They take a holistic, big picture view on those maps using stream guidance and soil moisture. Re: if you got 1" of rain over 8 hours that will help lower this more vs. 4" in 30 min. I have a client that works at USGS so I hear some stuff coming up. They get people writing them, on FB, when they occasionally post these maps. "It just rained and I am in D1, whaaa" and they explain it in a reply. Much of their FB is shake/earthquake stuff which is fairly interesting.