Meteorological aspects of the 31 March 2009 Coffs Harbour flash flood

Abstract

Flash flooding from short duration, torrential rain occurred in the city of Coffs Harbour on the New South Wales north coast during the afternoon and evening of 31 March 2009 when totals of more than 300 millimetres were recorded in the hills immediately to the west of the city. Meteorological analysis of the event showed that several factors combined to produce the flooding. These included a moist, convergent low-level airflow onto the hills five to ten kilometres inland from Coffs Harbour, a strongly backing vertical wind profile favourable for broadscale ascent, and the presence of a mid-tropospheric trough to the west which assisted upmotion and reduced the static stability in the region. A southwesterly surface flow, induced by the orientation of the hills near Coffs Harbour, also enhanced the backing of the wind with height (and implied ascent through warm air advection) further intensifying the vertical motion in the trough and was coincident with the heaviest period of rainfall. This slow moving pattern provided an efficient dynamical mechanism that anchored the rain over the hills for several hours, similar to a Coffs Harbour severe flash flood event thirteen years earlier. Model forecast guidance of these factors, combined with using a multiple of approximately three times the maximum ensemble-predicted rainfall amounts over the highest topography nearby, is capable of providing valuable information to forecasters of the potential for dangerous flash flooding in the Coffs Harbour Creek catchment.