Probable y posible son palabras diferentes con definiciones diferentes.
Probable: greater than 50% chance of occurrence, close to 100% Posible: less than or equal to 50% chance of occurrence
You could say "It is probable that I will make it back to my house without getting into an accident, but I should still be careful because it is possible that I might crash."
The difference in English is equivalent in Spanish.
Hope this helps.