Monday, December 10, 2018 - 12:00pm
2 different parties on Mt. Esha triggered D1 avalanches on Saturday. These were triggered a few hundered feet down from the ridgeline, were less than a foot crowns, but went wall to wall in the chutes that they were triggered in. These were recently deposited windslabs from the winds that started up the night before. Fortunately no one was caught.
In addition to these skier triggered avalanches on Mt. Esha, another skier triggered avalanche occured on Mamoth Mtn on Saturday, and other natural windslab avalanches occured in other parts of the forecast zone. These are detailed in a seperate observations on the ESAC site.
Winds that began friday night were very unsubstantial for Sierra standards, only in the 30-40mph range. But these speeds are actually ideal for wind slab formation. The difference here is that typically very strong winds happen during storms for us, which blow much of the loose snow into windslabs during the storm. Then after a few days after the storm these windslabs stabilize, and when winds increase again into this moderate 30-40mph range there isn't much loose snow left for transport, so widespread wind slabs don't form again (but some more isolated rare ones do). Another factor that very well could have played in here, is surface hoar formation that took place during the calm winds period that followed last week's Tue-Wed storm. Again, a rare concern for the eastern sierra, as periods of calm like we just had last week are rare, especially combined with the asmopsheric conditions which included moisture in the air, which were ideal for surface hoar formation (this is essentially winter-time frozen dew that looks like feathers growing out of the snow). We do get surface hoar formation, but it is very fragile, so it is typically destroyed quickly with usualy Sierra Winds. Again, we got atypical much less-strong winds, which likely were still strong enough to destroye this surface hoar high up right below ridgelines. But where the blown snow was deposited further down slope, this surface hoar was not destroyed, as wind-blown snow gently settled ontop of it. Hence these skier triggered avalanches on Mt Esha, as well as on Mammoth Mountain, were triggered several hundred feet down the slope, rather than the usuall trigger location just below the ridgelines.
This isn't just academic, buried surface hoar can persist for a very long time. The typical 1-3 day period that it takes for wind slabs to stabilize in the Eastern Sierra doesn't account for a persistent weak layer that is buried underneath. Slopes that weren't skier triggered or that didn't slide naturally that do have this buried surface hoar underneath a recently deposited windslab could remain sensitive and dangerous for quite some time. The challenge here is knowing which slopes actually had surface hoar formation, and of those that did, where did it not get destroyed??
Keep in mind, this buried surface hoar hypothesis is not confirmed. We haven't been able to do a pit profile near these slides yet to try and actually identify buried surface hoar under the surface wind slabs. What we do know is that surface hoar has been reported in many high areas prior to the Tue-Wed snowfall.
If you are venturing into steep alpine terrain, keep this discussion in mind when making your decisions. Better to play it safe if you don't know. Take some time to investigate the upper layer of the snowpack in safe places, you don't have to dig deep! just below the recent wind deposit. The distribution will be highly variable, so better to chech out many different places quickly than taking a half hour digging a pit to the ground. Quick handpits, a quick stabilitiy test, a close look at what kind of snow that recent windslab is sitting on is all it takes.
While windslab avalanches right now will not likely be large, they could lead to a nasty fall and partial burial, and even a full burial if a terrain trap is involved. If surface hoar is indeed buried, and we get more snow later this week, or more wind deposit, avalanches could become larger and more dangerous.
PLEASE be safe out there and PLEASE PLEASE let us know what you see!!
This just shows the importance for being aware of the recent weather history, and checking the ESAC avalanche advisory.