Weather 18 November 2016
The off-balance synoptic picture continued into this week. The South Atlantic high pressure cell is in its customary location west of Cape Town, slightly displaced to the south, with a strong core reading of 1032 mB. The southern Indian high, still very weak at the beginning of the weak, reading only 1016 mB, grew marginally in density to 1020 mB, but still considerably weaker than the South Atlantic high. The southern Indian high was also dispersed over about 5000 km of ocean from west to east, further reducing its influence on the weather in southern Africa.
These two high pressure cells are the drivers of many other elements of local African weather. The outer rim of the southern Indian high drives the zonal flow, east to west, from the Indian ocean into central Africa. The leading (eastern) rim of the South Atlantic high drives the south to north airflow over the southern African sub-continent. These two systems have to meet somewhere and that is usually over Namibia. Sometimes it can only be discerned by taking barometric readings but often, especially during summer, a clearly visible convergence zone forms with cloudiness north and east of the convergence line, and clear skies, south and west of this line.
This phenomenon was again readily observed this week with windy but clear conditions over the south-western quadrant, and hot, humid conditions over the north-east. The convergence line remained static, running more or less from Terrace Bay, through the interior to Aminuis. North of this imaginary line, clouds were visible with isolated showers. South of this line there was zero activity other than consistent windiness.
Between the two high pressure cells, a moderate low pressure area formed at the 500 mB level. That is roughly at about 18,000 feet. Whereas mid-level conditions over Namibia and Angola were fairly static, a strong low pressure system south-east of Port Elizabeth created a strong north to south airflow from Angola to the Eastern Cape. This advective system started in southern Angola where it was weak, cut across the Kavango and Botswana into the South African interior, where is gradually became stronger the closer it got to the low pressure system south of the continent. It brought some precipitation to the north and north-eastern quadrant, and some spectacular activity in Botswana south of Kang. But the real action was over the South African highveld where most of the precipitation was released.
In Namibia over the central plateau, the days were again very hot, with a concentration of energy in the Karas region, along the Botswana border, and across the entire Angolan border from the Kaokoveld to the Zambezi. The low pressure system was most active in the north-west with considerable cloudiness over the Kaokoveld. Further east, advection was strongest in a broad band from Eenhana to Rundu, and the areas south of this point of entry, towards the Botswana border.
The Inter-tropical Convergence Zone is still fragmented in East Africa but in Angola it is visibly growing in strength. Its southern boundary is now only about 400 km north of the Namibian Angolan border.
On Saturday, the South Atlantic high pressure cell starts slipping around the southern Cape. Ahead of the high’s leading rim, is a strong low pressure system. During the weekend, the low is located about 1000 km south of Madagascar. This enhances the north to south airflow but the activity is restricted to South Africa.
It is only by Monday that the South Atlantic high has migrated far enough to the east to envelop the sub-continent, remaining offshore. This forms a high pressure ridges which effectively obstructs the north to south airflow. The effect is a “damming up” over Namibia, an increase in atmospheric depth, and positive expectations for local rainfall.
Precipitation is indicated for most of Namibia, including the central and northern Namib, on Monday and Monday night, with limited rainfall still possible over the interior during Tuesday.