Weather 23 September 2016

What Happened
Local weather continues to be dominated by the South Atlantic high pressure cell. In fact, most of southern Africa was under high pressure control for the duration of the week with only a weak mid-level trough running through southern Angola, Babwatwa, Botswana and South Africa’s Limpopo province, manifesting towards the end of the week.
The South Atlantic high is fairly static in its position with the core about 1000 km out to sea. On the eastern side of the continent, the southern Indian high is situated south-east of Madagascar but much farther away. The cores of both highs more or less straddle the 35°S latitude. These are “normal” positions for the two dominant highs that determine the sub-continent’s weather and it is also typical for the transition from winter to summer.
Meanwhile, the equinox was this week Wednesday night. This is when the sun is perpendicular to the equator and when the number of sunshine hours per day over Namibia passes the 12 hour mark. In practical terms, it means the Namibian surface, from north to south, everyday receives more solar energy through irradiation that what it loses overnight through radiation. The result is a slowly warming environment.
The best evidence for this is the gradual appearance of the so-called heat low over the central basin of the southern African sub-continent. While the cold dense air from the south-west (Atlantic Ocean) controls surface conditions in Namibia, the lighter, warmer air from the heat low, opposes this intrusion. The locus of the heat-low currently is western Zambia but as the summer progresses, it shifts to the south, eventually making its mark over the Kalahari basin. The airflow from the south-west (cold) is anti-cyclonic (descending) while the continental airflow (warm) is cyclonic (ascending). These two systems have to meet somewhere and this is most of the time over Namibia. Where they meet, a so-called convergence zone forms usually running from about Ruacana, through the interior to Mata Mata. In the convergence zone the colder, southern air stays on the surface while the warmer, northern air slips over it, almost like two huge bubbles, the one moving over the other. This becomes visible as a convergence line and it is east of this line that clouds form when there is sufficient convection. This dynamic interaction is all driven by solar energy. This week’s weather was a classic textbook example of this volatile exchange.
By the end of the week, the weather has settled into its customary pattern with the South Atlantic high situated offshore Saldanha Bay in the Western Cape, the southern Indian high some distance south-east of Madagascar, and the heat-low intrusion from the north presenting itself during the afternoons when the temperatures in the north climbed to well above 30°C with the central and southern interior coming very close to it.
What’s Coming
The incumbent South Atlantic high makes landfall over the weekend but the local effect is limited. As the high slips across the southern Cape, in its wake it makes space for warmer air to intrude from the north.
This comes from southern Angola across the Kunene river and into the northern Namib covering the coastal plain. The cyclonic circulation over the sub-continent grows in strength which will see a reasonable amount of moisture entering Namibian airspace at the 500 mB level around 18,000 feet. While the weekend will be typically warm to hot in the south, hot over the interior and hot to very hot in the north, the high low interaction will become more defined as from next week, with clouds appearing over the north-eastern quadrant from around Eenhana to Babwatwa, moving to the south-east as far south as the Gobabis district.
Overall conditions are neutral so local weather should follow a fairly conventional synopsis. Light Precipitation is possible over the north-eastern quadrant. The Zambezi will be very hot, especially in the afternoons.