Sherwins: Persistent Weak Layers, Unreactive Wind Slabs

Submission Info
Saturday, March 10, 2018 - 2:15pm
Red Flags: 
37° 36' 28.7244" N, 118° 59' 26.1672" W
Snowpit Observations
More detailed information about the snowpack: 

Toured up the standard skin track behind Mammoth Rock on the Sherwins to the top of the Hose to track and test the persistent weak layers that have been most reactive in that area. No whumphing or shooting cracks. No recent avalanches observed. Probing found two primary facet layers separated by a decomposing crust at the bottom of the snowpack from 8600' to near the top of the ridge at 9800'. Depth of the layer varied with overall snowpack depth but was generally found between 60 and 100cm below the surface. The general structure of the overlying snow seemed similar throughout our tour as well: the 0301 storm snow is mostly a single layer that goes from F hard at the top to 1F hard just above the top facet layer. Two sets of snowpack tests targeting the facets: one on a NE aspect mid-slope at 9465' where snow depth was 115cm, ski pen = 15cm and boot pen = 30cm, and a second on a NNE aspect at 9767' just below the summit ridge where snow depth was 165cm, ski pen = 15cm and boot pen = 30cm. Both areas were in large openings in the trees on about 30 degree slopes. Lower site results: CTM SC down 78cm at the bottom of the upper facet layer. Upper site results: CTV SP down 97cm at the bottom of the upper facet layer, CTE SC at the same depth/layer, ECTP 24 in the same layer (see ECT video). Hand shears in shallower areas where a pole could cut through the snow to the facet layer failed either on isolation or with easy force.

Stomped on cornices and ski cut a few of the wind slabs at the top of the ridge that were deposited between 0307 and 0309 with no results. Hand shears on the leeward (NE) side of the skin track ridge that had been previously wind loaded failed within the wind slabs with moderate to hard force, sometimes planar, sometimes rough.

Light S winds at the ridgetop, but no blowing snow. Weather went from totally overcast, but hazy, to very light snow that began as we turned to descend at 335pm. Warm day in general: Tair = 1 degree C, snow surface temp = 0 degrees C. The snow was warming and transitioning to moist down to a depth of about 15cm, dry below that. No signs of rollerballs or wet instabilities anywhere. Most surfaces had thin melt-freeze crusts or were soft but heavy and almost moist. Snow cover below about 9000' on all aspects but N is melting out quickly (see photo of bare slopes next to Mammoth Rock).

No other signs of instability today.

Snowpack photos: 
All but N aspects are melting quickly below about 9000'
Snowpit videos (tests, etc): 
Weather Observations
Blowing Snow: 
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