Dust in the Free State (South Africa)

By Lee-ann Simpson and Tonie Rossouw (South African Weather Service)

In the animation, from 08:00 UTC until 12:00 UTC, the dust can be seen suspended over central parts of South Africa.

The central and eastern parts of South Africa (see map in Figure 2, below right) are generally known as summer rainfall regions, and receive most of their rainfall between October and March, in the form of convective precipitation. The month of September is extremely dry over this region, as no significant rainfall would have fallen for many months.

The central parts of South Africa, and the Free State Province in particular, are very successful and popular farming areas — the Province is known for its rolling fields and cultivated lands. Due to the lack of rain over the South African winter months, this farming region will be marked by the abundance of loose soil; open farming lands without any crops, and generally a lot of dust.

Dust storms that are large enough to be detected on satellite imagery are not a common occurrence in South Africa, this case is a very good example that such phenomena are indeed happening in South Africa, under certain conditions. The forecasting of dust storms over South Africa is not a common practice and is very difficult to do.

Dominating weather systems

The synoptic pattern on 19 September included a cold front, which was situated on the south-west coast of South Africa, with and upper air trough over the western parts of South Africa. Ahead of the approaching front, the surface winds were north-westerly, and increasing in speed as the surface trough and associated front moved eastwards. The approaching troughs, in both the upper air and at the surface; mid-level moisture, and instability were also resulting in the formation of convection over the central parts of South Africa.

Forecast

METARS issued by the weather office in Bloemfontein (located in the area of the dust storms), reported winds gusting up to 65km/h (40mph) during the morning of 19th. The forecaster included a trend with the possibility of dust storms as part of the METAR.

A case study of the event, done by Mr Tonie Rossouw in Bloemfontein, showed the NWP output can accurately simulate the environmental conditions that could occur, but only forecast experience in this case, could detect the possibility of dust storms developing.

Satellite Imagery

The Dust RGB was used to locate the area where dust storms had occurred. The following images and loop show a clear area of dust to the south-west of the main cloud band.

In the Dust RGB (Figure 3) the suspended dust can be detected at 09:00 UTC, behind the cloud band. The red circle indicates the area of interest.

By 11:00 UTC (Figure 4) the winds were still strong north-westerly at the surface, and the suspended dust is clearly visible in two regions over South Africa. The most prominent dust storm being over the Free State Province, with a weaker signal being detected on the border between South Africa and Botswana.

At 11:30 UTC (Figure 5) both dust storms are still present. These dust storms persisted into the early afternoon, and the convective development which moved over the region, resulted in more gustiness at the surface, and the persistence of the dust storms. Due to the cloud cover caused by the convection, the dust was not detectable on satellite imagery during that period.

Additional content

Dust RGB showing dust over South Africa on 29 August 2013
EUMeTrain ePort page for 19 September 2013