Wind slabs, storms slabs, shooting cracks to 80' and natural avalanches below treeline.

Update on Jan 8: Having returned to this area today it is now apparent that the failure layer in these avalanches and shooting cracks was most likely buried surface hoar. While in the area making the below observations on Jan 5, I did note some surface hoar grains at the old/new interface, but at the time they were extremely difficult to isolate and remove from under the storm slab. At the time I believed that the two previous days of cloud cover and warming air temperatures had caused the surface hoar to fall over prior to burial. I though that the surface hoar was an artifact at the old/new interface with the loose grained near surface facets underneath the surface hoar as the actual problematic weak layer. Today's observations indicate that was not likely the case. See http://www.sierraavalanchecenter.org/fri-01082016-1100-reactive-buried-s... - Brandon Schwartz

Widespread instability of storm slabs below treeline on Andesite Ridge between the Hole-in-the-Ground TH and the ridgetop. Instability was greatest below treeline and near treeline with storm slabs up to 6 inches deep avalanching naturally on NE-E-SE aspects. Skier triggered shooting cracks up to 80' were observed at treeline around 8,000'. Digging out the shooting cracks revealed that the slabs were failing at the old/new snow interface where poorly developed, but fairly loose grained near surface facets existed.

In wind loaded areas near treeline and above treeline, wind slab instability was more difficult to trigger, but slabs up to 1 foot deep and 15' wide were remotely triggered on some of the smaller terrain features. Larger terrain features were not tested. Pole probing revealed areas of wind load up to 2 feet deep.

All wind and storm slabs were noted to avalanche on a density change (4F slab over F hard weak layer) within the storm snow. One wind slab was observed to step down to near surface facets at the old/new interface. Storm slabs were noted to crack to the depth of near surface facets, but slab avalanche activity was occurring higher in the snowpack on the density change within the new snow.

Photo 1: Natural storm slab avalanche below treeline (SE aspect, 7,700', slope angle greater than 35 degrees). One of three natural below treeline storm slab avalanches observed.

Photos 2&3: Remotely triggered wind slab avalanche near treeline (ENE aspect, 7,900', slope angle in upper start zone of 48 degrees).

Photo 4: Intentionally skier triggered wind slab avalanche with step down on near surface facet layer (NE aspect, 8,100').

Photo 5: Shooting crack 80 ft long connecting from tree to tree to tree, etc. Skier triggered at treeline, failing on near surface facets at the old/new snow interface (ENE aspect, 8,050').