A human skydiver has a terminal velocity of about 120 mph. A single snowflake's terminal velocity would be conciderably lower - around a metre/second. The avalanche moves much as electricity does - the speed of a single electron is very slow at 0.3 mph, but push one in at the 'start' and you have, almost instantaneously one exiting the 'finish'. This can also be demonstrated with marbles in a tube. The snow at the top of the mountain will start to slide, pushing the snow beneath and so on. It is this movement that is the speed of the avalanche, not the single snow drop. The single snow drop is not faster than the skydiver, but the overall movement of the snow is.

Hope this has ansered your question.

Harry

An avalanche or rockfall is mainly air. The mixture of lumps and air constitutes a fluid denser than the air around it, so it slides downhill, accelerating until the drag of the fluid against the hillside matches its excess weight.  This limiting speed is akin to the "terminal velocity" of the skydiver, 120 mph or 200 km/h, which it may exceed, depending on the slope and density.

A rockfall at the Elm in Switzerland in 1881, composed of fragments a few centimetres in diameter, reached in excess of 300 km/h with an average 155 km/h. It sloshed up the opposite hillside then flowed down along the valley floor, "destroying houses and everything in its path, killing 115 people". (Principles of Geology, Gilluly, Waters, and Woodford).Earthflows underwater give rise to a particular type of sedimentary rock called "turbidites", evidence of even greater energies.