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.
Ice jams can also cause flooding during late winter and early spring.
Ice jams are unpredictable and can form or break up without warning. Be alert to any rapid rises or falls in the river levels. A rapid rise in a river may indicate that an ice jam has formed downstream of you. In contrast, a rapid fall in a river may indicate that an ice jam has formed upstream from you. Always remember that an ice jam breaks can break up suddenly, and flooding of downstream areas can occur without warning.