What could cause equator-level rainfall in a northern region? And what sort of effect might it have on the climate/ecosystem?

I had an idea for a part of one of my world projects that would be a sort of boreal jungle or swamp, not arctic or near-arctic, but more like britain or ireland, but I'm struggling to think of what could cause the necessary level of rainfall at that latitude, and its especially unclear what kind of environment it would create, given the closest real-world examples still aren't nearly that wet.