Below are a couple details from the large cornice that fell on the southwest
face of Magnum mid-day Saturday, March 2nd.
This cornice grew during the February 25-27 storm where 3+’ of snow fell with
moderate wind and mild temperatures – ideal for cornice growth. Saturday’s calm
winds and sunny skies tipped the balance by warming the snow and increasing the
plastic deformation. This added enough stress to cause failure. The cornice
broke at 3:15 while people were on the Ridge but no one was in it’s path
which is very lucky.
The weight of the cornice falling on the snow below was a great test to the
snowpack. Resulting avalanches triggered were:
– One pocket that broke in the Oct facets directly under the impact of the
cornice and did not propagate further (old deep slab concern). Good news.
– Two mid-slope pockets that released and propagated ~50′. These failed
at the bottom of the Feb 25-27 storm snow that had not fully settled and bonded.
This old/new snow interface has gained in strength substantially since then.
*Take home point – Cornices continue to be a very real concern and many continue
to hang in the balance. The fact that none of our buried layers of concern (Oct
facets or various weak snow above crusts below 3,000′) propagated or were
triggered points to a strong snowpack in this area.