Snowpack question

I'm not familiar with that incident so I can only offer vague speculation.

Anytime shear stress on a slab exceeds the friction or support holding it in place you can have a slide. Late in the season there are three possible faults I can think of:

1. Rapid warming and melt of upper layers over lower density lower layers with the possible lubrication of running water between the slab and existing ice layer or other potential be surface.

2. Rain on snow event that added substantial mass to the upper snow pack increasing shear stress.

3. A lower part of the snow slope that had been supporting the upper portions by compression could have melted out rapidly. Especially if there were recently sun exposed rocks that could have accelerated melt.

Again, these are just guesses and there are probably several other scenarios that could explain it that I'm failing to think of or some combination of the above. However, for a compacted,avy controlled and heavily skied run to slide there had to have been a rapid increase in shear stress or a similarly rapid decrease in slab support.

Rick Blair wrote:Dave, On a slight tangent, there was an avalanche fatality at Arapahoe Basin in 2005. I never read a good report on what exactly were the mechanics behind it. Obviously an extremely popular run, 1st Alley into Pali, skied all season, avy controlled, packed and late spring conditions. I had heard someone mention flowing water under the snowpack possibly separating the snow from the bed surface. Could not find it on COAIC database. What exactly went on there? This presents another method avalanches can be created, although extremely rare.... hopefully.