unlikely vs improbable

unlikely and improbable both are adjectives.

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

As adjectives, unlikely and improbable are synonyms defined as:

unlikely and improbable: not likely to be true or to occur or to have occurred

As adjectives, unlikely and improbable are synonyms defined as:

Other synonyms of unlikely include unbelievable, unconvincing.

unlikely (adjective)	improbable (adjective)
has little chance of being the case or coming about	not likely to be true or to occur or to have occurred
not likely to be true or to occur or to have occurred	having a probability too low to inspire belief
having a probability too low to inspire belief	too improbable to admit of belief