Shooting cracks, collapsing, propagating test results and VERY thin coverage in the Sherwins

SNOWPACK OBSERVATION
Mammoth Rock
Submission Info
Forecaster
Saturday, December 26, 2020 - 2:00pm
Red Flags: 
Whumphing noises, shooting cracks, or collapsing
Recent loading by new snow, wind, or rain
37° 36' 38.1492" N, 118° 59' 32.0388" W
Snowpit Observations
More detailed information about the snowpack: 

I made my way up to Sherwin ridge above Mammoth rock today to get a look at the new snow from last night’s storm and to gauge the extent of recent wind loading in the area. leaving the car around noon the storm was breaking, skies were starting to clear, and the snow had slowed to S-1. Temperature was below freezing, and the winds were honking. Consistently in the moderate realm with strong to extreme gusts that were moving snow around even at lower elevations.  Uneven snow surfaces made for interesting skinning on the ascent to Mammoth rock as I transitioned between very stiff supportable windboard to drifts of soft snow and back again.

New snow totals seemed to average around 2”-4” and there was a notable (p+ hard) but thin (3mm) rain crust below about 9000-9500’.  I suspect the precipitation started on the wet side of things yesterday afternoon before the temperature dropped. 

As I made my way to the ridge weaving between exposed bushes and rocks the wind speeds increased and snow was blowing around in many different directions locally, leading to sporadic drifting. I noted small sensitive wind deposits on several terrain features below tree line. Most of these ranged from 2-6” in depth, 4f to 1f in hardness overlying f+ new snow, and were reactive to my weight. However, I did not encounter anything large enough to knock me off my feet at lower elevations. I also observed many areas where the snow cracked under my feet as I broke trail, highlighting the upside-down nature of the more wind effected areas. On the ridge top I noted consistent strong to extreme winds out of the SW. I also noted ample fresh deposit on the lee ward side of the ridge line. Most of this deposition was occurring well below the ridge top and I measured 4-8” 1f deposits overriding 4f snow. No recent avalanche activity was observed.

Along my approach I initiated a notable collapse a little below 10,000’. I heard a loud wumph, as a large shooting crack broke in both directions from where I stood (approximately 40-50 ft in length), and the ground settled below my feet. I dug in about 20-30 meters away on a similar slope. (9,900’, NE aspect, 30° slope angle) (See pit profile #1 below for more details) At this location I found 41 cm of snow with very similar structure as has been found in many other areas across the range. (4f- 1f Dec snow on top of fist hard well-developed facets on top of a melt freeze crust near the ground) I performed multiple stability tests that showed propagation propensity. I conducted two ECT tests both of which propagated under moderate force. I also conducted a PST which initiated after 25 cm of cut and resulted in a vertical block fracture. (PST SF 25/120)

 I Also dug in at around 9,200’ on a NE aspect and a 28° slope as I made my descent. My goal here was to see if the facets near the ground were more rounded and condensed at lower elevations. I did find a very similar structure in this location albeit a slightly deeper more supportable snowpack (64 cm HS) The melt freeze crust below the concerning facets in this location was thicker and much more substantial. However, the facets resting on top of this melt freeze crust are still fist hard and well developed and showed propagation potential in my stability tests. (ECTP 21) (See Pit Profile #2 for more details)

Surface conditions were mostly predictable above 9,500' with the new snow providing some relief along my descent, but it is still VERY shallow. I stuck mostly to gully features and did my best to avoid the rocks that are out there, but I still managed to leave some p-tex behind. Below 9000’ things really get dicey and I chose to walk in more than one location.

 

Snowpit or crown profile photo or graph: 
Snowpack photos: 
Snowpit videos (tests, etc): 
Weather Observations
Blowing Snow: 
Yes
Cloud Cover: 
50% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Strong
Precipitation: 
None
Air temperature trend: 
Cooling
Wind Direction: 
Southwest
Accumulation rate: 
None
More detailed information about the weather: 

Skies were broken mid day and mostly clear by late afternoon. SW winds were strong to Gale force thoughout the day and blowing snow was noted at all elevations.

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