During 29 August 1992 an intense extratropical cyclone passed south of Adelaide, South Australia. Rain from the cold conveyor-belt cloud system ahead of the low/trough system cleared during the afternoon; however, in the late evening a small area of convection appeared southwest of Adelaide at the head of the comma-head cloud which by this time was wrapping around the western side of the low system. This area of convection moved eastwards, and as it passed over the Adelaide Hills produced intense localised rainfall and flash-flooding in the Torrens River system. Two people were drowned. The synoptic evolution of the extratropical cyclone is described with some emphasis on the tropopause undulation, or isentropic potential vorticity (IPV) anomaly, which was associated with this system and its relation to the upper-level jet/trough patterns and the low-level thermal and moisture fields. It is shown that shear on the cyclonic side of a strong northwesterly jet strongly modulated the IPV structure, and so resulted in strong warm advection in the upper troposphere ahead of, and strong cold advection immediately upstream of the IPV anomaly. This cold advection in the upper troposphere moved into phase with lower-tropospheric warm, moist advection at the head of the comma-head cloud as it moved around the western side of the low pressure system, producing a localised area of convective instability. Itis shown that this destabilisation was resolved in the assimilated grid-point analyses of the Australian Bureau of Meteorology, and so this paper proposes a mechanism to which forecasters should be alerted, and which can be resolved in currently available operational products.
Mills et al. (Fri,) studied this question.