Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 2–3
Prominent weak layers are buried in the upper 1-3 feet of the snowpack. Yesterday a skier triggered a large avalanche in the Southern Bitterroot (click here for information). These weak layers exist throughout the forecast area, and you could expect similar avalanches in the Seeley Lake region if you find the right place to trigger one. Last week we saw evidence of a widespread natural cycle on persistent weak layers across all three regions.
Avalanches on these layers are becoming more difficult to trigger as weak layers adjust to their new load and the slab above them thickens. Conversely, this means any avalanche you do trigger will be large - something you do not want to be caught in. We’ve reached a point where we aren’t seeing warning signs of instability, such as collapsing, so triggering an avalanche could be the first sign of instability that you see.
It is difficult to determine where exactly these avalanches may occur, and where on a slope you’ll find a trigger point. We are getting our weakest stability test results on northerly slopes, and most of last week’s natural avalanches occurred on northeast, east, and southeast aspects. But, a variety of buried weak layers including surface hoar, facets, and crusts have been found in snowpits on all aspects at middle and upper elevations. It is possible to remotely-trigger these avalanches from below, beside, or above steep slopes. So, completely avoiding this problem includes avoiding terrain threatened from above.
A portion of the crown from a large avalanche in the Seeley Zone. The full extent of this crown was 1500 feet wide. This avalanche occurred naturally on buried weak layers in a wind loaded area.
(2/28/2026) Photo: Ben VandenBos