ominous vs forbidding

ominous is not a noun while forbidding is a noun.

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

As adjectives, ominous and forbidding are synonyms defined as:

ominous and forbidding: threatening or foreshadowing evil or tragic developments

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

ominous (noun)	forbidding (noun)
	an official prohibition or edict against something

ominous (adjective)	forbidding (adjective)
threatening or foreshadowing evil or tragic developments	threatening or foreshadowing evil or tragic developments
presaging ill fortune	harshly uninviting or formidable in manner or appearance