A flood watch is perfectly adequate. Flash floods occur very quickly, and generally happen in specific locales- low lying areas(esp near rivers), densely populated urbanized areas, etc. Given the localized nature of developing storms that may produce flash flooding, and the specific areas that are susceptible to it, it is difficult to put a 'broad' flash flood watch in effect. Thus flash flood warnings are issued in these localized areas when a storm capable of producing such is imminent.