A comparison between the broadscale environment associated with cool season tornadic thunderstorm occurrences over southern and western Australia with the environment associated with Californian tornadoes reveals many similarities. Although some of the Australian cases occur along a leading cold frontal band, the majority occur behind a front in the cold sector of an intense mid-latitude cyclone, comparable to the "prototypical" pattern associated with northern and central California tornadoes.
The kinematic environments in both Australia and California are characterised by considerable wind shear in low-levels, concentrated in the layer beneath 850 hPa. Typical wind profiles turn about 45 degrees (anticyclonically) from the surface to 850 hPa as wind speeds increase from 5-10 m/s at the surface up to 15-25 m/s at 850 hPa. There is also strong speed shear in the layer from 400 hPa to 300 hPa. Strong topographic channelling of the low level flow contributes to the low-level shear and convergence in the Californian cases. Although topographical influences on the low-level flow are less obvious in the Australian cases, the preferred regions for occurrence near the north/south oriented coastlines of South Australia and Western Australia may also be regions of enhanced shear and convergence due to frictional effects. The thermodynamic characteristics of the environments of both regions are similar in that the tornadoes occur in environments of relatively low buoyant convective available potential energy (CAPE), with values typically less than 500 J/kg.
These results add to the growing body of observational and modeling evidence that supports the occurrence of tornadoes in situations with relatively modest CAPE values. Forecasters need to be aware that strong tornadoes can occur in weakly buoyant situations if there is sufficiently strong low-level wind shear. The typical warm season high CAPE/high shear conceptual model for tornadogenesis can be quite inappropriate for cool-season events. Whereas shear values from the surface to 4 kilometres or values of Storm Relative Helicity (SRH) from the surface to 3 kilometres are used for warm-season forecasting, these studies show that shear and SRH calculations from the surface to 1-2 kilometres may be more appropriate for cool-season events. Standard stability indices that emphasize 500 hPa data as input are likely to be misleading if applied to these situations.