An example of ominous is a dark cloud before a thunderstorm.
Ominous black clouds; ominous rumblings of discontent.
Her ominous tone did not register with him.
There was an ominous silence.
His ominous farewell silenced her.
Gabriel's ominous warning suddenly made sense.
The door closed with an ominous boom that echoed throughout what sounded like a massive but empty chamber.