Date of Observation: 12/18/2021
Name: Zach Guy
Zone: Southeast Mountains
Route Description: Mount Whetstone, South to east to north aspects up to 12,200 ft.
Observed avalanche activity: No
Avalanches: Some old slides and ski triggered a 2” wind slab on a north aspect ATL
Weather: Cold, light winds, clear skies.
Snowpack: In wind-sheltered below treeline terrain, we observed continuous collapses while breaking trail, some localized and about a dozen rumbling collapses that propagated up to 100 ft. The culprit was the 12/6 surface hoar layer, a little more than a foot deep below a faceting soft slab. As we ascended to near and above treeline, the signs of instability stopped; slabs were denser and the surface hoar changed to just facets/depth hoar. Stability tests on a low angle east aspect at 11800 ft produced a mix of propagating and non-propagating results below a 2 foot slab (4F to 1F hard) on the 12/6 crust facet complex. (ECTP26, ECTN26, ECTX, PST55/100 END). See profile. Snow surfaces are faceting at all elevations, with small surface hoar growth below treeline. The 12/15 surface hoar layer is also well preserved below a few inches of snow, at least 10mm in size. Most of the alpine start zones where we traveled were scoured from the big wind event on 12/15, big erosional features and bare ground exposed in places.
Photos:
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- Example of a localized collapse, where the weak layer failur didn’t propagate far, perhaps because the slab is decaying now. We got plenty of these.
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- Example of a rumbling collapse and shooting cracks across a slope. Got plenty of these too.
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- The snowpack structure (WEAK!) in a location where we triggered a rumbling collapse
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- 12/15 surface hoar buried by a few inches. And near surface facets and surface hoar on the snow surface.
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- Study Plot Profile. (12/6) is the problematic layer
