A synoptic-mesoalpha-scale climatology of flash-floods in the Sydney metropolitan area

A synoptic climatology of 94 flash-floods occurring near Sydney from 1957 to 1990 is presented. Flash-floods essentially occur with four types of synoptic flow: easterly troughs, lows, pre-frontal flow and post-frontal flow. All types are associated with upper-level short waves or deep cut-off lows. The first two, which are subtropical excursions, are responsible for the bulk of Sydney’s rain and dominate the numbers of flash-floods. They exhibit weak diurnal and seasonal modulation with preference towards the warmer months. These systems typically have little convective available potential energy (CAPE) (~ 1000 J/kg), but also have little convective inhibition. The other two types occur in association with the northerly margin of extratropical fronts. They exhibit strong diurnal and seasonal modulation with preference towards the warmer months. Typically, they possess much more CAPE (~ 1800 J/kg) but given considerable convective inhibition, convection is triggered by the lifting over the shallow cold front and concentrated in mesoscale convergence lines.