Prediction of extreme rainfall for the Coffs Harbour catchment

On 23 November, 1996 a flash flood struck Coffs Harbour, New South Wales, Australia, resulting from rainfall totals of up to 400 mm, in just 4. 5 hours, recorded in the 50 km2 Coffs Harbour Creek catchment. 1-four hour rainfall totals exceeded 500 mm at some higher elevations. One person was killed and there was an estimated damage bill of A30 million. The event was sim-ulated using the non-hydrostatic version of the University of New South Wales HIRES mesoscale model, which predicted rainfall totals similar in pattern and amount to the few observations avail-able. The synoptic and mesoscale mechanisms responsible for the heavy rain were well-defined for this event. Focussing on these mechanisms, a perturbation methodology was used to vary the initial conditions provided to the HIRES model, within the range of known present climate values. The ensemble of HIRES model rainfall forecasts from these perturbed initial conditions suggests that, even under current climate conditions, the Coffs Harbour flash flood rainfall totals could have been much higher. For exam-ple, the largest 24-hour model-predicted rainfall amounts over the hilly, data-sparse parts of the Coffs Harbour catchment were 2. 5 to 3. 5 times that predicted for the near-coastal station of Coffs Harbour Airport. This was true for both the unperturbed model prediction and for the highest 16 per cent of the perturbed model runs. Such information is valuable as it has significant implica-tions for quantitative precipitation forecast (QPF) estimates and associated flood mitigation strategies. Finally, the perturbation methodology was also applied to four other flood events in 1991, 1989, 1977 and 1974, and the results are briefly compared and contrasted with the November 1996 storm.