Guest Contributor | May 31, 2021 | 0
Weather overview and short-term outlook to Wednesday 27 January 2021
Visual: Computer-generated map of expected precipitation over central and southern Africa for Friday afternoon, 22 January.
With the departure of the last remnants of Chalane from the middle and upper atmosphere over Namibia, the weather picture reverted to a fairly normal midsummer stance with widespread showers in the north-east, north and north-west up to the Opuwo area. Afternoon temperatures over the central plateau, the Kalahari and the south-western quadrant were hot to very hot, indicating mild high-pressure control in the alto levels above 35,000 feet.
After the unprecedented rains over much of Namibia south of Etosha, two issues came up. The first was rainfall in Owambo and northern Kunene, which has been disappointing so far and almost completely absent in Kaokoland. The other issue is whether the wetspell could be repeated.
Rainfall conditions in the North changed significantly during the week as the strong low-pressure system that moved in from the Zambezi, gradually migrated to the west, covering Owambo in wide swathes of scattered showers. Rain was also reported in the eastern sections of Kaokoland but nothing comparable to the rain in the rest of Namibia over the previous two weeks.
This week’s visual was widely circulated in social media but it is not clear what the source of the information is. Nevertheless, it still provides a very good visual of the interplay between high pressure (anti-cyclonic) circulation over South Africa, and the low pressure system pushing in from the tropics. Furthermore, it indicates the locality of Tropical Cyclone Eloise as it is about to make landfall in southern Mozambique.
The big question on everybody’s mind is whether Eloise’s impact on local weather can again reach Namibian airspace like Chalane did two weeks ago.
This is where the visual becomes important. It shows a clear line between the high pressure and the low pressure areas. It also shows a convergence line that runs from east to west through central Zimbabwe, central Botswana and central Namibia.
As Eloise advects millions of cubic metres of moisture into the upper levels, copious rain will continue to fall north of the convergence line while south of it, it will be relatively dry. The big unknown is how the position of the South Atlantic high, as it migrates around the continent, will allow Eloise a trajectory to the west and to the south.
Leading rainfall forecasts are not unanimous about the immediate future, other than indicating that a typical summer rainfall pattern will persist north of the convergence line.
There is every possibility that Eloise will reach the Namibia Botswana border by next Tuesday, but how much of its strength and moisture it will retain, is pure guesswork. It is not known now whether it will lead to a second round of widespread, high intensity rainfall for the whole of Namibia.
On the Radar
At this point, the 30-day Southern Oscillation Index of the Australian Bureau of Meteorology is perhaps the most reliable indicator of expectations for the short to medium term.
In the past four weeks, this index literally went off the charts twice. Its northern axis first had to be extended past 15 and then past 20. This indicates a very strong effect from the current La Nina in the Pacific Ocean. The index is now at 18.5, indicating an above average chance that southern Africa will continue to receive good rains.
For the next three days, the rain bias lies over Namibia’s northern half. This means that from Windhoek northwards, there will be daily cloud formation with scattered thunder showers. Over the southern half, there will also be some cloud formation above the escarpment but the chances for rain are slim.
By Tuesday, there should be a marked change in conditions, preceded by a lot of windiness, especially at night. This is the approaching outer rim of Eloise. She is expected to reach at least up to Buitepos but how far she will move into Namibia will only be known towards the end of next week.