oThongathi tornado has been rated on the Enhanced Fujita scale
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Durban — The South African Weather Service (SAWS) has concluded that last week’s tornado in oThongathi can be rated as an EF3 on the Enhanced Fujita scale.
This was contained in an extensive statement by the Saws covering details on the tornado, what a tornado is and the Enhanced Fujita scale.
SAWS explained that 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 situated 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 KZN, experienced warm and moist atmospheric conditions aiding in the development of severe thunderstorms where these cold and warm air masses met.
Additionally, strong low-level wind shear (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 severe storms moved across parts of the province, was one of the primary contributing factors in the development of the supercell thunderstorms and tornadoes,” SAWS explained.
The SAWS explained that a tornado is a violently rotating column of air that develops from the cloud base of a severe thunderstorm and extends to the ground.
It said most tornadoes develop from strong and violent thunderstorms called supercell thunderstorms. Tornadoes are the most formidable windstorms on earth and unleash devastating destruction along their trajectories. Tornadoes typically form in environments where certain atmospheric conditions come together. Some factors include wind shear, which involves significant changes in wind speed and direction with altitude, and atmospheric instability, where warm, moist air near the ground is overlaid by cooler, drier air aloft.
Additionally, cold fronts or cut-off lows help the warm, moist air to rise. Sufficient moisture in the lower atmosphere is also necessary to fuel thunderstorms that can produce tornadoes. 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. Most tornadoes in the previous century (1905-1999) occurred over the eastern provinces, especially around Gauteng and KZN.
The SAWS said one of the tools to assess the intensity of tornadoes is the Enhanced Fujita (EF) scale, introduced in 2007 as an updated version of the Fujita-Pearson (F) scale.
“It assigns a rating to a tornado based on estimated wind speeds inferred from observed damage by utilising various damage indicators. This EF rating system enables the classification of tornadic events. 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,” SAWS explained.
It said one of the highest-rated tornadoes in South African recorded history was the Welkom tornado of March 20, 1990, rated as an F4 on the Fujita scale, resulting in damage to 4 000 homes, and was one of the costliest in history. In 1999, an F4 tornado struck Mount Ayliff. This tornado threw cars and trucks into the air and left 95% of the population in the area homeless.
Last week, the SAWS issued a media statement saying at least two tornadoes were confirmed in KZN on June 3. One of the tornadoes called a “wedge” tornado because it was 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. Meteorologists rely on specific radar features to identify severe thunderstorms like supercells.
“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. A supercell thunderstorm is a severe type of thunderstorm that contains a rotating updraft (rising air) and can be identified using a weather radar with characteristic features such as the hook echo on the reflectivity field and a rotational signature (mesocyclone) on the Doppler velocity field. Such thunderstorms are also known to produce tornadoes at times (but not all). 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,” explained SAWS.
It further explained that a tornado signature on the Doppler velocity field may at times be detected, although limitations exist including the proximity of the storm to the radar. No significant Doppler velocity signature was noted in this case despite its close proximity to the radar, however on the reflectivity field a possible tornadic signature known as a debris ball was noted. Such signatures occur because of the debris (often including highly reflective material) that is picked up aloft by the tornado and thus causes a “ball-shaped” echo. Debris balls are often associated with stronger tornadoes, EF3 or greater on the Enhanced Fujita Scale.
“The supercell features developed from around 15:30 local time and dissipated at around 16:41, soon after moving off the coast. The path of the supercell thunderstorm is indicated below, while the second image indicates the path of the possible debris ball (most likely the path of the tornado),” SAWS continued.
“Significant to severe damage was observed over the eastern parts of the oThongathi area and towards the coast.
“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, with estimated wind speeds of 225 to 265 km/h that occurred within the tornado’s path,” SAWS concluded.
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