Persistent Slabs
- Alpine
- Treeline
- Below treeline
- Likelihood
- Possible
- Size
- 2–3
There is a persistent weak layer of surface hoar (weak feathery snow crystals) and facets (weak sugary snow) at the base of 2 to 4 feet of recent storm snow that has not finished adjusting to the heavy storm snow load above it. While likelihood is decreasing, triggering a large to very large persistent slab avalanche that fails several feet deep remains possible today. Human triggered avalanches and signs of instability over the past two days include:
- A remotely triggered persistent slab avalanche on Saturday near a cliffband above Watson Lake (NE aspect near 8,300') with whumpfing triggered by the same party earlier in the day.
- A remotely triggered a persistent slab on Friday in old-growth trees on Hidden Peak (West Shore Tahoe). The party stated, "[the avalanche] propagated so far I could not see the other side of the path." This slope had seven previous tracks on it.
- A second persistent slab avalanche on Friday on Hidden Peak (West Shore Tahoe) in the trees at 7500 ft.
- Two persistent slab avalanches triggered on Friday on Jakes Peak (West Shore Tahoe).
- On Friday, widespread whumpfing and collapsing was reported from the West Shore Tahoe, Trimmer Peak (South Lake Tahoe), Blue Lakes (Carson Pass), and more isolated snowpack collapses on Donner Summit.
- On Saturday less widespread whumpfing and collapsing was reported from the Echo Summit area, Mt. Watson area, Mount Rose area, and Carson Pass/Blue Lakes areas.
- Numerous very large natural avalanches occurred during the storm on N-NE aspects, some of which had impressively wide propagation.
Persistent slab avalanches have less predictable behavior in response to triggers. Sometimes you trigger a persistent slab avalanche from a distance while traveling below or next to a slope in lower-angle terrain. Sometimes you trigger a persistent slab avalanche after getting fully out onto the slope, often with previous tracks on it. This type of avalanche often breaks much wider than expected. Avalanches over the past two days have occurred in sheltered, tree-covered terrain that people often think of as "safe." This same persistent weak layer may also lurk in more exposed terrain where wind-blown snow has created a much thicker slab above it.
Slope scale uncertainty continues concerning this avalanche problem. Many people may not see any evidence of a persistent slab problem today. Others may trigger a very large avalanche. For this type of avalanche problem, snowpit test results can be hit or miss and sometimes misleading. Sometimes a large avalanche on a seemingly stable slope is the first and only clue you get. If you are luck enough to get indications of snowpack collapse or whumpfing sounds beforehand, mother nature has given you clear indications of unstable snow in the immediate area.
Stepping back from the type of terrain where the problem exists, utilizing other aspects or traveling only on sub 30 degree angle slopes not connected to steeper terrain represents the most reliable way to reduce exposure and the potential consequences of a persistent slab avalanche.
(2/14/2025) Photo: Ming Poon
This avalanche was remotely triggered on a NE aspect of Hidden Peak (West Shore Tahoe) in below treeline terrain.
