Persistent Slabs
- Alta quota
- Limite del bosco
- Sotto il limite del bosco
- Probabilità
- Possible
- Dimensione
- 1.5–2.5
Some details on our tricky Persistent Slab avalanche problem:
What and where is it? You could trigger 1.5-2.5 foot thick avalanches that fail on buried persistent weak layers—sugary facets or crust + facet combinations—near the bottom of the snowpack. Until proven otherwise, assume these old, weak layers of snow exist on all upper elevation slopes and on middle elevation slopes facing the northern half of the compass (W-NW-N-NE-E aspects). Slopes where the wind recently formed drifts are especially suspect. In the craggy Sawtooth Mtns, you may also find stiff surface snow where it "spindrifted" to aprons at the bottom of chutes and couloirs.
How can you recognize it? The old crusts and sugary facets are easy to feel with your hands, ski pole, or snowmobile track (see photos and videos below). You may not see obvious signs of instability like recent avalanches, snowpack collapsing, or cracks shooting out in front of you until it's too late.
How can you manage it? Sadly, the only surefire way to manage this problem is by avoiding avalanche terrain where more than about a foot of denser snow lies above the weak, older crusts or facets. Short of that, you can reduce your risk by avoiding large avalanche paths and those with ugly consequences if you are caught in a slide. Persistent Slab avalanches can break wider and higher on slopes (above you!) than you might expect. You could remotely trigger avalanches from gentler terrain below, above, or to the sides of steep slopes.
ADDITIONAL DISCUSSION: Recent snow showers and wind may have formed thin, fresh wind slabs in exposed upper elevation terrain. While small, these slabs could be very easy to trigger due to the weak snow (facets with or without a crust / surface hoar) buried beneath them.