Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 1–2
Recent Avalanches: There were four human-triggered avalanches on Seattle Ridge yesterday. Luckily, none of these caught or carried anybody. These were 6 to 12" deep on average, and up to 200' wide. All of the avalanches were triggered remotely. More info here and here.
Yesterday gave us some clear feedback that the weak snow that formed during the beginning of the month and was buried this week is touchy. Between the multiple remote triggered avalanches on Seattle Ridge mentioned above, and what we were finding as we traveled along Eddie's ridge (details here), it is clear we have a persistent slab problem. The main culprit is a layer of unusually well-developed surface hoar, although there is also a layer of weak, faceted snow that exists at a similar depth.
The biggest factor controlling stability today is how deep this weak layer is buried. The most dangerous conditions exist where we got the most snow last week, which is near the north end of Turnagain Pass, Girdwood, and Portage. We also commonly find deeper snow near the Seattle Ridge headwall. We will also likely find dangerous conditions on upper elevation slopes that were loaded by the strong easterly winds that were blowing through Friday evening. Terrain near the south end of Turnagain Pass that received less snow may still produce avalanches, but these should be on the smaller side and are unlikely to be big enough to bury a person.
The snowpack is complicated, and we're seeing clear warning signs that it's dangerous. If you are confident in your snowpack assessment, it may be reasonable to ease into steeper terrain, but layers of buried surface hoar can be very difficult to assess, often giving mixed feedback over very short distances. Given the uncertainty in the distribution of our current weak layer, the best bet is to avoid traveling in steep or consequential terrain, especially in areas that saw the most loading during last week's snow and winds.
Glide Avalanches: We're continuing to see glide avalanche activity throughout the forecast area. These avalanches are large and unpredictable, so the only way to manage this problem is to avoid spending time under open glide cracks.
Cornices are large and sensitive following the new snow and wind over the past few days. There have been multiple reports of cornices falling on the back side of Seattle Ridge (details here), and similar conditions likely exist along most ridgelines. These may occur naturally, or they can be triggered from ridgelines. You can reduce your exposure by avoiding traveling under large cornices, and giving them plenty of space if you are traveling along ridgelines.