ENVIRONMENT NEWS - A tornado was observed on a farm in Heilbron, Free State, between 15:00 and 16:00 on Monday, 24 November.
The South African Weather Service (SAWS) confirms that thunderstorms in the area matured in severity from the afternoon, as a severe thunderstorm warning had been issued in advance.
There were gusts around the region with identified shearing, convergence and moisture forming a convective cloud that resulted in a supercell at a later stage, creating favourable conditions for a tornado to form.
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 like the Highveld, Free State, Mpumalanga, and KwaZulu-Natal.
Below is an analysis, integrating the observations with existing literature, plus some conditions and forward-looking thoughts.
Research and climatology
Climatological hotspots align with where the tornado occurred. Historic and contemporary analyses show that many tornadoes in South Africa occur in Gauteng, the Free State, KwaZulu-Natal, and parts of Mpumalanga.
According to our previous research, there is even a kind of “tornado belt” in the eastern provinces, including high-density activity in the Highveld regions.
Seasonality
Spring and summer signal is well supported. Historical event analyses (Goliger et al., 1997) 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.
Dynamic drivers
Moisture, Shear and Convergence. Research on a notable tornadic supercell on 11 December 2017 (Highveld) found that conditions were driven by strong low-level moisture and significant boundary-layer vertical wind shear, exactly the ingredients observed in this case.
Supercell characteristics in the Highveld
A relevant recent study (Liesker, Dyson & Becker, 2024) looked at warm-season left-moving supercells over the Highveld. They found that these 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 1h 12 min, with a horizontal speed of 41 km/h, and travelling about 49 km, 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.
Impacts
There were no further observed impacts or reports from the tornado and it looks like it occurred in an open field and resulted in minor or no damage in the area.
Conclusion
Based on these findings, the SAWS has classified the event as an EF0 tornado. An EF0 is the weakest category on the Enhanced Fujita Scale, with estimated wind speeds of 105–137 km/h. Such tornadoes typically result in minor damage, such as broken branches, shallow-rooted trees being pushed over, and light damage to roofs.
While EF0 tornadoes are relatively weak, they remain dangerous weather phenomena, and the public is urged to always exercise caution during severe thunderstorms.
Compiled by: Lehlohonolo Thobela, Forecaster
Approved by: Ezekiel Sebego, Manager
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