Extreme winter weather conditions expected by the South African Weather Service

The South African Weather Service is continuing to monitor the weather system that is expected to cause a range of extreme weather conditions over the majority of South Africa as from today, Monday, 6 August 2012 onwards.

The areas most likely to be affected are as follows: Western Cape, Eastern Cape, Northern Cape, KwaZulu-Natal, Free State, Mpumalanga and Gauteng as well as the southern parts of North West province. In other words, the only province not to be directly affected by these extreme winter conditions will be Limpopo province.

The cause of the above-mentioned extreme weather conditions will be the combination of a well-defined cut-off low pressure system (located in the upper regions of the atmosphere), interacting with a cold frontal system. An additional feature associated with this weather pattern, is the presence of a strong surface high (also known as an anticyclone) that will be ridging very strongly overland, immediately following the passage of the cold front. This process of surface ridging is expected to advect (i.e. move horizontally) very cold air from our sub-Antarctic southern ocean areas. Daytime temperatures for South Africa are thus likely to drop significantly over the majority of the interior between today and tomorrow, with many areas likely to experience daytime maximums of only 10˚C (or even less!) by tomorrow.

The public is thus warned and advised that, particularly in terms of low temperatures, this system is expected to be quite severe. Furthermore, the public is also urged to be aware that (apart from the bitterly cold conditions) a range of severe/extreme weather phenomena can be expected to develop in association with this rapidly-developing system. These extreme phenomena predicted by SA Weather Service forecasters include heavy rainfall, snow (heavy in places over the north-eastern high ground of the Eastern Cape) as well as gale-force winds, combined with very rough seas in the open ocean off the coast of the Western Cape and Eastern Cape.

For parts of the open ocean, seawards of our southern Cape coast, it is also likely that sea conditions could deteriorate beyond the ‘very rough’ category and into the ‘high’ seas category. In the case of the former, very rough seas are defined by total sea of 4 to 6 metres, whilst in the latter case, high sea conditions are defined by total seas of 6 to 9 metres. Ships (regardless of size) would be best advised (if possible) to modify their sailing schedule and to make every effort to avoid traversing the region between Cape Agulhas and Plettenberg Bay for the next 24 hours.

Snowfalls are expected to continue over the Western and Eastern Cape high ground, Lesotho mountains and southern high ground of the Northern Cape. By tomorrow, Tuesday 7 August 2012, further snowfalls are expected to spread into (all of) the Free State as well as the western and southern high ground of KwaZulu-Natal.

Very cold conditions are expected to persist today (Monday) over the southern high ground of the Northern Cape, high ground of the Western and Eastern Cape and southern parts of Free State. By tomorrow, the entire interior of South Africa (with the notable exception of Limpopo province) is expected to be very cold.

Heavy showers and thundershowers are expected at times along the southern coast and interior today, associated with an upper cut-off low that might result in a possible heavy rain along the coastal areas later today. By tomorrow, Tuesday, 7 August 2012, the cut-off low will migrate towards the south-eastern interior of the country, with associated deep convection (showers and thundershowers) over the eastern parts of the Eastern Cape. Tomorrow, the sustained ridging action of the surface high pressure system will continue feeding low level moisture into the south coast region, possibly resulting in further heavy rain in the morning over the Cape south coast and adjacent interior.

Figure 1: METEOSAT 9 Satellite image (Meteosat, “Day Natural” RGB at 10h15SAST). © EUMETSAT 2012.

By Wednesday, the SA Weather Service expects that the upper-air cut–off low will begin moving off the country towards the east, thus weakening the overall impact of extreme weather on South Africa by this time, when isolated to scattered showers are likely to be limited only to the south-eastern and eastern extremities of the country. On Wednesday, daytime temperatures are still likely to be cold over most parts of the country. The slow recovery of diurnal temperatures, day by day, will continue into the latter part of this week and the public can ultimately look forward to more mild, sunnier conditions being more prevalent across a large part of the country towards the coming weekend (11th and 12th August).

- SAWS