Persistent Slabs
- Alta quota
- Limite del bosco
- Sotto il limite del bosco
- Probabilità
- Possible
- Dimensione
- 1.5–2.5
Depth hoar on some northerly aspects near and above treeline continue to fail stability tests. Earlier this week, a large natural avalanche occurred on the NE face of Fremont Peak. The avalanche occured as wind built a slab over weak faceted snow, producing a failure near/at the ground. Steep NW, N, and NE aspects above 10,500' should be considered most suspect, especially cindery slopes with few rock anchors and depth hoar. Some westerly slopes may also have this problem, see January 2nd pit from Allison Clay. We have a lack of evidence of depth hoar on easterly slopes, and untill we gather more data, these slopes should be considered suspect. Many easterly slopes were skiied in Humphry's Cirque over the past few days, however this is not solid evidence that a persistent slab avalanche will not be triggered on an easterly slope. A traveler could also trigger a persistent slab avalanche from thin areas on the slope, where it is easier to affect the buried weak layer.
Curretly, triggering this persistent slab is a low likelihood, high consequence scenario. The only sure way to stay safe is avoiding suspect terrain or evaluate carefully by digging pits, performing stability tests and treating this condition with great respect and prudence. As we collect more information, this problem may warrant changing to a deep persistent slab problem. It does appear that this problem is trending that way, but we need more data. Deep persistent slap problems are reserved for low likelihood/high consequence, or high level of uncertainty scenarios; and when the average slab increases to approximately 1.5 meters thick and is at least 1F hardness above a persistent weak layer.
Be on the lookout for a second layer of concern, a rain crust that formed on January 1st. We have limited data, but this layer seems to be only present on W, SW, S and SE slopes near and below treeline. Limited observations on January 5th indicate that this layer is becoming less of concern.
