A portent is a foreshadowing, usually of something important, significant and possibly calamitous.
I don’t think it necessarily implies a bad happening, but it does seem that great change is uncomfortable and perceived as a negative experience to most people.
There are several portentous indicators for future change in our society and economy, for example. One such omen is the price of fuel. Sooner or later, something will have to change, at least in my personal life. I can’t continue the current trajectory without some change. Either more money will be deposited into my account (not so likely) or I’ll have to drive fewer miles (not likely), or I’ll have to drive the same miles less often (most likely).
In Symbiosis, my work-in-progress novel, I’ve used portents to foreshadow change. Some are more obvious in the first draft and I’ll likely refine them during editing because they’re, well, obvious. I like subtlety. The indicator to the most significant change is subtle, but the character endpoint is very distant from her starting point.