Example : 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.

Synonyms : baleful, forbidding, menacing, minacious, minatory, ominous, sinister

Example : A heavy sky.

Synonyms : heavy, lowering, sullen