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Weather 30 September 2016

What Happened
The early part of the week was dominated by a well-developed trough in the middle layers running diagonally across southern Africa from Angola through Namibia and Botswana into central South Africa. This trough developed ahead of a frontal system which crossed the southern tip of the continent from Tuesday to Thursday. This created a considerably pressure differential between the low pressure in the mid level and the high pressure on the surface. Not only were the two systems readily identifiable by the different vertical strata they occupied, they were also geographically separated. The trough dominated in the north east while the front controlled the south west. This roughly split the whole sub-continent into two quadrants, north-east and south-west, with divergent conditions between the two opposing systems.
The most typical way to identify this systemic clash is to observe the reciprocal interaction between the two, from day to night. This is typical spring weather for Namibia. In the period immediately after the solstice, the amount of solar energy received during the day exceeds the amount of energy lost during the night. The visible effect is the direction of the airflow. The days typically start with a cooler air intrusion from the east but as the sun works its magic, the airflow veers to north-east, and later in the afternoon due north. This leads to warm or hot afternoons depending on the latitude.
The moment the sun sets, the energy input is removed from the system and conditions swing around to be controlled by the surface high pressure intruding from the south-west. The direction of the airflow changes to west, and then south-west, and the night temperature comes down rapidly. This see-saw pattern of the local weather is usually only broken by early November when solar irradiation and the anti-cyclonic circulation over the continent become the main features. By the end of the week, the frontal system has departed the continent with only a few vestiges visible south of Madagascar. It was replaced by the outer rim of the South Atlantic high pressure cell as this followed in the wake of the frontal system. As is usual, it brought cool nights to the Karas region, windy conditions over the interior by night, and clear skies. At this point in the season, a noticeable anomaly is the fragmentation of the upper atmosphere over eastern Africa. The interior of the southern African sub-continent gets its moisture from the Mozambican Channel and the Indian Ocean farther north. There is a huge conveyor system, driven by the southern Indian high pressure cell that depends on evaporation in the Indian Ocean. This system then advects moisture across the continent in a broad path running across Tanzania, Mozambique, Zambia, the southern Congo, and Angola. From there the path recurves across Namibia, Botswana and South Africa.
For this path to work efficiently, there must be low pressure control over eastern Africa. This is currently not the case and the upper level high pressure control acts as a buffer preventing sufficient moisture to enter the continent.
What’s Coming
By the end of the week, the core of the South Atlantic high has gained in strength but is still offshore from the west coast. Solar heating over the sub-continent prevents it from having much local influence as it slips around Cape Agulhas on its way to the east. By Sunday the high pressure cell is south of the continent which allows some space for the lower pressure from the north to intrude Namibian airspace. By Sunday night there is again a considerable pressure differential between the Kalahari and the Namib which should lead to very windy conditions along the southern Namib coastline on Monday.
From Tuesday onwards the whole Namibia is under low pressure control which will bring very hot days, mild nights and windy conditions over the entire north/south length of the escarpment. It will be sweltering in the North from Ruacana along the entire Angolan and Zambian border up to Katima.

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