Strong Pacific storms with very deep moisture fields and long durations, along with large moisture fluxes often result in flash flooding and heavy snow in southern California. This is especially problematic since these oceanic moisture fluxes can interact with rugged terrain. Possibly the worst case scenario involves a long period of moderate to heavy rain (moisture duration problem), followed by a burst of very high rainfall rates in deep forced frontal or free convection (moisture depth problem), supplemented by strong upslope and/or convergent flow (moisture flux problem). With this in mind, a moisture “depth/duration parameter” and a “peak storm 850 mb wind speed parameter” was created.

It was found that during the 1998-2002 cool seasons (October through May) for depth/duration parameters of 4 or more, there was flash flooding during more than 60 % of the events (5 of 8 events). With freezing levels of 6000 feet (about 1800 m) or less at least 12 inches (about 305 mm) of mountain snow fell. For depth/duration parameter values of 2 or more and peak 850 mb winds of 35 knots (about 18 ms^-1) or more, there was flash flooding during more than 60 % of the events (2 of 3 events), and either flash flooding or urban and small stream flooding 100 % of the time (3 of 3 events).

The moisture depth/duration parameter and peak storm 850 mb wind speed parameter combine to form a moisture depth/duration/flux flash flood potential index (FLINDEX) for forecasting flash floods and heavy snow. For FLINDEX values of 45 or more with depth/duration parameters of 2 or more there was flash flooding nearly 60 % of the time (5 of 9 events), with a foot or more of mountain snow over 60 % of the time (6 of 9 events). The data suggests that these parameters and FLINDEX can be used as a first guess for predicting flooding or snowfall, and will be discussed.