In tropical storms and hurricanes, wind speeds and surge are not the only danger—flooding and flash flooding have claimed the most lives in tropical cyclones from 1970 to 1999 (unlike the historic 2005 hurricane season, where storm surge claimed thousands of lives). Flash flooding will occur in creeks, streams and urban areas within hours of torrential rain. These floods can reach heights of 30 feet or more. Streets can be turned into rivers, and underpasses become deadly.
During the 20th century, floods were the number one natural disaster to cause the loss of lives and property, according to the USGS. Flooding can occur in a number of situations, including heavy downpours in strong thunderstorms, or during the spring when snowpack is melting. Record flood events have occurred along the Mississippi and Missouri rivers in years such as 1927, 1973, 1993 and 2011. During the spring, snow melt from winter snows flows into rivers in the upper U.S., most of which converge into the Mississippi River. After winters of extreme snowfall, rivers can swell far into their flood plains and wreak havoc on river towns.