Assessing the flash flood threat is important since flooding is one of the top two weather tragedies that kills people. The ingredients for a flash flood are:
1) A nearly stationary frontal boundary
2) High values of precipitable water. On a precipitable water chart, values above 1.75 inches are considered high. The climate of the region also plays a role. A dry climate with little vegetation will produce more runoff than a humid climate with dense vegetation. Often on Precipitable Water charts, the percent of climatic PW is given. If the percent of climatology is over 100%, then there is more precipitable water for that region than normal (if you have Chaston's weather maps book, see page 71).
3) An unstable atmosphere, widespread region of CAPE
4) Weak upper level winds, storms generally move with the mean 700 to 500 mb wind (look at upper level winds on 700, 500 and 300 mb analysis charts)
If mid and upper level winds are weak, storms will remain stationary or move very slowly.
5) Previously saturated soils
6) Snowmelt along with rain (can more than double runoff)