Flash floods generally develop within about six hours of their immediate cause. According to the National Weather Service, common triggers include heavy rain—especially from slow-moving thunderstorms or storms that repeatedly train over the same area—plus ice or debris jams and levee or dam failures.
Urban pavement and steep or rocky terrain raise the risk because water runs off faster than soils or storm drains can absorb it, so streams and low-lying areas can rise rapidly. In some western U.S. settings, flooding can appear far downstream from where the rain first fell.