Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Very Likely
- Size
- 2–3
Persistent slab avalanches could be very large and could easily bury or kill you. These dangerous and surprising avalanches can catch even the most experienced backcountry travelers. These avalanches will be easiest to trigger and biggest at mid to upper elevations. You can trigger them from long distances away. They may run naturally, especially near and above treeline, and may come down from slopes above you. Keep your distance from any slope steeper than 30 degrees. Make sure you're not standing below or adjacent to steeper slopes. Use clues to stay out of avalanche paths, like trees stripped of uphill branches or openings in vegetation that run up slopes. Watch for obvious signs of instability like recent avalanches, whumphing collapses, or cracks shooting through the snow.
About 1.5 feet of snow is in the forecast with winds strong enough to transport snow at upper elevations. This could tip the balance on an already weak snowpack. You can find a layer of weak facets about 2-3 feet below the snow surface that was buried in early December. This rests on a crust on many slopes which is especially thick below 5,500ft. Observers have reported numerous collapses, propagating test results, and even avalanches on this weak layer in the past week. On the 18th, observers reported triggering a size D1.5 avalanche in the Cuthroat drainage on a S aspect at 5,300ft.
(12/18/2020) Example of a widely propagating persistent slab that surprised experienced backcountry travelers in the Stevens Pass zone. The slide was triggered from below.
