Persistent Slabs
- Alta quota
- Limite del bosco
- Sotto il limite del bosco
- Probabilità
- Likely
- Dimensione
- 1.5–2.5
What is it? You could trigger 1-4 foot thick avalanches on buried weak layers (sugary facets or crust + facet combinations). The steady stream of snowfall and windblown snow has many slopes teetering on the edge and waiting for a trigger. Wind-loaded slopes have a thicker, more cohesive slab and are the most dangerous.
Where is it? Conditions are most dangerous where wind slabs and drifts formed above the faceted snow or crusts + facets mentioned above. However, the snowfall from the past couple of weeks has become cohesive enough to act as a slab on most sheltered slopes. In the steep, craggy Sawtooth Mountains, slabs often form on the aprons of chutes and couloirs and in cirques beneath large cliff bands. The distribution, character, and sensitivity of the weak layers vary from slope to slope—your best bet is to assume this problem exists until proven otherwise.
How can you recognize it? Wind-loaded slopes have stiffer, denser snow that keeps your skis or sled from sinking deep into the snowpack. Wind-stiffened snow could be hidden beneath the fresh, fluffy powder. Snowpack collapsing and cracks shooting out in front of you are obvious signs of instability.
How can you manage it? You can reduce your risk by avoiding slopes steeper than 30 degrees—especially large avalanche paths and those with ugly consequences if you are caught in a slide. Persistent weak layers can fail unpredictably and break significantly wider, deeper, and higher on slopes than you might expect, producing much larger avalanches. You could remotely trigger avalanches from gentle terrain above, below, or to the sides of steep slopes. Avalanches could be triggered by the second, third, or fourth rider on a slope.
ADDITIONAL DISCUSSION: You could trigger smaller wind slab avalanches or loose snow slides (sluffs) in steep terrain. Some of these avalanches could release naturally today. While most of these "new snow" slides should not be large enough to bury you, they could trigger the larger, more powerful persistent slabs discussed above as they move downslope. Sadly, you cannot assume steep slopes are safe after triggering small slides involving only the recent snowfall.
(3/3/2023) This large avalanche - over 1200 feet wide - was remotely triggered by a snowmobiler in the western Smoky Mtns and created a 20 foot deep debris pile in places. SE aspect near 9700'.