portending evil or harm; foreboding; threatening; inauspicious

an ominous bank of dark clouds.

having the significance of an omen.

adjective

auspicious , happy , lucky , promising , propitious

adjective

apocalyptic , augural , baleful , baneful , clouded , dangerous , dark , dire , direful , dismal , doomed , doomful , fateful , fearful , forbidding , gloomy , grim , haunting , hostile , ill-boding , ill-fated , impending , inauspicious , inhospitable , lowering , malefic , malificent , malign , minatory , perilous , portentous , precursive , premonitory , presaging , prescient , prophetic , sinister , suggestive , threatening , unfriendly , unlucky , unpromising , unpropitious , apocalyptical , fire-and-brimstone , grave , hellfire , dour , foreboding , menacing , monitory