Weather 27 November 2015

What Happened
The surface high pressure control over the southern half of Namibia gradually gave way to an intrusion of lower pressure from the north. This lead to the expected mid level trough, relatively weak, but still active enough to bring limited cloud cover to the interior from Tuesday onward. During the first part of the week, it was only the north-eastern quadrant and the Botswana border that were exposed to very hot conditions. The rest of the country, while still very hot, did not cross the 36oC threshold.
Against all the negative expectations as a result of the strong El Nino in the equatorial Pacific, one positive feature has been growing on the local weekly synopsis, the so-called Inter-Tropical Convergence Zone. While generally weak or poorly defined in the southern hemisphere during the austral winter, it is now relatively well demarcated running across the entire African continent from Senegal in the west to Tanzania and Mozambique in the east.
As of this week, the southern extremity of the ITCZ is hovering just north of the Angolan border within a few hundred kilometres of Namibian airspace. This is a good sign. Usually, by the end of November, the ITCZ is about midway through Angola on its southward journey towards the Angolan Namibian border. So it is more ore less in place, with only a short leap to complete before the solstice. It is notably well-developed, crossing the sub-continent from the Atlantic Ocean to beyond Madagascar into the Indian Ocean.
As the high shifted to the east, the anti-cyclonic low pressure circulation resumed its intrusion from the north, building in intensity as the week progressed, leading again to excruciating temperatures on Thursday and Friday.
This cycle has repeated itself three times over the past six weeks, every time manifesting a very distinct pattern of a high intrusion from the south with cool nights, then windy conditions with the wind backing from east to north, and then finally, after about two days of a northerly airflow, excessively high temperatures over Kunene, Owambo, Kavango, Zambezi, the Botswana border and the Kalahari. Usually, a day later, hot conditions also appear over the southern Namib but this is due to the katabatic flow from the escarpment down to the coastal plain, and not as a consequence of the intrusion of Angolan air.
By the end of the week, the next approaching South Atlantic high pressure cell is situated about 1000km south-west of Cape Town with the outer rim, the 1016mB isobar, just offshore from the Western Cape. Barometric pressure in the core is subdued reading 1028mB, but it is situated south of 40oS latitude, leaving ample space over Namibia for an intrusion from Angola.
What’s Coming
During this week the Southern Oscillation Index of the Australian Bureau of Meteorology registered a rather dramatic shift from a reading of around -21 to -5. This index is based on the barometric differential between Darwin in Australia and Tahiti in the equatorial pacific. It’s move this week is usually indicative of normalisation of sea surface temperatures but it is at the same time a “busy” index prone to large fluctuations. However, it is so far the first empirical indication that the brunt of the El Nino has passed. Unfortunately it is by no means an indication how long the waning phase will last.
The weekend again see hot to very hot temperature for the entire country. Barometric pressure continues to fall and by Sunday, the familiar mid level trough is present with a good expectation for rainfall north-east of the line running through Ruacana, Windhoek and Mata Mata.
By Monday, light rain is indicated for the entire country excepting the coastal plain, as the trough develops in strength and runs from Angola through Namibia into the Northern Cape. There is a reasonable expectation for continued cloudiness on Tuesday and Wednesday.