Not all floods are the same. Some develop over time, however flash floods happen quickly, and amazingly can occur without visible signs of rain. Causes of flash floods include:
Excessive rain – within minutes of precipitation or up to 6 hours after.
Slow-moving rain of a long duration, such as occurs with thunderstorms, tropical storms, and hurricanes.
A dam/levee failure.
A sudden release of water held by an ice jam.
Rainfalls or snowmelts that exceed the capacity of underground drainage pipes and sewage systems.
Floods after wildfires which reduce the capacity of the ground to absorb water.
Urbanization, such as paving, which reduces rainfall absorption and increases runoff.