Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1.5–2.5
What is it? You could trigger 1-3 foot thick avalanches on buried weak layers (sugary facets or crust + facet combinations). The steady stream of snowfall and windblown snow has some 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. 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 thin wind slab avalanches or loose snow slides (sluffs) in very steep terrain. 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.
(2/28/2023) NE face of Copper Mountain at 8,400'. This avalanche appeared to be remotely triggered by a skier from above. This crown in sheltered terrain (great skiing and riding conditions) is an excellent example of a slope that did not need wind loading to become unstable.