Rikus Grobler | Aug 22, 2017 | 0
14 February 2014
The week started with another prominent weather anomaly. A cyclic low pressure cell developed early in the week over the northern half of Namibia. Its core was situated more or less above Etosha and its influence extended from the Kunene Region right across the northern regions up to the Botswana border. In the south it extended to Windhoek. It circulated moisture from Zambia in a very large circular motion bringing light to heavy falls to the Kaokoveld, Owamboland, Okavango, Erongo Region, Otjozondjupa Region and to Omaheke Region.
Vestiges of this tropical low pressure system travelled further south along the western escarpment, being deflected by the approaching South Atlantic High, but remaining strong enough to take light rain to the very far south, all the way from Lüderitz in the west to the Kalahari in the east.
As the South Atlantic High made landfall by Wednesday, it cleared much of the tropical low pressure system on the surface levels, but airflow from the Mozambican Channel in the upper air, i.e. above 35,000 feet remained sufficiently strong to lead to cloud formation over the central plateau.
From the recorded rainfall figures (See page 3), the widespread nature of the system can be observed. The main impact was on Tuesday and consequently the highest falls were recorded on that day.
From Wednesday onwards the interior cleared with only a narrow recurved band from the Okavango in the north east across the central parts, across the Khomas Hochland, and back over the southern interior to the Kalahari in the south east, indicated for prospects of light rain.
The location of the low pressure cell over Etosha lead to conditions that are typically associated with patterns earlier in the season, notably December. This circulation could be seen on satellite images as a marked rotation but its influence was restricted by the strong zonal flow in the upper air above it. This zonal flow that originated in the low pressure system west of Madagascar, was driven by a weak high pressure cell located south of the island. The visible effect was airflow from west to east on the surface, but in the opposite direction, from east to west, in the upper air. This restricted further rainfall on Wednesday, Thursday and Friday.
Preliminary indications are that the apex of the rainfall season for the northern half, have passed. February and March are typically more favourable for rainfall over the southern half of the country but these totals can not compare to rainfall over the northern half.
A repeating succession of cold fronts south of the Cape of Good Hope indicates that the South Atlantic High may weaken for a short while, allowing just that small space required for tropical inflows to resume their activity. More rain is indicated by leading forecasts for next week, but the showers will be isolated with limited intensities.
The Inter-tropical Convergence Zone, having shifted east for about two weeks, is migrating back west and sits over southern Angola. This creates the potential for an invasion of moisture into Namibian airspace depending on the strength of the South Atlantic High.