However, typically, the greatest threat of flooding is in the spring when heavy rains and snowmelt often combine to produce excessive runoff. Ice jams can also produce flooding during the late winter and early spring. There are several different types of flood warnings. Here are some definitions. Widespread heavy rain, possibly coupled with snowmelt, is often responsible for this type flooding.
Ice jams can also cause the major rivers to flood.
Heavy, slow moving, summertime thunderstorms are usually the cause of this type of flooding. Dam breaks can also cause flash flooding. In the fall, winter, and spring, leaves or snow may block the drainage systems, contributing to this type of flooding. To determine the potential for flooding, the National Weather Service monitors hydrologic conditions throughout the year. During the late winter and early spring, the National Weather Service, in cooperation with other federal, state, and local agencies and cooperative observers, surveys the snowpack weekly to help determine the potential for spring flooding.