Updated-Natural Wind Slab Avalanche on Elephants Hump

We went back to revisit the Elephants Hump avalanche as the weather improved and we could do further investigation of the avalanche.  This natural avalanche event was a very wide propagating avalanche that connected multiple aspects through lower angle terrain via shooting cracks.  The crown line was measured from above at 740' wide and ran downslope up to 300'.  Minimum slope angle was measured at 32 degrees.  2 main avalanche paths were connected via shooting cracks through lower angle terrain as the avalanche changed from a N aspect to a ENE aspect.

The hard wind slab was pencil hard.  Below the hard slab was 4F hard near surface facets that were served as the weak layer of the avalanche.  The gale force SW winds on Tuesday night through Wednesday wind-loaded this area and then caused the weak faceted (sugary) snow to fail naturally. It looked like this avalanche occurred sometime early Wednesday morning.

This is the only avalanche that we know of that has occurred on these near surface facets.  This location had ideal conditions for the near surface facets to exist, not get destroyed by previous wind scouring, and then get buried by wind loading on Tuesday night/Wednesday.  This layer has already started to show signs of gaining strength with bonding of the slab to the faceted snow below.

Initial avalanche observation from Wednesday Dec 18:

We observed a natural wind slab avalanche that was thought to have occurred sometime early this Wednesday morning.  Shifting strong winds in the Carson Pass area over the last 4 days have left many above treeline areas scoured with hard wind slabs in other locations.  Strong to gale force SW winds started blowing on Carson Pass via remote weather data yesterday afternoon.  This avalanche location is along a lower ridge below Elephants Hump in mostly near treeline terrain at 8400', northerly aspect, with a convex rollover.

Human triggered wind slabs were forecasted for today in near and above treeline terrain.  What makes this wind slab somewhat unique is the propagation that was associated with it.  This wind slab had horizontal propagation along the ridge and went around a slight corner to another aspect that also released.  The avalanche was at least 200' wide and ran down slope 30 to 200'.  The crown and bed surface were partially covered up by additional wind loading.  The hard slab nature of the wind slab and that the slope was actively reloading did not allow for safe entry on to the slope.  Shooting cracks went from the avalanche and on into lower angle terrain.  A collapse was felt on a hard slab while investigating the avalanche.

The wide propagation, shooting cracks, and collapse have all the ingredients of a faceted grain type failure below this wind slab.  It is thought that previous surface hoar or near surface facets may have somehow survived the recent winds and become buried under the wind slab.  Grain type identification was not possible with the winds and conditions despite best efforts.  Further observations are needed to determine weak layer grain type and how widespread this may be.