I think the conditions you described were responsible for an approximately one foot release on St Helens tuesday. SE aspect at around 7500'. I didn't notice extensive propogation where we were but there was definitely settling with each step in certain areas and apparently enough cohesion to create a crown maybe 100' across where it did release. It was amazing how slopes only a few degrees more southerly didn't seem to have any of this crust settling.
Marc
Interesting Marc. That's why sometimes acknowledging weakness is sufficient to suggest caution. It's (sometimes) just not possible to test widely enough to be comprehend the variability and predict how the snowpack will behave.
Another interesting aspect to this is the quality of shear. Just having collapse and even propagation doesn't necessarily mean that the coefficient of static friction is sufficiently low that the slope will slide. When frozen, a weakness like this one (which is made of refrozen grains that have become faceted and possibly even melted and refrozen more than once) is often a fairly rough surface - i.e. poor shear quality. So, it would take a substantial load to overcome the rough shear and enable avalanching. However, when sufficient faceting takes place, the ice grain roughness is replaced by sugar, which of course overcomes the shear roughness and essentially eliminates it.
But the lesson here is that warming can not only weaken a weak layer and crustal bonds, but it also lubricates the shear plane and makes the rough ice and faceted grains less rough. It rounds and lubricates them. So, you can have a stable situation when the ice grains aren't too faceted and when the crust and weak layer are frozen, but with warming all that can change.
