How can I talk about probability in the future in French?

There are several ways to express something that will possibly happen in the future.

One of them is to use adverbs with the future tense to show that you are not sure that the event will actually occur. For example, if you say Il va pleuvoir demain, it means that you are certain that it will rain tomorrow. You can add words to this sentence to express uncertainty or probability: Il va peut-être pleuvoir demain (It will maybe rain tomorrow), Il va probablement pleuvoir demain (It will probably rain tomorrow)... 

You can also use sentence structures that are constructed with the subjunctive form of the verb, usually introduced by que: Il se peut qu'il pleuve demain (It may be that it will rain tomorrow), Il est possible qu'il pleuve demain (It is possible that it will rain tomorrow)...

Another way is to use the conditional form of the verb devoir. Devoir has different meanings, but in that case it is used to express the idea that something has a good probability of happenning: Il devrait pleuvoir demain. Native speakers often use this kind of sentence with the adverb normalement: Normalement, il devrait arriver à 10 heures (If everything happens as planned, he should arrive at 10).