The specific percentage probabilities depend on context but in short; 

 - Possible means it *can* happen, (be it unlikely or likely - however it is more often used for things which are unlikely)
 - Probably means it is likely to happen

Related terms;

 - Definite means it **will** happen.
 - Improbably means it could happen but it is unlikely
 - Impossible means it cannot happen


## Use of probable or possible for things which are definite
It would feel very unnatural (and give the wrong meaning) to say that something definite was probable or possible and should be avoided. For example I would say "it is definite that I am using a computer to type this" not "it is probable I am using a computer to type this". This is because the strongest term should be used to describe the likelihood

However; when used as a condition it becomes acceptable. For example "If it is **possible** that it will be sunny tomorrow we will go to the park" or "If it is **probable** that it will be sunny tomorrow we will go to the park", if the weather reporter gives a forecast of 100% chance of sun then you still go to the park.

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  [1]: https://i.sstatic.net/D93eA.png