Weather 22 July 2016
The general synoptic picture for the week progressed from a typical mid-winter pattern to a more intense dynamic vortex-driven system by the end of the week.
The southern Indian high pressure cell was more or less in its customary position south east of Madagascar with an extension that reached towards the sub-continent linking up with the continental high over South Africa, Zimbabwe and Botswana. The South Atlantic High pressure cell was close to the coastline, its core slightly further north than usual, and it had a significant impact on weather in the Western Cape, and in the southern Namib including Oranjemund, Lüderitz and the western half of the Karas region.
Between the two oceanic high pressure cells, a strong vortex developed only a few hundred kilometres south-east of Cape Agulhus. This intense low pressure system collapsed the continental high. By the end of the week, only a small remnant remained in the Mozambican Channel. It reduced the barometric pressure over the entire southern Africa to around 1020mB with a mild airflow from north to south. Although the nights remained cold, the daytime temperatures rose to the middle twenties with the effect that afternoons were pleasant.
The South Atlantic high however, gained in strength and started slipping towards the south as it approached the continent’s western coastline. The outer fringe of this approaching high pushed a frontal system causing extensive rains over the Western Cape, the southern Cape, and later in the week, even over the Kwazulu Natal coastline, and over the South African interior. It also extended northward along the western coastline reaching Oranjemund during Wednesday with a spill-over onto the coastal plain up to the escarpment.
The South Atlantic high is the animal to watch. By the end of the week, its has developed a strong core which reaches from the Western Cape out across the South Atlantic for at least another 4000 km. It drives a strong cold front which reaches the Cape by Friday, extending towards the north with a dramatic impact on the Karas region. For the first time this winter, the continental high is absent meaning there is no obstruction between the cold front and the continent.
By Saturday the 540 dam line where it is zero degrees on the surface, moves slowly across the South African interior. From the source of this intense cold, the wind blows towards the north dropping nighttime temperatures in Karas to below zero. This continues through Saturday night with the system now covering the Keetmanshoop and Karasburg district as well as the eastern half of Omaheke along the Botswana border.
Saturday night will see many place from Karasburg in the south to Otjinene in the Otjozondjupa region go below zero. Frost is inevitable.
The cold conditions continue through Sunday with the effect being felt as far north as Grootfontein and as far west as the Khomas Hochland. Sunday night also has the potential for frost over large areas in the eastern half of the country.
By Monday next week, the core of the intense high has migrated to eastern Africa but a prominent ridge extends back over Botswana. It means continuing cold for Namibia, especially in the south and the east. It is only by Tuesday next week that a low pressure system enters from Angola, bringing milder days to the interior and a relief from the cold nights.
It must be noted that conditions west, north and east of Etosha are expected to remain mild with warm days.