A simple question, with a complex answer: The sky cannot contain more than about 50mm of rain at any one time (laws of physics). The key in a flood event is that it happens over a period of multiple hours, and that moisture is fed into the system (like a conveyer belt). This event is a type of cutoff-low, and has remained mostly stationary for the last 48 hours, so the peak rainfall has been concentrated in one area. The system will eventually move offshore, and the peak rainfall will move slightly south along the coast today.