Severe weather over South Africa and Botswana

By Estelle de Coning (South African Weather Service) and Jochen Kerkmann (EUMETSAT)

During the weekend of 4–6 November 2005 heavy rainfall across South Africa brought welcome relief after very hot (heatwave) conditions over most of the country. The persistent heavy falls lead to local flooding in many areas over the southeastern parts of the country where rainfall figures in excess of 150mm in 24 hours were reported at East London.

Over the northern parts of the country large amounts of rain fell in short periods of time leading to local flooding and considerable damage was caused by strong wind. Electricity cables were damaged which led to power outages, roofs were blown off and informal settlements were destroyed. In some villages east of Polokwane (23.8S and 29.4E) a 10-year-old boy drowned when his shack washed away, three women were injured when their houses collapsed and 60 newly built houses were destroyed. West of Polokwane power lines were damaged: five of the pylons were damaged and one blown over, each of these weighing about 20 tons.

On 4 November rainfall amounts over the northern most province of South Africa (Limpopo, see Map of South Africa) were less than 10mm, but on 5 November rainfall figures were between 20 and 50mm. The rain of these two days left the ground fairly moist in this area and, thus, an area of cloudless, rain-cooled surface existed over the province in the early hours of 6 November.

Convection started WNW of the Limpopo province on the surface dry line and migrated towards the E-N-E (along with the upper air flow) and first entered the area of interest during the early afternoon. At 11:00 UTC the RGB composite image shows the first explosive development with strong updrafts on the western border of the moist area (see Interpretation below upper left image).

Thirty minutes later, a second severe storm develops further to the northeast (see upper right image). The bright yellow colour indicates cold, thick ice clouds with small ice particles (strong updrafts). At 12:30 UTC, both storms show strong development (strong cooling rates, strong increase of storm area, small ice particles) with signals of a U-shaped storm (see IR10.8 image, 12:30 UTC). Using IR10.8 colour enhancement the cloud top temperature of these very severe storms is indicated at -85 °C.

In the following hours, the southern storm moves into the Limpopo province, crossing exactly the area where the damage occurred to the electricity towers (between Elisras and Pietersburg, see HRV image). The northern storm, which appears equally strong in the satellite images, crosses the most eastern parts of Botswana. On the satellite images one can derive many details of this huge thunderstorm.

In particular, one can clearly see overshooting tops and a high-level plume of small ice particles (high reflectivities in the NIR1.6 and IR3.9 channels) drifting away from the overshooting tops in a southeasterly direction. The northern storm also has indications of rotation of the clouds (see animation of RGB composite VIS0.8, IR3.9r, IR10.8). However, no surface reports are available from Botswana to confirm the severe character of this storm.

It is interesting to note that after the passing of the storms new moisture streaks left by the heavy convection are clearly visible in the infrared images and, thus, also in the RGB composite VIS0.8, IR3.9r, IR10.8 (see Intrepretation below the image).