Flash flooding is a rapid flooding of low-lying areas and can come with no warning. Often a flash flood is attributed to heavy rainfall from a severe thunderstorm, hurricane, tropical storm. Melt water from a heavy snow season can also contribute to flash flooding.
Flash flooding can be exasperated when the soil moisture level is high. It is also for this reason that urban areas can sometimes be more prone to flash flooding than the suburbs or countryside. The concrete and asphalt found in urban settings can block the transfer of water channeling it to low spots very quickly.
You can use the FEMA Flood Map Service Center to determine the flood risk in your area.