Deep Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 1–3
The definition of a deep persistent slab: A thick, hard cohesive slab of old snow overlying an early season persistent weak layer located in the lower snowpack or near the ground. The structure is conducive to failure initiation and crack propagation. Typically characterized by low likelihood and large destructive size. Development occurs early in the winter and is characterized by periods of activity followed by periods of dormancy, then activity again.
Our snowpack is holding several concerning persistent weak layers that vary in depth from the upper third of the snowpack to the ground. Forecasters and observers are finding reactive and often alarming snowpack results on all of these persistent weak layers:
1. )The Christmas Rain Crust is 2-4 feet from the snow surface and varies in thickness from 4" at lower elevations to 1" or less above the treeline. The snow around this crust has shown up as small grain facets in several pit locations. In locations where a cohesive slab of snow sits above this interface, human-triggered avalanches remain possible. More observations from the Southern Wallowas are indicating that the New Year's Eve human-triggered avalanche near Fish Lake likely occurred on the Christmas crust in association with a layer of buried surface hoar.
2.) Buried Surface Hoar in the Western and Southern Wallowas has been found on Southeast through North through Southwest aspects near and below the treeline. While the distribution of this persistent weak layer is more specific- perhaps even spotty, a large avalanche would be the result of failure and propagation on this layer. Yesterday I found a layer of buried surface hoar at 7,000' on a N aspect near Fish Lake. It was not reactive in my two snowpack tests, but still well preserved in the snow, just above the Christmas Crust.
3.) Basal facets near the ground are most everywhere throughout our forecast area. Now buried up to 8 feet in many locations, these are the monsters in the basement. While becoming increasingly hard to trigger this layer, a small to large avalanche on one of the weak layers above could produce enough weight to trigger a very large avalanche failing on the ground. Areas, where we are more concerned with this happening, are zones where the snowpack depths are shallow- say 2-4 feet. Upper elevation areas that have seen snow stripped by wind and harbor shallower snowpacks are one location that I would be especially concerned with triggering one of these deep slab avalanches. Another location would be rocky areas in the mid-elevation band. Finally, in the lower elevations, where snowpack depths are shallower, the weight of a rider would be more likely to have a greater impact on this weak snow near the ground.
Utilizing slope scale evaluation before committing to terrain features greater than 30 degrees in slope angle will be critical to avoid a deep slab avalanche. There is a high amount of uncertainty when dealing with this type of avalanche problem, and it is recommended to hedge your bets with wide terrain margins. Excellent riding conditions exist on the north side of the compass in terrain that is less than 30 degrees. It is recommended to focus on the poor snowpack structure, not potentially inconsistent stability test results before committing to riding any slope steeper than 30 degrees.
(1/7/2023)
Photo shows likely trigger points that could create a Deep persistent slab avalanche.
Photo: VM
