You’re thinking of a pyroclastic flow from a volcano, and that only ranges between 200 and 500 mph depending on slope and gas composition.
“Avalanches have been clocked travelling at speeds of up to 360 km per hour (225 miles an hour), which is the top speed of a MacLaren F1 or Ferrari Enzo. Imagine a line of these supercars, often up to several hundred metres wide, racing towards you at top speed! A typical dry snow avalanche travels around 130 km per hour, and reaches this speed within five seconds after it fractures, with acceleration similar to a BMW 5 Series.”
http://www.snowshoemag.com/2008/10/09/avalanche-avoidance-tips/
Not all go that fast. Some merely hit 20 mph. Ever try running faster than 20 mph in the snow?