Possible means having the potential. Probable means likely.
Probable refers to what is likely to be done, to occur, or to be true; possible refers to what can be done, to occur, or to be true. If you say something is probable, you are expressing more confidence about it than if you say that it is possible.
Where is Danny? He’s probably at Lee’s place. They’re supposed to rehearse together for the show tomorrow. [It’s very likely that Danny is at Lee’s place. So you use ‘probably.’]
What can I possibly do to help her? [You are not stressing on one particular action that you’re very likely to take]