Overview for the week and 5-day outlook to Wednesday 28 March 2018
First Visual: Digitally enhanced satellite image of convection over southern Africa on Thursday 22 March 2018. Image is based on refraction, i.e.temperature differences at 18:00 CAT.
Source: Eumetsat at oiswww.eumetsat.org/IPPS/html/MSG/RGB/CONVECTION/
Second Visual: Eumetsat infrared satellite image of southern Africa on Thursday 22 March 2018 taken over the Greenwich meridian at 14:00 CAT.
Source: oiswww.eumetsat.org/IPPS/html/MSG/IMAGERY/IR108/COLOR/SOUTHERNAFRICA/
What Happened
With another cold front passing the southern Cape, conditions south of the Orange River this week displayed a distinctly wintery pattern. This was confirmed by the first widespread but light rain over large areas of the Western Cape and the southern Cape region.
Further north however, weather conditions are still dominated by a strong summer pattern witnessed by the position of the southern boundary of the Inter-tropical Convergence Zone (ITCZ) which has been sitting on the Namibia Angola border since the previous weekend.
The area in between is more or less the geographical space occupied by Namibia. This slice of southern Africa was marked by a strong zonal air flow on the surface from east to west, originating in the Mozambican Channel and reaching all the way across the continent up to the Namibian escarpment.
A weak low pressure system was present over south-eastern Angola which caused cyclonic circulation over this remote corner which in turn helped to bring moisture from the ITCZ into Namibian airspace.
The zonal push from the east coupled with the cyclonic circulation over southern Angola created a rare conveyor effect which advected moisture from southern Angola into Namibia, developing in intensity as the week progressed, eventually covering almost the entire country except for the coastal plain.
This pattern was detected in all layers of the atmosphere from the surface to as high as 45,000 feet.
It is a rare occurrence in Namibia to have ample moisture from the very low levels at around 8000 feet (750 mB) up to 45,000 feet (less than 200 mB) but when it does happen, it boosts convection and although much of the week’s rainfall was not the result of thunder showers, the pervasive tropical low pressure ensured that the various layers of atmospheric activity strengthened one another, producing showers that endured, in some cases for several hours.
Both images show clearly the concentration of moisture over southern Angola and Western Zambia, and the cyclonic effect which took this moisture into Owamboland and the Kavango in the northern regions.
Over the central high ground, the rainfall mechanics were slightly different, being lar
gely the result of the persistent air flow from the east where the upper atmosphere was saturated with moisture that primarily came from the convective power of the cyclone/tropical storm that migrated down Madagascar’s east coast.
During the second half of the week, another low pressure system developed over central South Africa, helping to reinforce the advective mechanism that drew moisture into the Namibian interior from further north This was most evident on Thursday and Friday as a strong cloud presence started forming in a broad swathe that lay more or less equally distributed over the Fish River from north to south.
What’s Coming
The weather split between early winter conditions in the south and summery conditions in the north continues during the weekend.
Another cold front is close to Cape Town and by Saturday evening, Namibia’s South will see some impact from this front which is at the leading rim of the approaching South Atlantic high pressure cell. Cooler, even windy conditions will prevail in the South.
Continued rainfall is expected for the Northern regions close to the Angola border with the main concentration in Kavango West, Bushmanland along the Botswana border, Bwabwata and the Caprivi section of Zambez
i.
By Monday the country is split into the familiar southern half northern half pattern. While the south remains under the influence of the high crossing the Cape, air flow over the northern half will be predominantly north to north-west. There is a 50/50 chance for light precipitation over the northern half.
Overall, upper air conditions are relatively unstable indicating that showers can develop literally anywhere in the northern half, with the bias in the central and central-northern areas. Moisture levels will remain high, so only limited convection has the potential to produce showers. This implies that rainfall will most likely be the result of classical thunder showers and not the presence of a tropical low pressure system as was the case this week.
