The two key elements to flooding are rainfall intensity and duration. Intensity is the rate of rainfall.
Duration is how long the rain lasts.
Rivers, bayous, and streams may overflow their channels and banks. Streets and areas which are normally dry may be submerged. Flooding can be brief, or last for an extended period of time. Rivers and streams can flood areas downstream where there may not have been any particularly heavy rainfall. Storms may continue to develop and travel across the same area repeatedly, causing localized flooding.