Is It Going to Rain Today in Chicago?

Three layers go into the answer above. For the next two hours we use a 15-minute nowcast, which is close to radar extrapolation — that is what lets us say rain starts in twenty minutes rather than «this afternoon». Beyond that we use the hourly model blend, flagging any hour with a 40% chance or more, or with measurable accumulation. And for the rest of the week the daily probabilities take over. A probability of 40% does not mean it rains for 40% of the day; it means four times out of ten, a spot in this area sees measurable rain during that hour. The most useful trick is still the oldest one: watch the radar. If a band is moving toward you at 30 mph and sits 60 miles out, you have two hours, whatever any percentage says.