Synonyms for Threatening (same or very similar meaning)
WordNet sense 1 (darkened by clouds):
heavy, sullen, loweringWordNet sense 2 (full of or covered with clouds):
cloudyWordNet sense 3 (threatening or foreshadowing evil or tragic developments):
baleful, forbidding, menacing, minacious, minatory, ominous, sinisterWordNet sense 4 (causing alarm or fear):
alarmingFrom the ODE community, based on WordNetadd/edit