Sign
Noun and の adjective
ぜんちょう
この雲と風は嵐の前兆です。
The clouds and wind are signs of a storm.
A before omen is something that tells you about something that will happen before it happens. Really this is just an omen or sign.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.