Warn somebody in the past about something that will really happen.
Do it in your own voice or in the voice of somebody else.
Boris. Congratulations, you are prime minister. It’s a big thing, and you deserve to enjoy the moment. But I want to let you know that Brexit is not going to be your biggest concern in 2020. In fact, you will give it very little thought. More important than Brexit will be the pandemic of coronavirus. This is a deadly virus rather like flu. It will start in China – may even have started already – and spread rapidly around the world. I would never have thought that this was the kind of thing you could deal with, but that’s a reflection on me, not you. Because actually you will handle it quite well. You will very sensibly fortify yourself with experts. And you will find yourself fighting a war that justifies your inner Churchill. I don’t mean that to sound bitter. I mean that you will do well. Not perfectly – who could do that? – but a lot better than I would ever have expected. And better than I would have done. I wish you well, Boris. Yours, Theresa
Something like that. Or much better please!