[Alneyan]

You mean a negative value? It should improve the odds for this event to target this system, which may be a good thing if you have good events. (Otherwise, you do not want to increase the odds for a Star Destruction in your systems)

However, I wonder if a given event only targets the systems where it can actually do anything. Would a Warp Close event target systems without wormholes? It doesn't make much sense, but you ought to check in case.