Well, take the actual weather observations for the past year. Take the actual weather observations for the last week. Overlay and slide this week of observation

Well, take the actual weather observations for the past year. Take the actual weather observations for the last week. Overlay and slide this week of observations on top of the observations of the past year, until you find the window that matches "the best" — then take the day right after this window and predict that the weather will be just like that (or maybe try to tweak the values a bit). I wonder how poorly this thing operates, and whether taking several years of history to look at would help much.