"unlikely" can be used as either an adjective or an adverb; however, "unprobable" can only be used as an adjective. If we want to use it as an adverb, we should change it to unprabably.
So as an adjective, we can use unlikely as: It's unlikely to rain tomorrow. Extending the deadline seems to be a unlikely thing, as the professor is harsh. As an adverb, it can be used as: It may, not unlikely, rain tomorrow. Not unlikely, she'd study abroad.
As for improbable, we can phrase the sentence like: It's not improbable that she would study abroad. Keeping drizzling for a couple of weeks is improbable here.