Winters aren't harsh - if it snows, that tends to come as a rude shock to public transport! February is actually the coldest month. But temperatures CAN be close to freezing, the air is damp because we're on a big island, so it can feel like kind of miserable nose-drippy cold.
So it depends on what kind of temperature you're used to. Just wrap up warm, really, and expect rain, though that isn't as common as stereotypes might lead you to believe.