Heads-up and Omen

Heads-up noun - The act or an instance of telling beforehand of danger or risk.

Omen and heads-up are semantically related. In some cases you can use "Omen" instead a noun phrase "Heads-up".

Omen noun - Something believed to be a sign or warning of a future event.

Heads-up and omen are semantically related. Sometimes you can replace phrase "Heads-up" with "Omen", this terms are similar.