Don't have time to work out a complete tutorial but if you want to stay completely inside AE try Particular with a custom particle that looks like rocks, maybe a bunch of them, line up a floor for the particles to roll down and go from there. A thorough knowledge of how Particular works will be required. Start by learning the basics.
If it were me I would just start that kind of effect in a 3D app like Blender, or if you have it C4D. C4D lite that comes with AE doesn't have the tools to create an avalanche but you can easily do it in Blender.
BTW, just a suggestion on forum educate, a bump to your thread only 15 minutes after the original post doesn't help generate answers. This forum is not moderated and if anyone has ideas and time they usually will post.
Looking at reference footage of avalanches might help you. To me, it looks like you could get this using the Aux particles and two instances of Particular. You need to set up your main particles to emit in a kind of rolling motion. I'd probably use the bounce Physics with gravity and not much bounce and an emitter that moves down the "hill". Use the copy with property links option to make a second version. Hide that layer for now.
One particle layer is going to be the main snow body and the other is going to be big puffs of snow on impact.
Snow Body layer would need to have no main particles visible (make them tiny), but the aux system would need to be active and set to be white cloudy puffs (probably). You could use this as a reference for the basic motion: Preset Detail - Avalanche - RED GIANT PEOPLE, but you might need to tweak it as needed. You also will probably want more aux particles. You may or may not need to enable shading with the shadowlet option to make it look right. Have it kill particles once they hit the floor. By the way, your floor layer needs to follow the basic slope of your hill as best as you can.
The Big Puffs layer represents the impact of the pouring snow on the existing snow. Again, you don't need to see the main particles (which should be matching the motion of the Snow Body layer), but the Aux particles should be created on bounce rather than just streaming from the main particles. These will need to be cloudy puffs as well, but they need be pretty transparent and have decreasing opacity over their life and while increasing their size.
I guess now you don't have to do it yourself , I was trying to do the same project for my new year and Christmas customers and I found this awesome free after effects plugin which has this effect for free. I hope it’s gonna’ be useful for you as well
I did this one with particular :
It needs more adjustments but particular should be OK if you don't need an "atomic cloud like" avalanche... or if you can't spend 3 weeks working on an avalanche shot in pure 3D.