Johannesburg - The South African Weather Service (SAWS) said late on Tuesday, the development of the first thunderstorms of the season was eagerly awaited by residents of the Free State, Gauteng, and North West.
SAWS says that these storms, however, were characterised by widespread reports of strong, damaging surface winds across these provinces as well as very modest amounts of rainfall.
“Radar and satellite remote sensing data provided little or no indication of the possible severity of storms; however, a feature that proved to be significant was that, given the very dry conditions at the surface, the convective cloud base of the thunderstorms was at an abnormally high altitude above the ground. While this type of storm development can occur at any time of the year, early summer storms are notorious in terms of frequently being associated with strong, damaging winds caused by ‘dry microbursts’. Moreover, such storms are not typically associated with delivering much rainfall at all, which explains the popular term ‘dry thunderstorms’,” said SAWS.
SAWS said a dry microburst is a localised column of sinking air (also known as a downdraft) within a thunderstorm and is usually less than or equal to 4 kilometres in diameter.
“Microbursts can cause extensive damage upon reaching the Earth’s surface and, in some instances, can be life-threatening. In the case of a dry microburst, the precipitation evaporates aloft within the downdraft, causing the downdraft air to become colder and denser, thus accelerating the cold air towards the ground under the influence of gravity. This phenomenon is typically associated with high-cloud-base thunderstorms, as was the case yesterday. This weather phenomenon has the capability of uprooting trees as well as lifting off entire building roofs, as indicated in the photos of storm damage below,” read a SAWS statement.
According to SAWS, earlier on Tuesday afternoon, it was observed that wind speed exceeded 50 knots (114 km/h) over Harrismith, Free State, between 4.15pm and 4.35pm SAST, which is consistent with the typical strength of a dry microburst.
“Later in the evening, towards 9pm, strong surface wind gusts of 40 to 46 knots, in association with thunderstorm activity, were reported over the central and southern parts of the North West. Much later, in the period between 9pm and 10pm, reports of extremely strong winds as well as widespread blowing dust were reported at many localities across Gauteng, including (but not limited to) Soshanguve, Mamelodi, the greater Pretoria area, and Centurion. Wind measurements suggest gusts slightly in excess of 100km/h at these localities. Interestingly, although thunder was heard, generally very little precipitation arrived at ground level,” said SAWS.
SAWS says that the areas favourable for thunderstorm development have shifted further to the east since Tuesday.
“The SAWS therefore expects isolated thunderstorms to redevelop over parts of eastern and north-eastern provinces today, however no further storms are anticipated over North West, Free State or Gauteng. Some of these storms, especially over the eastern and north-eastern parts of Limpopo, are expected to be at risk of developing dry microbursts later today, Wednesday, September 20, causing strong and damaging surface winds,” added SAWS.