Persistent Slabs
- Alta quota
- Limite del bosco
- Sotto il limite del bosco
- Probabilità
- Possible
- Dimensione
- 1–2.5
Yesterday I found myself in the Northern Elkhorns doing observations at low elevation locations. I dug a snowpit on a north northwest facing slope at 6655' . The snow at this location was about 4.5' deep, quite a bit shallower than what I have been finding higher up at treeline and near ridgelines. While traveling through the snow at this location I observed no signs of instability. I put in my snow pit and it became obvious that we have multiple persistent weak layers buried throughout the snowpack at this location. The Christmas rain crust is buried about half way down at this location and has thin, weak, faceted layers above and below it. Compression tests performed in this snowpit revealed weakness at this layer. Columns failed using moderate force. The shears occurred in the faceted layer above the crust and were mostly smooth (CT18RP⬇️70cm). No propagation was observed on any stability tests I performed yesterday. WAC forecasters traveling in the Blues yesterday were seeing similar snowpack characteristics that I observed in the Elkhorns. Meanwhile, WAC forecasters in the Northern Wallowas, yesterday, observed buried surface hoar about 14" down from the snow surface at 6400' . Stability tests they performed at their location exhibited propagation and sudden planar shears (instability red flags) on that buried layer of surface hoar. That said, we are still concerned about what we have been seeing and what we have been hearing about throughout other areas of our forecast zone this past week. Myself and other WAC forecasters have been observing propagation in our snowpit stability tests. Also, in the Observations area of this website you will find an interesting post about a snowmobile triggered avalanche that occurred in the Southern Wallowas in the Little Eagle Creek area. This was a persistent slab avalanche that failed on a layer of buried surface hoar and was big enough to injure or kill a human. Luckily no one was injured in the accident. The best way to eliminate the risk of triggering a persistent slab avalanche is by staying off slopes steeper than 30 degrees, staying out from under slopes steeper than 30 degrees, and staying off slopes connected to slopes steeper than 30 degrees. Give the new snow plenty of time to stabilize after this recent storm. Until this problem goes away it is important for us recreationalists to remain patient and disciplined by maintaining conservative terrain choices even when instability appears unreactive.
(1/25/2023)
Snowmobile triggered slab avalanche approximately 3' deep on north aspect at 7400'. Weak layer was buried surface hoar.
