14 March 2014
The most noticeable feature of the week’s progression of synoptic charts is that the core of the high pressure cells in the so-called subtropical high pressure belt, has shifted south by about one thousand kilometres. This shift open up some very promising late season possibilities.
Of course it must be remembered that unless there is a solid reason for the high pressure cells to move south, they will just as quickly move back north, and our weather will revert to a more typical late summer pattern.
Earlier in the week, the South Atlantic high pressure cell was very dominant. This can be noticed on the surface by the cool, and in the South, even cold early mornings. A prominent but weak low pressure cell formed just south of the Mozambican Channel, and another weak high pressure cell lingered far south-east of Madagascar.
The strong presence of the South Atlantic high pressure cell, and the marked zonal flow all the way from the Indian Ocean across Mozambique, Zambia, southern DRC and Angola into Namibia from the north created a trough (low pressure funnel) which brought heavy rains to the Zambezi, northern Botswana, Limpopo Province in SA, and further south across the eastern parts of South Africa.
The spill-off from this was intermittent cloud cover over the Kavango Region which pushed into the Namibian interior with widespread but isolated and scattered showers the result. The observable pattern was one of “lots of jealousy showers (Jaloersreen)”.
However, later in this week, the impact of the South Atlantic High diminished as it migrated to the east. By Thursday its influence over the eastern sub-continent could be seen on synoptic maps as it brought in colder air from the south. This high, when it passes the southern Cape, becomes a massive conveyor of cold air from south to north, opposing the weaker low pressure area in the Mozambican channel, but at the same time, creating a prominent trough in its wake. This trough typically runs from Angola, across Namibia and Botswana into the central interior of South Africa. It is the rain-bearing conveyor belt for the interior of southern Africa, and it grew in strength as the week progressed.
The inflow of moisture from the north is pushed along by the high pressure area over the eastern half of the sub-continent. Strong zonal flow from the Indian Ocean persists. As the wind veers from north-east to north-west, rain prospects for Namibia improve radically.
Various forecasts are unanimous in indicating rain for the north western areas (Kaokoveld) shifting south and east towards the Kalahari following the low-pressure trough line. During the weekend rain prospects keep on improving with rain indicated for most of the country during this period. The whole of next week is also forecast to receive some showers, at least for the eastern half of Namibia, i.e. north and east of the new convergence line. However, the high pressure cell south of Madagascar remains somewhat weak, possibly creating the space for the trough to stay longer and penetrate our southern regions. Next week may be a pleasant surprise, possibly bringing more precipitation than is currently expected.