Flash flood events happen quickly. In the recent Kansas City incident, for instance, the rain began about 7pm. Flooding began around 8pm, and most of the fatalities occurred between then and midnight. By 1am the water had receded enough to begin searching for the victims. As the incident unfolds, dispatch is overwhelmed by the sheer volume of calls, and available rescue resources quickly run out.
Time is as much an enemy as the water.
Rescues will be done or not by the people on the spot, since there is little time to bring in outside resources an excellent argument for training as many people as possible. A good preplan really helps. If you have just a few hours warning you can pre-dispatch units to known trouble spots. The easiest rescue is one you don t have to make. Since you know which streets are going to flood, you can block them off.