A flash flood is a flood that occurs within six hours, or less, of heavy or excessive rainfall, a dam or levee failure or water released from an ice jam. The heavy rain can be produced by a severe thunderstorm, hurricane or tropical storm. The water from melting ice and snow can contribute to flash flood events during the spring. The topography of an area can also enhance the flood risk. Steep and hilly terrain can produce rapid runoff that can cause stream and river levels quickly to rise. Urban areas are especially prone to flash floods since the added concrete does not allow water to easily seep into soils, causing it to pond and collect quickly in low points.
Source: Jamestown, CO Cut off by 2013 Colorado Floods | Wikimedia