As a precursor to earthquakes, well water sometimes becomes muddy.
彼の突然の沈黙は、何か悪い知らせの前兆だった。
His sudden silence was an omen of some bad news.
It is said that signs of the big earthquake appeared beforehand.
I sensed an omen that something bad was about to happen.
A headache might be a sign of illness.
They say there are always warning signs before a volcanic eruption.