Flood Forecasts are published based on data that meets a high accuracy bar. Notifications are sent only for Flood Forecasts with an even higher confidence bar.
Notifications go to people who have their smartphone location services turned on, have installed the Google Search application and who are located in supported areas that are flooded or expected to flood. If you aren’t near a supported area that’s flooded or expected to flood, you won’t get a Google flood forecast notification.