I think you have misunderstood how Avalanche works.
Firstly it has a priority of -4, so is nearly guaranteed to go last.
What it actually does is it doubles in power if the user has taken damage in that turn. So it is basically an Ice type Payback, with a little more power and a decreased priority.
So to answer your question, if the opponent switches, then Avalanche will have 60 power, because the user was not damaged in that turn (assuming we are talking about single battles).