Harbinger (noun)

1. A person or thing that indicates or foreshadows a future event.
2. A person or thing that precedes and gives an indication of the approach of another.

Origin:

Middle english herbergeour, from old english herebeorgian, from here "army" + beorgan "to protect".

Examples:

1. The crocuses are a harbinger of spring.
2. The stock market crash was a harbinger of the Great Depression.
3. The early arrival of the geese is a harbinger of winter.
4. The peace treaty was a harbinger of a new era of relations.
5. The company's poor earnings report was seen as a harbinger of more bad news to come.