Severe thunderstorms with golf ball size hail and flash floods hit Gauteng province of South Africa in late November.
15 November 2020
28 November 2013
by Edwin Thema (South African Weather Service)
Massive thunderstorms were triggered over the central parts of South Africa by the low level convergence of moist tropical air over the east of South Africa and the dry air from the western interior. The brightness temperature difference BTD of IR12.0-IR10.8 image (Figure 1) is used to illustrate the moist air to the east and dry air to the west, in a cloud free area.
There was other dynamics on the day that favoured the development of severe thunderstorms. In Figure 2, the midnight sounding for Irene station situated in the northern parts of Gauteng shows moist conditions in the lower atmosphere, becoming drier in the mid levels, known as loaded gun sounding.
The sounding also shows a great deal of wind shear, both directional, with the wind direction swinging from north-easterly in lower atmosphere to westerly in the mid-levels, and speed shearing.
The presence of wind shear is also shown in Meteosat-10, Natural Colour RGB image overlaid with 700 hPa winds and 500 hPa pressure lines. It clearly shows the Gauteng area with wind at 700 hPa crossing the 500 hPa pressure lines at a greater angle, close to 90 degrees.
These severe thunderstorms were driven by winds at 500 hPa level than the usual 700 hPa winds, and they moved in a north-easterly direction along the periphery of the upper high pressure system affecting areas in south-western Gauteng (Randfontein, Roodepoort and Krugersdorp) during lunchtime and northern Gauteng (Mamelodi, Soshanguve and Hammanskraal) later in the afternoon and early evening.
Tshwane municipality in Gauteng was declared a disaster zone after these severe storms ripped through parts of the area, with golf ball sized hail and flash floods that resulted in damage to a number of households and infrastructure. The storms tore apart roofs and blew out windows.
The enhanced IR10.8 and HRV sandwich product (Figure 4 and animation) shows there were overshooting tops over these storms, with the cloud top temperature going to a low -79 °C. The cold U-shapes over the cloud show the extent of the severity of these storms.
The animation shows the progression of these storms, an outflow boundary that flows to the opposite direction of these storms, and how the cold U-shape evolves to becoming a cold ring at times.
The radar reflectivity over Gauteng showed high values going up to 70 dbz for relatively large areas on this day (Figure 5) — according to SAWS interpretation, anything above 60 indicates very heavy showers and large hail.
The Severe Convection RGB animation, Meteosat-10, 28 November 09:00–15:15 UTC shows bright yellow colours over these storms and it resembles the amount of small ice particles formed from strong updrafts in the clouds.
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