Weather 25 September 2015

What Happened
Local weather oscillates between winter and summer. The most notably feature on the week’s synoptic progression was the absence of a strong South Atlantic high pressure cell and the persistent southern Indian high south-east of Madagascar.

The week started with considerable cloudiness over the eastern half of southern Africa. This blanket of cloud covered a large part of South Africa, Zimbabwe, and eastern Botswana. By Tuesday, the same system has spread towards the west covering northern Botswana and western Zambia. From there it spilled over into Namibian airspace with scattered clouds over the interior, and some substantial cloudiness over the Kavango and Bwabwata.
By Wednesday the South Atlantic high was reduced to a small, weak core some 3000 km offshore from Oranjemund. The southern Indian high, however, remained strong and was displaced towards the north by a good 1000 km. Its 1024mB isobar was almost touching the south-eastern tip of Madagascar for most of the week.
It must be expected therefore, that the influence of the southern Indian high over the southern African subcontinent was enhanced while that of the South Atlantic high was diminished. But the atmosphere is a four-dimensional space meaning it develops laterally, vertically and continuously. This was amply illustrated by the colder nights in Namibia’s south-western quadrant, and the hazy, hot conditions over the northern and north-eastern half.
The impact of the South Atlantic high is strongest on the surface and over the southern Namib and Karas Region, i.e. those areas in close proximity to the high pressure cell. But with such a small, restricted and weak core, the impact was limited to nighttime.
The southern Indian high however, being much stronger and situated further north, had a much stronger impact. First was the cloudiness over eastern Africa, and later in the week, the cloudiness over Angola, Zambia and the northern half of Namibia. But while the South Atlantic high tends to hug the ground, the northern rim of the southern Indian high has a more pronounced impact in the middle and upper levels. As its rotates over land, it warms up losing its density leading to lower pressures.
Thus the interior was marked by low pressure conditions, with higher pressures over the south-western subcontinent. This also reflected in the cloud level with its base around 18,000 feet and its tops hardly exceeding 25,000 feet. Indeed a thin blanket to cover such a large area.
What’s Coming
The local weather over the next few days will be determined by the strength and relative position of the southern Indian high pressure cell. It is expected to remain stuck south-east of Madagascar with a relatively strong core reading of 1028mB. Its 1024 mB rim will remain in close proximity to the island itself.
The southern Indian high forms a very effective barrier against the approaching South Atlantic high but the massive volume of air has to go somewhere and the only way is up. This creates a large area over the southern Cape where vertical velocity will be enhanced meaning very windy, even stormy conditions during Saturday night and Sunday. The southern Cape may be far from Namibia but as the weather goes, it is only a stone-throw, and it has a major impact on local weather especially above 20,000 feet. The low pressure conditions over the southern Cape leads to a pronounced trough at around 25,000 feet running all the way from southern Angola, across Botswana and into the South African interior. This trough will lead to very hot conditions over northern Namibia, especially over Bwabwata, generally lower pressure over the interior, and considerable cloudiness over the north-eastern quadrant.
In the meantime, the South Atlantic high continues on its relentless migration towards the east despite the stagnant position of the southern Indian high. This leads to a northward push by the South Atlantic high and by Tuesday next week, it will feel like winter again in the Karas Region. Colder nights can be expected as far north as Otjiwarongo with very windy conditions over the escarpment.