Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1.5–2
What is it? Over the weekend, skiers and riders triggered several 1-2 foot thick avalanches. Many of the slides appeared to fail on a thin layer of surface hoar, and crust + facet combos exist on some slopes lacking the surface hoar. While none of the slides were "huge", around half were large enough to bury or injure people, and all were capable of carrying someone into a gully, through trees, or over cliffs.
Where is it? The recent avalanche activity was focused on NW-N-NE aspects between 8000-10,000' in elevation (middle and upper elevation terrain—view graphic here). The avalanches released in sheltered and wind-affected terrain. Very steep slopes over about 35* are most likely to produce avalanches.
How can you recognize it? The surface hoar layer is thin, and the crystals are relatively small, making it difficult to identify this layer in snowpits. In the terrain described above, assume this problem exists until proven otherwise. You may or may not experience snowpack collapsing and/or cracking while traveling. These red flags are obvious signs of instability.
How can you manage it? Surface hoar is a sneaky weak layer. These avalanches may break wider than expected or higher on the slope above you. You may be able to trigger a slide after several riders have safely climbed or descended a given slope, and there's an outside chance you could remotely trigger an avalanche from gentler terrain above, below, or to the sides of steeper slopes. Avoid middle and upper elevation slopes on the northern half of the compass that are steeper than about 35* to significantly reduce the chances of finding trouble. Steer clear of large wind drifts or slabs in very steep terrain. Picture where even a small avalanche could carry you before committing to playing in large or consequential avalanche paths.
ADDITIONAL DISCUSSION: In addition to the persistent weak layers in the top few feet of the snowpack, there are more deeply buried layers to keep on your radar. These facets (often above, below, or in between crusts) are buried beneath dense, 2-4+ thick slabs—making them difficult to trigger but capable of producing very dangerous avalanches. While triggering them is unlikely, it can't be ruled out in steep, rocky, wind-affected alpine terrain. Slopes like these have variable snow depths, with thinner areas of the snowpack that allow us to impact deeper weak layers.
(1/15/2023) This avalanche was triggered by a snowmobiler in upper Stanley Lake Cr. at the very northern end of the Sawtooths. The rider managed to get off the slab and was not caught. The slide likely released on a persistent weak layer that buried in early January. 8000', NE.
