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Weather 13 February 2015

What Happened
A broad but shallow trough stretching from Angola to the Northern Cape, brought a few isolated showers on Wednesday and Thursday to many areas from Kunene to Karas, particularly in the south-eastern corner of the country along the Botswana border and the Orange river. However, informal reports indicate that precipitation, generally were less than 5mm in any of those spots where light rain occurred. The week started still very much under high pressure control. The core of the South Atlantic high pressure cell remained weak early in the week, and it was located again some 1000 km north of its customary position, which should be more or less in line with Saldanha Bay in South Africa. By midweek the core has shifted south and increased in intensity to 1024 mB as it shifted south in its trajectory from west to east, slipping past the continent some 600 km south of Cape Agulhas. This movement of the South Atlantic high provided some relief to the interior of southern Africa, particularly the western half which includes Namibia. The result was the formation of a weak trough, east of the convergence line, from south-western Angola, again all the way through Namibia, crossing southern Botswana and into South Africa, where eventually, the north south airflow lead to significant precipitation in the south-eastern regions. However, as has happened several times since November last year, the weak core of the South Atlantic high pressure cell causes it to disperse over a much wider area than usual and by Thursday morning this was evident again, as the 1016mB line reached very far north, approaching the equator. This debilitating condition is foreseen to keep repeating itself until the sea surface temperature between Africa and South America has returned to normal. Currently, there are good indications that the warmer water is receding but there is still a small area, about halfway between the two continents, where the temperature is elevated by two degrees Celsius. Since the affected area is so large, literally thousands of kilometres across, temperature changes happen very slowly. As the week progressed, the synoptic pattern became increasingly complex. Towards Friday, the South Atlantic high has started moving around the continent, with two low pressure areas, one south of Madagascar and the other some 2000 km east of the island. South-east of Madagascar lay the somewhat stronger core of the southern Indian Ocean high pressure cell, too far to have a direct impact on the subcontinent’s weather on the surface, but strong enough to create a significant zonal flow from east to west, in the upper air between 33,000 and 38,000 feet. This lead to an interesting anomaly. Usually the upper air zonal flow is from west to east. This is well-known by airline pilots as this flow reduced Windhoek Johannesburg flying time by anything between twenty minutes and half an hour. Tuesday, Wednesday and Thursday, this typical flow was reversed, (as a result of the southern Indian high), and pilots now had a tailwind from Johannesburg to Windhoek.

What’s Coming
The advective machinery which conveys moisture from the Indian Ocean across the continent in a very wide arch, until it reaches Angola, and then Namibia, is intact. But this conveyor functions only in the mid-level altitudes between 15,000 and 30,000 feet. Its operation can be readily seen in the cloud cover that enters Namibian airspace from the north-west but remains restricted to this level. The clouds appear flat almost like a blanket with little convection indicating that the normal structure required for substantial thunderstorms, is absent. This condition continues over the weekend. As the South Atlantic high passes Agulhas, the wind should be strong southerly to south-easterly along the southern Namib. Advection remains limited so the weekend will present clear skies for almost the entire country. Windy conditions will be experienced below the escarpment or in close proximity to the escarpment on the interior side. Only in the most remote north-western corner of the Kaokoveld will some intrusion of moisture occur. By next week, the windflow will shift to east, then to north-east and the interior will again be very hot as anabatic compression releases energy close to the surface. This should affect the flatlands north of the Waterberg, around Mariental, and across the Kalahari east of Keetmanshoop. These conditions will remain in place over the entire country, backed by the influence of the next approaching South Atlantic high. None of the leading forecasts expect any rain, anywhere for the whole of next week.

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