A flash flood happens when there is a large amount of rainfal in a short period of time. The ground can't absorb the large amount of water, so the water overflows on the ground, and that is how the flash floods start. All that water that doesn't get absorbed by the ground will stay above the ground and travel to the lowest point and that is how the flash floods start.
Another way flash floods start is when a large amount of rain happens in a short period, and the rain flows down into a body of water such as a stream, river, creek, lake, etc. When there is too much water flowing into a body of water, the body of water can't handle all of the charge, and the excess water will begin to overflow over the banks of that body of water.
Flash floods also occur on roadways. When there is a large amount of rain that occurs in a short period (such as very heavy showers that occur during thunderstorms), and the storm drains can't handle that sudden amount of water trying to go into the drains, then flooding will start on the roads.
Foe example, two years ago, there was a tornado here in Brooklyn. The storm that produced that tornado had such a high amount of rainfall in a short period of time that the storm drains on my block could not handle all of that water, and then the entire block had about 2 feet of floodwater on it. That storm, if I remember correctly, dropped about 2 inches of rain in about 40 minutes.
So, basically, a flash flood is when a large amount of rain occurs in a short period of time, and either the ground, a body of water or a roadway can't handle that sudden and large amount of rain, and a flood starts to occur in a very short period of time.
The most likely time a flash flood will occur is when you have something like a severe thunderstorm that will give you a lot of rain in a short period of time. Also, flash floods can happen almost anywhere.
