Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 2–3
What is it? You could trigger a 2-4 foot thick avalanche on buried weak layers (sugary facets or crust + facet combinations) on some slopes. Many large natural avalanches released last week during the big storms. The likelihood of triggering a slide is decreasing, but the dense slab makes the consequences severe.
Where is it? Last week, far more natural activity occurred in the western Smoky Mtns and southern Sawtooths than in the mountains closer to Stanley.The Persistent Slab problem is worst on middle and upper elevation slopes facing W-NW-N-NE-E-SE. On slopes with multiple or thick crusts in the upper 2 feet of the snowpack, slab avalanches are much less likely.
How can you recognize it? Unfortunately, unstable slopes probably will not provide obvious signs of instability like snowpack collapsing or cracks shooting out in front of you. Snowpack stability tests are likely to be inconclusive and are of limited use. In low probability/high consequence situations like this, there's a good chance the first sign of instability you'll see is the avalanche you trigger.
How can you manage it? Sadly, the only surefire way to manage this problem is by avoiding avalanche terrain. 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. The chance of triggering large avalanches appears to be highest on shadier, more northerly aspects. Persistent weak layers can fail unpredictably and break wider and higher on slopes than you might expect. You may be able to remotely trigger avalanches from gentler terrain above, below, or to the sides of steep slopes. Several people could ride or ski a slope before it avalanches; tracks do not mean a slope is guaranteed to be safe.
ADDITIONAL DISCUSSION:
- While weather stations didn't record impressive winds in the past 48 hours, observers reported seeing some wind-blown snow. Also, E winds can form wind slabs in places you're not used to seeing them. Watch for thin, fresh drifts in upper elevation, wind-exposed terrain.
- Despite the wintry weather, spring is officially here. As such, even brief periods of sunshine can be intense enough to trigger loose snow avalanches on steep, sun-drenched slopes. Rocky, protected areas with thin snowpacks less than a couple of feet thick are most suspect.
(3/16/2023) This very wide avalanche was observed in West Beaver Cr, near Smiley Creek at 8,200-8,800'. The crown is an estimated 3,500' (2/3 of a mile) wide.