1.
forewarning, omen, premonition, precursor
A sign or event that precedes and indicates something to come. Often implies a more concrete or observable warning sign.
Related Words
Synonym:
予兆 (omen (more abstract))
Synonym:
兆候 (symptom, sign (clinical))
Synonym:
前触れ (harbinger)
Synonym:
兆し (sign, first indication)
Compare:
凶兆 (bad omen)