| What it is | Slab over a long-lasting weak layer |
| Weak layers | Surface hoar, facets, depth hoar |
| Why deadly | Hard to predict; remote/wide triggering |
| Persistence | Days to weeks or months |
A persistent slab avalanche releases on a persistent weak layer — such as buried surface hoar, facets, or depth hoar — that can linger in the snowpack for days, weeks, or even months. These are especially dangerous and hard to predict because the weak layer is buried and stubborn, slides can be triggered remotely, and they can propagate far and break large. They are a leading cause of avalanche fatalities.
This is general educational information, not avalanche training. Heed the forecast’s persistent-slab warnings and take a certified course.
The stubborn weak layer
A slab avalanche on a long-lived weak layer like depth hoar in the snowpack; contrast the shorter-lived wind slab.
Frequently asked questions
What is a persistent slab avalanche?
A persistent slab avalanche is a slab that fails on a persistent weak layer — a stubborn weak layer like buried surface hoar, faceted grains, or depth hoar that sticks around in the snowpack for a long time. Because the weak layer doesn't quickly heal, the danger can persist for days to weeks after a storm.
Why are persistent slabs so dangerous?
They're hard to predict and forgive few mistakes: the weak layer is buried and hidden, slides can be triggered remotely or from low-angle terrain connected to steeper slopes, fractures can propagate long distances, and the resulting avalanches can be large and deep. Stability tests and clues can be inconsistent, so they fool even experienced travelers.
How do you manage persistent slab danger?
Respect the avalanche forecast when a persistent slab problem is listed, give the weak layer a wide margin by choosing lower-angle and less consequential terrain, avoid steep slopes connected to bigger features, and remember that 'getting away with it' once doesn't mean the layer is safe. This requires solid avalanche education and humility.
Sources
- Persistent slab problem — Avalanche.org
- Persistent weak layers — American Institute for Avalanche Research and Education