probable vs improbable

probable is a noun but improbable is not a noun.

Word	Noun	Adjective	Verb	Adverb
probable	Yes	Yes	No	No
improbable	No	Yes	No	No

probable (noun)	improbable (noun)
an applicant likely to be chosen

probable (adjective)	improbable (adjective)
likely but not certain to be or become true or real	not likely to be true or to occur or to have occurred
apparently destined	having a probability too low to inspire belief
	too improbable to admit of belief