Sherwins - Sensitive windslabs, whoomphs and shooting cracks

SNOWPACK OBSERVATION
Sherwins Mammoth Rock approach
Submission Info
Forecaster
Tuesday, December 22, 2020 - 11:00am
Red Flags: 
Recent avalanche activity
Whumphing noises, shooting cracks, or collapsing
Recent loading by new snow, wind, or rain
37° 36' 35.0784" N, 118° 59' 31.5564" W
Snowpit Observations
More detailed information about the snowpack: 

Went up along Mammoth Rock to the top of Sherwins this morning to see how if the dramatic overnight SW winds laid down any fresh windslab, and to see how widepsread and reactive the persistent facets near the bottom of the snowpack are.  Short answer: yes in very specific areas, fairly widespread, and quite reactive.  Long answer:

Last nights SW winds have definently done a number to the Sherwins, creating a thin stiff breakable surface windboard over much of the terrain even away from the side ridge and into the trees.  In hand pits and test pits, this thin layer failed easily indicating to us to be wary of any place there may be a thicker deposit. The only place we found it to be thick enough and concerning was ~50ft below the top of the Sherwin Ridge where we triggered a hard slab ~50ft wide that ranged from 2" -12" thick.  See detailed avalanche observation for pics and more explanation: https://www.esavalanche.org/content/small-windslab-triggered-below-sherw... (Of note, a ski cut just above this pocket of slab produced no result because of the strong winds that deposited snow further down the slope than lighter winds would have.)     

The snowpack is still very shallow in the Sherwins, and we hit quite a few rocks on our slow descent down.  It ranged from 8" to about 1.5' except in very limited areas that were more windloaded.  This shallow snowpack is keeping the buried facte layers near the bottom of the snowpack alive and well, and throughout the tour we could feel them with our poles.  However, the facet layer is in two parts.  The very bottom is the early November storm and apears to be starting to round and become denser.  The upper layer however is not showing signs of rounding, and is still quite loose.  The two layers are seperated by a thin ice crust which is gradually decomposing (in some areas quicker than others).  All of our ECT tests today propogated at the top of this upper loose layer.  We also felt several whoomph/slope collapses and some shooting cracks, the most significant one spanning about 10meters.  See attached snow profile where this crack occured, along with photo showing the depth of the crack down to the top of the upper loose facet layer (ECT test propagated with 12 taps - ECTP12).  

 

Snowpit or crown profile photo or graph: 
Snowpack photos: 
Whoomph and shooting crack
Cross-section of shooting crack extending down to facet layer
Wind-stripped top of Sherwin Ridge
Snowpit videos (tests, etc): 
Weather Observations
Blowing Snow: 
No
Cloud Cover: 
Clear
Air temperature: 
Below Freezing
Wind Speed: 
Light
Precipitation: 
None
Air temperature trend: 
Wind Direction: 
Southwest
Accumulation rate: 
None
More detailed information about the weather: 

Dramatic overnight and early morning SW winds shut-off near day break, with only light gusts while we were out.  Air temps hovered just below freezing before noon up to the ridgetop.   

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