Portent (noun)

1. An omen or a sign of something impending, especially something momentous or sinister.
2. An event or occurrence that is regarded as a warning or a sign of something significant to come.

Origin:

Middle english, from latin portentum, from portendere, to portend.

Examples:

1. The comet was considered a portent of impending doom.
2. The sudden silence was a portent of the coming storm.
3. The stock market's sudden drop is being seen as a portent of an economic recession.
4. The portent of war was obvious to all who had eyes to see.
5. The portent of the black smoke from the volcano was a sign of an impending eruption.