Probably is more likely than possibly. If something will probably happen, then we may assume that the odds are that it actually will happen. If we consider the event will possibly happen, then the chances would be believed to be much less.

Personally, I am bad about using them both for the same meaning. They both mean a likely occurrence. However, according to the dictionary, probable is followed with the word likely where possible is followed with the word may. In which case, you would want to use probable in circumstances leading towards a positive outcome instead of an undecided outcome.