Weather 13 November 2015

What Happened
The dominant synoptic feature for most of the week was the persistent low pressure core which hung over Botswana. This is the first real appearance this summer of a so-called heat low which is the direct outcome of solar irradiation, combined with the flat land of the Kalahari basin, and the lingering high-pressure control in the areas surrounding this core. All territories bordering Botswana were affected by this heat low. The same very hot conditions reigned in those areas of Namibia, South Africa, Zimbabwe, Zambia and Angola adjacent to Botswana. In Namibia the surface temperature was exacerbated by its geographic position on the western rim of this heat low, meaning that the local energy is augmented by the warmer, moister airflow from the north, i.e. from Angola. On the surface however, conditions were still controlled by the South Atlantic high pressure cell. Although its core did not make landfall, as a week ago, its outer rim at the 1016mB isobar, slowly expanded up the South African west coast and reached Namibian territory by Wednesday evening. By Thursday it covered the southern Namib leading to a marked pressure differential between the ocean and the interior where the heat low remained in situ. This brought fresh to strong south-westerly winds to the southern Namib up to the escarpment.
With the pronounced low pressure core over Botswana and low pressure conditions over much of southern Africa, a strong anti-cyclonic circulation was present in the mid and upper levels of the atmosphere. On the western rim, i.e. over Namibia this lead to another mid-level trough (low pressure band) which again typically ran from southern Angola across Namibia in a skew line to the South African interior. The resultant airflow was very prominently from the north at about 18,000 feet which again brought scattered clouds on a daily basis. The apex of this system developed on Wednesday over the interior plateau, and over sections of the northern Namib, with scattered, isolated, but very brief showers reported from many localities in Kunene, Erongo, Khomas, Omaheke and the eastern half of Hardap up to the Botswana border. Another notable synopritc feature was the rapid collapse of the southern Indian high pressure cell as it exited its position south of Madagascar. By Thursday the core of this high was more than 2000 km east of Madagascar with a very modest reading of only 1018 mB. This has been expected for some weeks given that sea surface temperatures in the Indian Ocean are generally about 1.5oC above normal for this time of year. If the temperature of the Indian Ocean waters remain elevated, the southern Indian high will weaken and relocate further south by some 1000 km. At this point, it is only speculative, but it does seem that a very large part of the equatorial and southern Indian Ocean is set to remain warmer at least throughout the summer. In theory this should lead to much enhanced evaporation and convection over east Africa. The first indications of this happening could be seen this week in the thickness of the atmosphere. For most of the early part of this year and throughout the winter, upper level high pressure control was the standard feature over east Africa. That posed a major obstacle to advection from the Indian Ocean, the source from where all the moisture of southern Africa comes. This week, the thicker atmosphere indicated a breakdown of the upper level high pressure control, corroborated by the collapse of the southern Indian high.
What’s Coming
Friday sees the first impact of the approaching South Atlantic high pressure cell but only over the southern Namib and Karas Region. This intrusion is however very brief but it does bring windy conditions to the south with a fresh south-easterly blowing from the Orange River Valley and becoming stronger over the coastal plain below the escarpment. It takes about a day for the high to cross the continent and by Monday the eastern half of South Africa is again under high pressure control on the surface. But it is only a weak high and local (Namibian) conditions are prone to show lower pressures, typically not exceeding 1012mB. The heat low remains over Botswana but its bias falls over the northern half. Of course, this means very hot days over the whole of Namibia for Sunday, Monday and Tuesday, but the blisteringly hot area shrinks to a much smaller core, covering only northern Botswana, Kavango East, Otjozondjupa where it borders Botswana, Babwatwa and the Zambezi. Light rainfall is forecast only for Kavango East, Babwatwa and Zambezi.