Severe Weather Section, Bureau of Meteorology, Perth, Western Australia


Bureau of Meteorology Research Centre, Melbourne, Victoria


Severe Weather Section, Bureau of Meteorology, Adelaide, South Australia


LEAD AUTHOR: Barry Hanstrum

POSTAL ADDRESS: PO Box 1370, West Perth 6872, Western Australia


Phone: 61 8 92632222

Fax: 61 8 92632297


Although the frequency of tornado reports in Australia has a maximum in late spring/early summer, there is a significant secondary maximum in mid-winter. In the 10-year period 1987-1996 40 per cent of all tornadoes occurred during the months from May to September. These so-called cool-season events can occur at any time of the day but are most common during the late afternoon. Cool-season events were confined to the southern and western parts of the Australian continent and occurred in two preferred areas, the southwest of Western Australia (WA) and the near coastal southeast parts of South Australia (SA). With the increasing population density in these areas there is a growing risk of loss of life as a result of a strong tornado impact. In order that timely warnings be issued it is important that forecasters be able to recognise the meteorological characteristics of the environment in which they occur.

A case study of an outbreak of 3 strong (F2) tornadoes over southwest WA on 7 June 1995 is presented. The tornado environment was characterised by low-level convergence associated with a secondary frontal line, strong low-level wind shear (surface - 850 hPa shear greater than 20 m/s) but with relatively low Convective Available Potential Energy (CAPE) of 300 J/kg.

An analysis of the observational data for all WA and SA events for the study period show that these characteristics were common to the cool season events, with much lower buoyancy and much higher values of lower-tropospheric shear than is observed in the warm-season tornadic thunderstorm environment. All of the "strong" tornadoes occurred with surface to 850 hPa shear values greater than 15 m/s. Typical values of CAPE were less than 500 J/kg.

Fields of the objectively analysed NWP gridded data from the Australian Bureau of Meteorology's Limited Area Prediction System (LAPS) for cases that occurred over southwest WA and SA were composited separately during the period from 1994 to 1996. The composite fields for both regions bore striking similarities. Significant features of the composites near to where the tornadoes occurred were: an intense mid-latitude low polewards with strong surface pressure gradient over the region, low-level convergence, relatively weak instability, and close proximity to both the axis of maximum surface to 850 hPa wind shear and the nose of the jet axis at 950 hPa.

A forecasting aid aimed at focusing on the features of the cool-season tornadic environment is being designed. The package will use analysis fields from the meso-LAPS model (resolution 25 kilometres), as well as forecast model output at 3-hourly intervals. Fields of the low level wind shear, low level convergence and low to mid-level stability will be made available through a McIDAS workstation. As well, regions where there is a coincidence of significant values of each of the fields will be highlighted in order that forecasters can better anticipate the threat of tornadogenesis and focus their nowcasting attention onto these areas. The technique will be trialed operationally in the WA Regional Forecasting Centre during the 1998 winter.