Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Likely
- Size
- 1–2.5
WAC forecasters continue to see persistent weak layers in the form of buried surface hoar, facets (weak sugar snow), and crust/ facet combos buried in the upper half of our snowpack, throughout the forecast region. When cohesive slabs sit on top of these persistent weak layers, the potential exists for large and destructive persistent slab avalanches to be triggered by humans. Ways to recognize if you have a persistent slab problem include the observance of collapsing snow or whumpfing sounds on the snow you are traveling through. When we are dealing with persistent slabs there are scenarios where there are no surface clues telling us there are persistent weak layers lurking below. Therefore, we have to dig down into the snow and look at the snow structure to find and observe the weak layers. We also have to perform stability tests to get an idea about the strength of the snow as well. Not all of our test columns are exhibiting propagation, but the observance of poor snowpack structure coupled with the observance of a fair amount of propagation in column stability tests is enough to make us uncertain about the overall snowpack strength throughout the region, especially where the snowpack is less than 3.5' deep. That said, we have had plenty of "bulls eye" information about the potential for persistent slab avalanches in the Blues and Southern Wallowas in the last couple of days in the form of observed avalanches. If I see avalanches in an area close to the zone I am considering, l leave my shovel in my pack and head for lower angle slopes where there is no chance of triggering an avalanche. When we see evidence of avalanches, that is all the info we need to know that slopes are dangerously unstable. These persistent weak layers located below the snow surface are getting tested by the added weight of recent new snow that has fallen since Saturday night, and then redistributed onto leeward facing slopes by the recent wind. The newly added weight of snow has, and continues, to put a lot of stress on these buried weak layers. Add to that the weight of a human, and especially the weight of a snowmobile and rider, that might be enough stress on the snow to make one or more of these weak layers fail resulting in a potentially large and consequential avalanche. There have been several recent avalances that have occurred in our forecast area since February 7th. I will mention 3 of them in this discussion. The first avalanche was triggered remotely in the Blue mountains. You can read about it HERE. The other two recent avalanches were remotely triggered in the Southern Wallowas Wednesday and Thursday. One avalanche was triggered on a northeast facing slope at 7600' . The crown was estimated to be 1'-2' deep and the weak layer was buried surface hoar. The second avalanche was remotely triggered as well on a southwest facing slope at about 8000' . Nobody was injured in any of these snowmobile triggered avalanches. See the photo attached to this discussion. Persistent slab avalanches are the cause of most avalanche fatalities. The best way to eliminate the risk of triggering a large and dangerous persistent slab avalanche is to stay off of, out from under, and away from slopes connected to avalanche terrain, especially today and into the foreseeable future. Now is the time to enjoy the good riding conditions that can be found on lower angle terrain.
(2/9/2023)
Photo of a remotely triggered persistent slab avalanche in the Southern Wallowas. This avalanche occurred on a northeast facing slope at 7600' . Avalanche occurred Wednesday or Thursday.
