Persistent Slabs
- Hochgebirge
- Waldgrenze
- Unterhalb der Waldgrenze
- Wahrscheinlichkeit
- Unlikely
- Größe
- 2–2.5
Today's primary concern is triggering a Persistent Slab avalanche on weak layers buried in the upper 3 feet of the snowpack. Where is this most likely?
- Exposed, windy slopes with variable snow depths and rocks sticking up above the snowpack.
- Slope angles in the "prime time" range where the majority of slab avalanches release, around 37-50 degrees.
- Slopes where wind frequently transports snow, forms drifts, or stiffens the snowpack.
- Middle and upper elevation slopes where surface hoar or facets exist, above crusts on some slopes. The problematic weak layers are more common on slopes facing the northern half of the compass. The weak layers can be subtle and difficult to see or feel in snowpits in some areas, and they are obvious on slopes with thinner overall snow depths.
The more bullet points you check, the more you should be thinking about triggering a large avalanche.
You probably won't experience obvious signs of instability like snowpack collapsing or cracking. When avalanches release, they could break well above you on the slope and be wider than expected. Avalanches triggered in shallow areas could involve much thicker slabs on the same slope.
Many slopes are stable, but the chance of triggering a large avalanche is not zero; this zone has a lot of terrain that checks all the "bad boxes" listed above. Determining if the chance of triggering a slide on a questionable slope is 1-in-20, 1-in-100, 1-in 1000, or 1-in-10,000 can be difficult for even the most seasoned backcountry travelers. However, we have higher confidence about the size of avalanches we can trigger: large enough to bury people. One way to manage risk is to carefully consider the consequences of being caught in an avalanche. Could a slide carry you into trees, over cliffs, or into a creek or gully where avalanche debris could pile up deep? Choosing terrain that reduces the consequences of an avalanche is much easier than definitively determining whether or not you can trigger an avalanche.