31 Oct 2014

What Happened?
Conditions this week were fairly conventional for this transitional period between the seasons. At the beginning of the week, the synoptic picture showed all the usual elements, more or less where they are expected. The strong vortex south of Madagascar has abated and shifted far to the east over the southern Indian Ocean. In its wake lay the high pressure cell south of Madagascar, i.e. east of the continent, while the South Atlantic high pressure cell circulated over the Atlantic Ocean with its core some three thousand kilometres west of Oranjemund. The only noticeable exception was that both high pressure cells were situated further north than usual at a latitude around 32o  South. On the surface, the presence of the South Atlantic high pressure cell continued to cause a southern half northern half split. Night temperatures in the southern half remained considerably cooler than in the northern half, and this offset persisted for the entire week. Over the Kavango and Zambezi regions, a lower pressure area induced by solar heat, persisted. This lead to some very hot days on Tuesday and Wednesday. By Thursday the so-called heat-low has extended towards the west, also raising temperatures in the Owambo basin. The only anomalous feature was the vortex in the southern Indian ocean. The location of this vortex had a marked impact on the strength of the high pressure cell south of Madagascar. The vortex considerably weakened the core of this high pressure cell creating space for the South Atlantic high to extend round the southern tip of Africa with a marked influence up the Mozambican channel, again exerting a high pressure control in the upper air as far north as Kenya. Because of the weakening of the first high pressure cell, by Thursday the South Atlantic high extended from the Mozambican Channel westward all the way to near the South American coastline. Throughout the week, it had a pronounced and lasting impact on the weather in the southern half of Namibia. By Thursday, the typical ying yang pattern appeared with surface lower pressure moving in from the north over the Kunene, and surface higher pressure moving in from the south over the Kalahari.

Squeezed between the two high pressure cells, a weak trough (lower pressure) developed extending from Angola to South Africa. However, it was only noticeable in the middle layers between 15,000 and 25,000 feet. It did bring in some cloudiness but almost zero precipitation. By Friday, the heat-low area has developed to cover the eastern half, taking the extreme heat experienced earlier in the week in Kavango and Owambo, to the south across the Kalahari.
What’s Coming?
The weekend starts with an intermittent weak trough (lower pressure band) indicated from Kunene to the Kalahari. This system rides on the edge of the high pressure cell south of Madagascar. As the weekend progresses it increases in strength, decreases in elevation and very slowly drifts towards the east. Windy conditions will prevail over the weekend on the escarpment from Damaraland south to Aus cutting across Karas to Ariamsvlei. By Monday the trough extends from southern Angola across the eastern half of Namibia into southern Botswana.
Starting late on Sunday, continuing during Monday and Tuesday, scattered clouds start appearing over the north-west, the north-east, the central plateau and south towards Aroab. These three days will be markedly cooler in the south and over the Namib. After Wednesday, the signatory low pressure circulation moves in from Angola covering most of the northern border but with the highest intensity in the west. Towards the second half of next week, the first real summer rainfall may occur although conditions are only favourable for the Kunene Region.