Usage patterns
前兆が現れるa sign/omen appears
It is said that signs of the big earthquake appeared beforehand.
前兆を感じるto sense an omen
I sensed an omen that something bad was about to happen.
〈事〉の前兆a sign of ~
A headache might be a sign of illness.
前兆があるthere is a sign/warning
They say there are always warning signs before a volcanic eruption.