just scolled the thread and checked out your test shots. They are looking quite nice, like the paths the particles are taking.
As I have a little bit of experience dealing with real-world avalanches I thought I would offer my two-cents…
FYI in the occurence of avalaches there are a few different types the two most common are:
Point Release (the avalanche starts from say a clump of snow falling from a tree), this somewhat resembles what you are trying to achieve. Usually these occur after fresh snowfalls, creating highly unstable conditions.
The second most common type is a slab avalanche ( can be a new snow soft slab or an older more condensed snow hard slab). As the name suggest, slabs are just that huge consolidated blocks of snow and are usually triggered by weighter objects such as explosives or skiers.
There certianly are many more factors than I wish to explian here but as with all avalanches from release zone through the track they generally “collect” more snow as they reach there apex.
So as it contunies down the steeper parts of the track the avalanche “grows” larger and larger. As the slope angle begins to decrease, this is when the speed of the avalanche begins to recede and growth slows down exponentially.
This may be something to take into consideration during your development process.
Will most likely be a cpu cruncher but setting your particle system to spawn on collision might make it look alot more believable.
Don’t know how much time you wish to spend on this but another fact is the track that is left after the snow has passed it quite visible from start to finish.
Hope this is of help and will check back to see how things are coming. Keep up the great work 
