Unexpected Flash Flood TRAPS Drivers on Highway! Flash floods are rapid flowing waters in low-lying areas typically caused by heavy rain fall, severe thunderstorms, hurricanes or tropical storms. They can make their way through hundreds of KMs of desert and it can take more than six hours between the rainfall and the beginng of the flooding in the lower lying areas, making them so unexpected for people in areas which might not even have taken notice about the downpours that caused them and the i