a sign that something is going to happen soon
harbinger - sign
A ‘harbinger’ is something that shows or indicates what is likely to happen in the future. It can be used both in a negative and positive context, though it is perhaps most frequently used in the former sense, with the phrase ‘harbinger of doom’ being quite common.
The recent job cuts are not a harbinger of doom. They are simply part of an ongoing process to streamline the business.
The arrival of flowers such as daffodils has for centuries been seen as a harbinger of spring.
Many see that company’s closure as a harbinger of things to come in an industry badly hit by the economic crisis.