ominous cloudsSyn:threatening, menacing, baleful, forbidding, sinister, inauspicious, unpropitious, unfavorable, unpromising; portentous, foreboding, fateful, premonitory; black, dark, gloomy; formal minatory; literary direful; rare minaciousAnt:promising, auspicious••ominous, fateful, forbidding, foreboding, portentous, premonitoryA sky filled with low, dark clouds might look ominous, but it probably wouldn't be considered portentous, even though the root words omen and portent are nearly synonymous. What is ominous is usually threatening and may imply impending disaster (an ominous silence), while portentous is more often used to describe something that provokes awe or amazement (a portentous show of military strength) or a very important outcome (a portentous moment for the American people). Like ominous, foreboding implies that something evil is coming (foreboding words that sent shivers through us), while forbidding suggests an unfriendly or threatening appearance (a dark, forbidding castle). Fateful and premonitory are less frightening words. What is fateful appears to have been inevitable or decreed by fate, with an emphasis on decisive importance (a fateful meeting with her ex-boyfriend; a battle that would prove fateful). Anything that serves to warn beforehand is premonitory, whether or not the warning concerns something negative (a premonitory dream about her father's death; a premonitory feeling about the exam).
Thesaurus of popular words. 2014.