A tornado sighting on farmland in Heilbron, Free State, on Monday afternoon has been verified.
The South African Weather Service (Saws) confirmed: “There were thunderstorms that matured to be more severe in the area from the afternoon.
“There were gusts around the region with identified shearing, convergence and moisture to form a convective cloud, resulting in a super cell at a later stage.”
The conditions were favourable for a tornado to form, which did between 3pm and 4pm.
Saws forecaster Lehlohonolo Thobela said the agency has classified the event as an EF0 tornado.
“An EF0 is the weakest category on the enhanced Fujita scale, with estimated wind speeds of 105km/h to 137km/h,” he said.
As it occurred in an open field, no damage was recorded.
The science of tornadoes in SA
Explaining the phenomenon in SA, Thobela said research has shown that moist, converging air, low-level shear and unstable environments colliding with cooler triggers are typical conditions conducive for tornadic and severe-convective activity in parts of South Africa such as the Highveld, the Free State, Mpumalanga and KwaZulu-Natal.
Climatological hotspots align with where the tornado occurred. Historic and contemporary analyses show many tornadoes occur in Gauteng, the Free State, KwaZulu-Natal and parts of Mpumalanga. According to research, there is a kind of “tornado belt” in the eastern provinces, including high-density activity in the Highveld regions.
Historical event analyses note a seasonal peak for tornadoes in the midsummer months (November to March), as investigated by the Water Research Commission in previous years. Importantly, tornadoes are not limited strictly to the core summer as there are documented events in spring (September and October) and late summer. This supports the sense that warm moisture environments with triggers from cool environments are critical indicators of tornadoes occurring during this time of the year.
Research on a notable tornadic supercell on December 11 2017 (Highveld) found conditions were driven by strong low-level moisture and significant boundary-layer vertical wind shear, exactly the ingredients observed in this case.
A relevant recent study looked at warm season left-moving supercells over the Highveld. They found the supercells are most common in October and November, which is early in the convective season (spring to early summer). Their preferred tracks average lifetime is about one hour and 12 minutes, with a horizontal speed of 41km/h and traveling about 49km, generally moving from the southwest, shifting more southerly as the season progresses.
There also seems to be some influence of topography, especially in eastern Mpumalanga and the Highveld regions, on where supercells form/propagate. This is consistent with the idea that the interaction of terrain, moisture and shear helps define preferred corridors for severe cell development which could explain spatial clustering of the severe thunderstorm reports.
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