Mammoth Crest: Testing Wind Slabs and Persistent Weak Layer

SNOWPACK OBSERVATION
Submission Info
Forecaster
Thursday, January 25, 2018 - 2:00pm
Red Flags: 
Recent avalanche activity
Recent loading by new snow, wind, or rain
37° 34' 46.92" N, 119° 0' 15.12" W
Snowpit Observations
More detailed information about the snowpack: 

Toured up the trees between Hammil and TJ Bowls to under the steep chutes of the Mammoth Crest to test newly formed wind slabs and the persistent weak layer buried under the new weight of the storm snow. Light to moderate S-W-NW winds with occasionaly strong gusts and consistent blowing snow visible across the Mammoth Crest and in localized areas below treeline all day. New drifts down low were F+ hard and up to 18" deep. Up higer, and on E-SE aspects, drifts were harder and deeper. It looked like winds with the bulk of the storm blew from the W and cross-loaded most of the Crest from above Crystal Lk (Jaws) to Duck Pass. Because of the cross-loading, areas like the N part of Red Cone might have seen less slab formation but that's just speculation.

Shovel tilt tests showed F+ hard wind deposited snow on top of F- storm snow even down at 9400' in the trees (see photo). Hand shears consistently failed with easy to moderate force either at the interface between today's storm snow and the new snow from the 20th or down deeper on facets above the melt-freeze crust buried on the 19th. A few W facing test slopes around 9900' showed short (>1m) shooting cracks but other test slopes at around 10,000' on E facing slopes had small D1 results at the interface between today's wind effected snow and the still soft strom snow from the 20th (about 10" deep). Several recent D2 wind slab avalanches were observed on the Crest (see avalanche observation for further information).

Performed 2 sets of quick snowpack tests to test the new snow/old snow interface and also the 0104 facet layer buried between 50 and 75cm down.

Tests 1 were done on a 9 degree, NNE slope at 9600' in a large opening in the trees where the snow depth was 123cm: CTE SC down 8cm in the strom snow, CTE SP down 25cm at the new/old interface (F+ over F-), CTE SP down 35cm on a thin facet layer under the melt-freeze crust buried on 0119, CTM PC down 45cm on another facet layer under a melt-freeze crust, CTM SP down 65cm on the 0104 facet layer (our "layer of most concern"), ECTP 26 in the 0104 facets also, PST 25/100 (End) in the 0104 layer down 65cm. Took 10cm interval temps at this test site too to see if faceting was likely, thus decreasing the strength of buried weak layers. Temperature gradient averaged 1 to 1.5 degrees C per 10cm from the surface down to -80cm below the surface (15cm deeper than the 0104 facet layer) which is sufficient to decrease strength and further faceting.

Tests 2 were performed on a 14 degree NNE facing slope just above treeline and below the Crest at 10,266' where the snow depth was 145cm: ECTP 9 down 30cm at the interface between F+ hard wind deposited snow from last night/today and the F- storm snow from the 20th, PST 45/100 (SF) down 80cm in the 0104 facet layer.

We didn't venture up into the steep terrain on the Crest proper thanks to continuous wind loading and recent (within the past 12 hrs). Skiing down in the low angled trees was pretty darn good!

Wind continued well into the afternoon blowing snow and creating fresh wind slabs along the Crest (see video taken at 3pm).

Snowpack photos: 
Shovel Tilt Test: F+ hard wind slab over F- hard storm snow.
Snowpit videos (tests, etc): 
Weather Observations
Blowing Snow: 
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