There is considerable uncertainty surrounding the frequency and intensity of extreme events, under current climate conditions, including the occurrence of rainfall amounts that produce severe flash flooding. On 23 November 1996, Coffs Harbour, a city on the east coast of Australia, experienced a flash flood which resulted from some of the most intense, short duration rainfall ever experienced in the area (Speer and Leslie, 2000). Rainfall totals of 400 mm in 4.5 hours occurred over the Coffs Harbour Creek watershed which is an area of about 50 square kilometers. One person was killed, caravans were wrecked, shops and houses flooded and there was an estimated damage bill of AU$30 million.

Predicted rainfall totals from a high resolution mesoscale model reproduced large rainfall totals close to the observed totals. There were several synoptic and mesoscale mechanisms responsible for the heavy rain. Focussing on these mechanisms, an observation perturbation methodology was used to vary the initial conditions of the high resolution mesoscale atmospheric numerical weather prediction (NWP) system, within error bounds represented by current climate. Model rainfall forecasts were generated using these perturbed initial conditions which showed that under current climate conditions the Coffs Harbour flash flood rainfall could be as much as 50% higher than the record observed totals of 23 November 1996.