Severe Thunderstorms and Tornadoes in Victoria
Tornadoes occur more commonly in Victoria than most people would expect. The Bureau of Meteorology's severe thunderstorm database for Victoria has 160 tornado reports dating from 1918. It is most likely that many tornadoes have gone unsighted or unreported. Tornadoes range in size from a few tens of metres across, up to around one kilometre in diameter. Because of this, damage is normally restricted to a small area, but is very intense.
Tornado at Leopold (near Geelong) 3rd October 2001. (Photo courtesy of Clyve Herbert)
Tornadoes are thought to be formed by the interaction between regions of strong updrafts and downdrafts of air within severe thunderstorm clouds. As a thunderstorm becomes stronger and develops an organised inflow, its main updraught may begin to rotate slightly. This is sometimes seen as broad rotation of the cloud base beneath the main updraught or in the circular nature of the wall cloud
Weaker tornadoes are formed primarily by "tightening-up" of a rotating updraught. They occur as the storm intensifies to a maximum and are found right under the updraught core, sometimes without a significant wall cloud. Weaker tornadoes are most likely during mid-summer storms but may also accompany squall lines and wintertime thunderstorms, mainly in southern parts of Australia. They are still significant events as they may produce narrow strips of severe wind damage.This example from Dimboola, Victoria occurred in December 1992. (Photograph courtesy K. Reynolds)
Stronger tornadoes typically occur with late spring/early summer severe storms and have a more complex cause. It is speculated that at a certain stage in the storms life-cycle, a particularly intense updraught pulse partially blocks the prevailing wind aloft and deflects air down toward the surface. The downward surge interacts with the updraught to produce a tight rotational motion (in much the same way as rolling a pencil between your hands). This "spinning motion" is then tilted into an upright position and enhanced as it moves torwards the ground as a tornado.
This photograph of a strong tornado, accompanied by a well developed wall cloud, killed two people near Sandon in Victoria in 1976. The point of contact with the ground is marked by a cloud of dust and debris. The large "prong" attached to the left of the wall cloud later formed a second funnel (Photograph courtesy by I. Kuiper)
Tornadoes are ranked using the Fujita F-scalewhich estimates wind speed based on the extent and severity of damage. Below is the Fujita scale with respective approximate wind speeds.
Tornadoes seldom exceed F2 in Australia, but these are still quite damaging and dangerous. One of the highest wind speeds ever actually recorded in a tornado (using Doppler radar) was over 450 km/h in Oklahoma, USA in 1999. Stronger winds are evident from the examination of the impact of many tornadoes, particularly in the USA.
In Victoria, tornadoes can occur any time of the year, although in Victoria tornadoes are more frequent in February, November and December. In winter in Victoria, tornadoes tend to be of the F0 and F1 size and are still quite destructive. Two examples of such destructive wintertime tornadoes were the Bendigo tornado in May 2003 and most recently the Noble park tornado in June 2004.
Recent examples of tornadoes in Victoria include: