The words likely and probable both express the degree of probability of something occurring.
They’re not vague words. Vagueness is a function of borderline cases—at what point does someone become tall? It follows that vague words such as promptly and material require that you assess circumstances from the perspective of a reasonable person. By contrast, likely and probable aren’t a function of reasonableness.
Nevertheless, likely and probable exhibit uncertainty of a sort that makes them problematic. In this post I said that likely might indicate a degree of probability greater than five on a scale of one to ten, or it might mean something different. The same could be said for probable.
“Although the term ‘likely’ connotes something more than a mere possibility, it also connotes something less than a probability or reasonable certainty.” State v. Green, 18 Ohio App. 3d 69, 72, 480 N.E.2d 1128, 1132 (1984).
“Probable and likely are synonyms.” Anderson v. Bell, 303 S.W.2d 93, 98 (Mo. 1957).