a sign of something about to happen
"he looked for an omen before going into battle"
Something which portends or is perceived to portend either a good or evil event or circumstance in the future, or which causes a foreboding; a portent or augury.
"The sudden silence before the storm felt like an ominous omen of the trouble that was about to unfold."
To be an omen of.
"The sudden storm rolling in was an ominous omen of a disastrous harvest year ahead."