formal avalanche education, carry rescue gear, and check the local forecast.
What a persistent slab is
A persistent slab is an avalanche problem in which a cohesive slab of snow rests on a ‘persistent weak layer’ — faceted grains, buried surface hoar, or depth hoar — that resists strengthening and lingers in the snowpack for days, weeks, or even months. When triggered, the slab releases on this weak layer.
Why they’re so dangerous
- The weak layer persists for weeks or months, so danger lasts long after the storm.
- It’s unpredictable — the snowpack can seem stable while still primed to fail.
- It can be triggered remotely or fail under a later party, even after others skied the slope.
- It tends to produce large, deep, deadly avalanches.
A week after the last storm, the snow surface feels settled and several parties have skied a slope — but a buried persistent weak layer remains. The next group triggers it remotely from the flats below, and a deep, wide slab releases — the hallmark unpredictability of a persistent slab.
How to manage it
With conservative terrain choices and patience. Because these problems are unpredictable and stability tests can mislead, the safest approach is often to avoid avalanche terrain entirely while a persistent slab exists, rather than hunting safe lines on it. Read the forecast, understand the buried weak layers, and give the snowpack a wide margin and time to heal. It’s a different beast from a quickly-healing wind slab.
The bottom line
A persistent slab is a cohesive slab over a lingering persistent weak layer (facets, surface hoar, depth hoar) that resists healing for weeks or months — among the most dangerous, unpredictable avalanche problems. It can be triggered remotely or by a later party and produce large, deadly slides well after a storm. Manage it with conservative terrain choices, patience, and a wide margin, not by hunting safe lines on it.
Frequently asked questions
What is a persistent slab?
A persistent slab is an avalanche problem in which a cohesive slab of snow sits on top of a 'persistent weak layer' — a layer of weak snow grains (like facets, buried surface hoar, or depth hoar) that resists strengthening and stays weak in the snowpack for a long time. When triggered, the slab releases on this lingering weak layer, producing a slab avalanche.
Why are persistent slabs so dangerous?
Several reasons: the weak layer can persist for weeks or months, so the danger lasts long after the storm that formed it; the problem is unpredictable and the snowpack can seem stable while still being primed to fail; persistent slabs can be triggered remotely (from a distance or low-angle terrain) and can fail under a later party even after others have skied a slope; and they tend to produce large, deep, and deadly avalanches. This combination defeats many of the usual stability cues.
How do you manage persistent slab problems?
With conservative terrain choices and patience. Because they're unpredictable and tests can give misleading 'stable' results, the safest approach is often to avoid avalanche terrain (slopes steep enough to slide, and connected terrain) while a persistent slab problem exists, rather than trying to find safe lines on it. Reading the avalanche forecast, understanding the buried weak layers, and giving the snowpack a wide margin and lots of time to heal are key. These problems demand humility and experience.
Sources
- Avalanche problems — American Avalanche Association
- Avalanche education — Utah Avalanche Center