threatening vs ominous

threatening and ominous both are adjectives.

Word	Noun	Adjective	Verb	Adverb
threatening	No	Yes	No	No
ominous	No	Yes	No	No

As adjectives, threatening and ominous are synonyms defined as:

threatening and ominous: threatening or foreshadowing evil or tragic developments

Other synonyms of threatening include baleful, forbidding, menacing, minacious, minatory, sinister.

threatening (adjective)	ominous (adjective)
threatening or foreshadowing evil or tragic developments	threatening or foreshadowing evil or tragic developments
darkened by clouds	presaging ill fortune