Weather 18 December 2015

What Happened
A strong frontal system, driven by the South Atlantic high pressure cell approached the Cape at the beginning of the week. This system increased in intensity and by Wednesday its core was lying offshore Oranjemund.
Although the core was rather mild, the high’s outer rim at the 1016mB isobar, covered almost two thirds of the South Atlantic ocean. The result was cooler night conditions in the south, clear skies, and windless days over the interior.
As the week progressed, it became evident that two systems of opposing origin, engulfed most of southern Africa. On the surface, the high’s intrusion caused dry conditions while above 20,000 feet asl, cloudiness improved towards the end of the week. This mid-level system also developed some strength towards the west, driven by the anti-cyclonic circulation over the continent.
By Thursday, moisture from Angola and Zambia started penetrating Namibian airspace, leading to improved cloudiness over Owamboland, Okavango, Babwatwa and Zambezi.
But still, the high controlled conditions on the surface over most of southern Africa. Later in the week, a weak mid-level intrusion from the north was evident beginning in southern Angola and running through the Kavango, northern Botswana, Zimbabwe and into Mozambique. However, precipitation on Namibian territory was limited. The system only developed its full potential over Zimbabwe and Mozambique, and only from Wednesday onward.
At this point, the southern limit of the Inter-Tropical Convergence Zone (ITCZ) is hovering on the Namibia Angola border in the western half of the sub-continent but in the eastern half, it has shifted to the southern tip of Madagascar. In this position, cutting across the continent from the Atlantic to the Indian Ocean, it is an effective barrier against further intrusion of the South Atlantic high pressure cell as it moves over southern Africa. Along the southern periphery of the ITCZ, a marked convergence line formed with some spectacular rainfall, but only over the eastern half of the sub-continent
Towards the end of the week, the core of the anti-cyclonic continental circulation has actually moved offshore into the Mozambican channel. This created a strong east to west zonal flow in the upper air covering Mozambique, Tanzania, Zambia, the southern half of the Congo, and Angola.
What’s Coming
Sea surface temperatures in the Atlantic have increase substantially over the course of the past two weeks. From a neutral stance, it has now shifted to about 1oC above the mean, with the biggest anomaly closer to the continent. This is the reason for the rapid weakening of the South Atlantic high pressure cell.
The weekend begins with weak high pressure conditions on either side of the sub-continent and a strong low pressure system covering most of Namibia, Botswana and the Northern Cape Province. This is a so-called heat-low and is normal for this time of the year. It brings very hot conditions on Saturday to the southern Namib, the rest of the Karas Region, and the Kalahari adjacent to the Botswana Border. The heat-low gradually shifts to the east, taking the excessive heat to southern Botswana, and then covering almost the entire Botswana from Sunday onwards.
With the heat-low centred over the Kalahari Basin (Botswana) the airflow on its western rim (Namibia) is predominantly from north to south. This brings a mid-level intrusion of moisture from southern Angola into Namibia which covers about two thirds of the surface area by Monday. It is only the southern Namib that will remain devoid of any form of cloud.
Low pressure conditions are expected to dominate local weather for at least the next four days. Although the intrusion from the north covers two thirds of Namibia, the rainfall forecast is not optimistic, indicating at best isolated thunder showers, spread over a very large area.
There is some enhanced convection indicated for Sunday night and Monday but only for Omusati and Ohangwena. For the rest, falls will be brief with less than 5mm per event. The next substantial intrusion of moisture is only expected with the full moon.