07 March 2014

What Happened?
If one divides Namibia into a south-western half and a north-eastern half, the separation line runs roughly from around Ruacana in the north west through Mata Mata in the south east. It is called a convergence line as this is where the South Atlantic high pressure cell meets the low pressure trough flowing from the northern interior, created by the heat over the land. This line shifts all the time in response to the strength of the two opposing systems, but the indicated position is where it is situated most of the time. When there is moisture in the air, it is easy to see on satellite imagery. When it is dry, or cold, its exact location must be inferred from other weather indicators.
This week the convergence line was very evident, reflecting typical late summer conditions. The first portent of the departing summer could be felt on Tuesday morning when it was noted that colder air has moved in over the southern half of the country. This is the influence of the South Atlantic high and not only does it bring cooler weather, it also greatly restricts widespread precipitation, especially at the alto levels.
Moisture continued to penetrate the Namibian airspace from its origins in Angola and Zambia but the effect is limited. It lead to good cloud formation but was restricted to altitudes not exceeding 35,000 feet. The result is isolated showers of very limited intensity. Although moisture circulates from the north across the central plateau, convection is limited by the colder, denser layer higher up, hence precipitation is also impeded.
The southwestern half remained in the grip of the South Atlantic high for the entire week. But the number of sunshine hours per day, still exceeds the night hours so the energy captured on the surface still exceeds the radiation loss during the night. Currently, Namibia gets about 12 hours and 30 minutes of sunshine per day, decreasing by 1 minute and 18 seconds per day until 21 March, at the equinox, when the two are equal. As the sun shifts north, its angle also changes, leading to an overall reduction in energy reaching the surface.
However, the amount of energy reaching the subcontinent daily is still sufficient to create a low pressure area over the interior, the so-called heat-low. This low pressure area, pushed along by a southerly flow over the eastern subcontinent, forms the low pressure trough that advects moisture from central and eastern Africa in a broad swathe over eastern Namibia, Botswana, and the adjacent provinces in South Africa.

What’s coming?
The South Atlantic high maintains its influence from the south west. Over the weekend, it pushes deep inland, displacing moist air all the way to the Okavango and Zambezi regions. Typical for the season, the high pressure cell migrates slowly to the east, bringing dry, sunny, but cooler conditions to the western as well as the southern half of the country.
Only by about Tuesday next week, has it moved over to the eastern half of South Africa, allowing the subtropical trough to develop and expand southward. From Tuesday onwards, conditions for rain improve but only for the north-eastern half. There is ample energy and humidity available to bring some impressive late showers to the Okavango and the Zambezi, but for the rest of the country, the window for rain is closing. One forecast expects the moisture to reach the Omaheke region and further south into the Kalahari but this view is not shared by the other forecasts. Very isolated and scattered cloud formation may still penetrate far into the south, but the expectation is only for brief showers.