09 December 2016
At the beginning of the week, the South Atlantic high pressure cell has moved around Cape Agulhas forming a contiguous high pressure area from the Kunene river mouth in the west, all the way around the southern tip of the continent up to the Mozambican Channel in the east. This high pressure presence had a remarkable impact on the entire sub-continent’s weather.
As the week progressed the high split, as expected, reforming the South Atlantic high west of the continent while the eastern remnant migrated to the Indian Ocean where it joined up with the southern Indian high pressure cell.
Meanwhile, surface conditions remained under the influence of a low pressure system that covered most of southern Africa above both the western and the eastern escarpments.
As the high south of the Cape moved to the southern entrance of the Mozambican Channel, it creates a strong south to north airflow leading to onshore high pressure control on South Africa’s east coast and along southern Mozambique. But it is such a powerful engine that despite its relative weakness, it influences weather conditions as far north as Tanzania. It is this high pressure impact from the south that has been responsible for the fragmentation of the atmosphere in East Africa since the winter ended.
The anti-cyclonic circulation that covers southern Africa during the summer follows a huge curve that originates in East Africa, then crosses central Africa and recurves into Zambia and Angola. From there it enters Namibian airspace. As the summer progresses, this pattern grows in strength and opposes the South Atlantic high which always approaches the sub-continent from the south-west. Where the two systems meet, a convergence zone forms, with the actual convergence line visible on 750 mB charts. This convergence zone, 90% of the time, is located over Namibia.
This week saw much moisture intrusion from Angola but with a relatively high cloud base around 14,000 feet. This intrusion was from the north-east, then backing to north and by Thursday to north-west. From the south-west, the South Atlantic high controlled surface conditions with slightly cooler nights over the interior. The high pressure surface conditions tend to restrict the impact of the northern intrusion, lifting the cloud base and consequently diminishing convective ability. Although much cloud was present over the central plateau and the north, convection was restricted and so was rainfall. Only isolated light showers occurred in a broad band from the Kunene region through the central parts and into the southern Kalahari.
The only exception was the north-western corner where good rainfall prospects were present.
A weak South Atlantic high is present west of the continent with a moderate southern Indian high located some 2000 km south-east of Madagascar. The co-called Indian Ocean Dipole is still neutral but the warmer water from the Arabian Sea is slowly migrating from north to south along the east African coastline.
The South Atlantic high lacks a prominent core but the cell itself spreads over an enormous area due west of Walvis Bay, covering about two-thirds of the South Atlantic Ocean. This indicates very static conditions west of the continent.
On the eastern side, the southern Indian high is slightly stronger, 1024 mB, but its well-defined core is displaced to the south.
Over the sub-continent, heat wave conditions remain in place over southern Namibia and southern Botswana. From Windhoek further north, there is much activity in the mid-levels at around 18,000 feet.
Rain is indicated for the northern half of Namibia for Saturday and Sunday. By Monday, both the surface and the mid-level activity indicate the presence of a mild trough that flows from Angola across Namibia into southern Botswana. This improves rainfall expectations for the whole country except the central and southern Namib.
The South Atlantic high starts moving in from Wednesday, pushing away the moisture over most of Namibia except Babwatwa and the Zambezi.