- baleful, forbidding, menacing, minacious, minatory, ominous, sinister, threatening, ugly - threatening or foreshadowing evil or tragic developments; "a baleful look"; "forbidding thunderclouds"; "his tone became menacing"; "ominous rumblings of discontent"; "sinister storm clouds"; "a sinister smile"; "his threatening behavior"; "ugly black clouds"; "the situation became ugly"
Antonym: unalarming (indirect, via alarming)