Weather 19 February 2016

What Happened
Over the past week, the Inter-Tropical Convergence Zone which cuts across southern Africa during summer, has see-sawed. Whereas its southern limit sat for about two months in a straight line across the subcontinent, it has moved south in the east, and slightly north in the west. It is almost like there is a kink in the ITCZ, favouring eastern Africa, while pulling away (by about 400 km) from the Namibian border. his move is corroborated by the early week synoptics. Monday saw a very typical arrangement with the South Atlantic high pressure cell west of the continent and the southern Indian high south-east of Madagascar. This is reminiscent of a typical late winter, early summer pattern except for one major deviation. The two cores of these dominant highs have both shifted a considerable distance southwards (about 1000 km), and are now straddling the 40oS latitude. This phenomenon was also observed in 2010. The southward displacement of the high pressure cores was expected. The water in the Indian Ocean is still some 1oC warmer than normal while the South Atlantic has many patches where the water is about 0.5oC warmer. Thus the colder water is found much further south and since cold seas and high pressure cells are partners in the same marriage, the logical consequence is that the high pressure cores follow the colder water and shifts towards the south. For Namibia, this opens up all sorts of improved expectations. If 2010 provides a cue, the relative position of the South Atlantic’s core determines the space available (on the surface) for the anti-cyclonic circulation from Angola to penetrate Namibian airspace and essentially to cover the whole country. But the week’s synoptic progression was very agile and by Thursday, a third high pressure cell has formed about 800 km south of Cape Agulhas. This created a rather unique set of conditions. Between the South Atlantic high, then far out at sea, and the high below the southern Cape, a strong frontal system developed. With the southern Indian high not too far from the continent, it blocked the usual south to north flow up the Mozambican Channel, which is typical for the leading rim of the South Atlantic high as it passes south of Agulhas. Over the sub-continent, this resulted in high pressure control on the surface over Namibia in the west and over South Africa and Mozambique in the east. Between these two systems, a weak mid-level trough developed, first over the Zambezi, then growing towards the west, bringing some inspiring rains to the Kavango and Owamboland. This trough followed the usual convergence line, and also produced very productive falls in Namibia’s east along the Botswana border, all the way south to the Orange River. The only limitation was that the showers were scattered, still not producing continuous rains from north to south. The effect is falls exceeding 25mm in one locality, while less than 20 km away, only drops are recorded, i.e. less than half a millimetre. It was also noticeable that the very hot conditions of the previous week have subsided somewhat. This is a direct result of the high pressure control on the surface.
What’s Coming
Low pressure conditions induced by heat developing again over Botswana. This leads to a thick atmosphere over the entire Kalahari basin, but spreading into Namibia’s eastern (Kalahari) and north-eastern (Bushmanland) sections. By Saturday, surface conditions across the entire sub-continent are under low pressure control from the Atlantic to the Indian. The approaching South Atlantic high may move slightly towards the north but this will be accompanied by a considerable weakening of its core. This allows ample space for the tropical low pressure system to move towards the south. The conveyor mechanism of moisture from the Indian Ocean is also reinforced by the low pressure conditions and substantial moisture enters the mid-level and upper levels of the atmosphere during the weekend. This builds up over Zambia, the DRC and Angola. By Tuesday evening next week, a mid-level trough forms again from Angola, across Namibia, into southern Botswana, and further south into South Africa. This trough holds much potential and against the background of a weaker, southward-replaced high pressure cell, there is much room for an intrusion from the north. This starts during Tuesday night, and should grow in strength during Wednesday. This intrusion of moisture from the north should penetrate deep into Namibia’s south, in the areas east of the escarpment, covering both the Hardap and Karas regions.

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