Learn French: Il va probablement pleuvoir ce soir.

You would say Il est probable qu'il va pleuvoir (or qu'il pleuvra). Because probable indicates a high likelihood, it is followed by the indicative mood, not the subjunctive. This matches the certainty expressed by the adverb probablement in our current sentence.

If I wanted to say it is unlikely to rain tonight, how would that change the grammar?

To express improbability, you would use the phrase il est peu probable que (it is unlikely that). Because this expresses doubt rather than certainty, it forces the verb into the subjunctive mood: Il est peu probable qu'il pleuve ce soir.

Why is probablement placed right in the middle of the verbs?

In French, when you have a conjugated verb followed by an infinitive (like the futur proche here with va + pleuvoir), short adverbs or adverbs of probability are usually sandwiched right between them. English speakers often want to put the adverb at the end, but French prefers Il va probablement pleuvoir.

Can I use ça instead of il here, as in ça va probablement pleuvoir?