Guest Contributor | Apr 21, 2017 | 0
A strong anti-cyclonic circulation in the middle and upper levels is the main feature of this week. The core of this massive circulation lay over south-eastern Angola at the beginning of the week, slowly migrating across the Zambezi to southern Zimbabwe. On its western edge it formed a prominent trough, advecting moisture from Angola into Namibia. As the system intensified, it developed towards the west, away from its core, bringing widespread, intense rains to northern Namibia, across the entire length of the Angolan border.
This intrusion from the north, followed the escarpment, moving south and east, taking the rains further south, and across the interior to the Botswana border. The result was scattered thunder showers over most of the interior, from Etosha further south across the central high ground into the Hardap and Karas region. This activity was seen clearly on satellite images on Tuesday and Wednesday. The perceived rainy conditions were corroborated by actual rainfall figures published by the Namibia Meteorological Service. From these it is clear that the core of activity lay in the Ruacana Okakuejo Eenhana triangle with substantial intrusions further east covering Rundu, Bagani, Kongola and up to Katima Mulilo. The secondary phase of this system spread further south, covering the rest of the country except for the central and southern Namib.
Towards the end of the week, although the upper level circulation remained strong, it was opposed by the approaching South Atlantic high pressure cell. The first signs of its impact on the surface level could be discerned on Wednesday afternoon when barometric pressures at Swakopmund, Walvis Bay, Lüderitz and Oranjemund started rising, with windy conditions along the southern Namib coastline.
By the end of the week, the encroaching high has supplanted the low pressure system from the north, all but obliterating the trough that was evident earlier in the week. Only the north-eastern quadrant remained positive for continued rain, with very little extension into the central and southern regions.
From the synoptic maps it could be seen that the southern Indian high pressure cell with its core south-east of Madagascar had weakened considerably. Immediately south-east of the Eastern Cape, a relatively strong low pressure system developed, aiding the speed with which the South Atlantic high spread over southern Namibia and South Africa.
By Friday, the skies are mostly clear with very little expectation of rain except for Kavango East and Zambezi. The core of activity has moved to eastern Botswana, southern Zimbabwe and central Mozambique, with only a remnant of the former trough remaining over the eastern parts of South Africa.
The weekend starts with a divided climate. Low pressure conditions over northern Botswana persist leading to temperatures remaining in the upper thirties over Babwatwa and Zambezi, moderate temperatures over Owamboland, and cooler temperatures over the rest of the country except Bushmanland (eastern section of Otjozondjupa) and the former Hereroland East (northern section of Omaheke).
Saturday brings a surprise to the southern half of the country, i.e. below the line running through Swakopmund, Windhoek, Gobabis and Buitepos. A strong cold front moves across the Cape during the weekend and the northern extension of this high pressure intrusion will extent to Botswana and Namibia.
Saturday night and Sunday should bring the first noticeable colder air, again from the south-east and later from the east, as one would expect during winter.
This high pressure system spreads across southern Africa, moving rapidly towards the east, displacing the low pressure system towards the north.
The outlook for Namibia is clear skies and dry, sunny conditions for almost the whole country. Only in the north-eastern quadrant may conditions develop for the odd, isolated, light shower.