A flash flood occurs when there's a great deal of rain in a very short time. While it rains periodically during different times of the year in the Valley of the Sun, in the Phoenix area, we experience flash floods typically during monsoon thunderstorm activity.
According to the National Weather Service, "Flash floods can be produced when slow moving or multiple thunderstorms occur over the same area.
When storms move faster, flash flooding is less likely since the rain is distributed over a broader area."
The Phoenix area has improved the urban drainage systems over the past three decades, but this is still a desert with a sprawling set of cities in it, and heavy rains don't drain quickly. Streets flood, and otherwise dry rivers and arroyos can become death traps in a matter of minutes.
Flash flood warnings will be issued on both local radio stations and TV stations. It's best not to be outside when these heavy rains are expected, since visibility is poor. When was the last time you checked your windshield wipers? Many people discover at exactly the wrong time that the heat has dried and cracked their wipers, rendering them useless.
Finally, don't drive through big puddles or through washes when there's water present. Sounds silly to say, but every year people are stranded in their vehicles just because they were sure they could make it (and were wrong).
As a result, Arizona has a statute affectionately called The Stupid Motorist Law. The gist of the law is that if government resources are used to rescue you from a flash flood in an area where you should have known that you weren't supposed to drive, you could be charged for the police, fire, helicopter and other expenses associated with the rescue.