oThongathi tornado rated EF3 – are tornadoes common in SA?

The oThongathi tornado in KwaZulu-Natal has been rated as an EF3 on the Enhanced Fujita scale, with estimated wind speeds of 225 to 265 km/h that occurred within the tornado’s path.

Following a South African Weather Service (Saws) investigation into the recent destructive weather, the service said that contrary to popular belief, tornadoes are more common in South Africa than what is generally perceived although the probability of them occurring is still very low.

On June 3, a line of thunderstorms developed along the border of the Free State and KwaZulu-Natal ahead of a well-developed cut-off low-pressure system over the western interior of South Africa.

“Cold and dry air swept in from the western parts of the country to the central interior, while the eastern parts, particularly KwaZulu-Natal, experienced warm and moist atmospheric conditions aiding in the development of severe thunderstorms where these cold and warm air masses met,” the Saws said.

In addition, strong low-level wind shear – wind shear refers to the change in wind speed and direction with height – was present over the coast and interior of KZN ahead of the line of severe thunderstorms. This process of shifting/changing winds in the lower levels of the atmosphere, as the line of storms moved across parts of the province, was one of the primary contributing factors in the development of supercell thunderstorms and tornadoes. Most tornadoes develop from strong and violent thunderstorms called supercell thunderstorms.

“Most tornadoes in the previous century (1905-1999) occurred over the eastern provinces, especially around Gauteng and KwaZulu-Natal,” the Saws said.

One of the tools to assess the intensity of tornadoes is the Enhanced Fujita (EF) scale which assigns a rating to a tornado based on estimated wind speeds inferred from observed damage by using various damage indicators. Most tornadoes in South Africa are weak, with a low EF rating of 0 to 2. The stronger and more destructive tornadoes (EF3 and EF4) occur every decade or so.

On June 4 this year, the Saws issued a statement that at least two tornadoes were confirmed in KwaZulu-Natal the day before. One of the tornadoes, called a ‘wedge’ tornado, because it is wider and larger than it is tall, resulted in severe damage in the oThongathi area.

An analysis of the Durban radar data revealed several features indicative of a 'supercell' thunderstorm, the Saws said.

“At least two thunderstorms could be classified as supercell thunderstorms, while another, despite exhibiting supercell characteristics, did not persist long enough to meet the required time criterion.

“The thunderstorm that passed over oThongathi and produced a tornado was confirmed to be a supercell, with a hook echo, although it did lack the typical rotational signature on the Doppler velocity field due to various limitations of the radar data.

“Several damage indicators were considered and utilised to assess the tornado impact around oThongathi and further down towards the coastline.

“Considering these key indicators, the Saws concluded that the oThongathi tornado can be rated as an EF3 on the Enhanced Fujita scale,” the Saws said.

The South African Human Rights Commission (SAHRC) earlier this week said the occurrence of the tornado can be classified as a disaster in terms of the Disaster Management Act, 2002.

“It is reported that to date, there have been approximately 10 fatalities and countless hospitalisations and injuries. Furthermore, the commission’s preliminary inspections reveal that approximately 1 500 persons have been displaced. Twenty schools in the area have reportedly been damaged.

“There is widescale destruction to public infrastructure, including power and cellphone towers, and bridges. The damage and demolition to the environment is extensive,” the SAHRC said.