ominous

"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"

"ill omens"; "ill predictions"; "my words with inauspicious thunderings shook heaven"- P.B.Shelley; "a dead and ominous silence prevailed"; "a by-election at a time highly unpropitious for the Government"