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28 February 2014

What Happened?
A strong and persistent zonal flow from the core of the low pressure vortex in the Mozambican channel, across Zambia and Angola, brought an extensive inflow of moisture from the north east. This cross-continent flow was assisted in the upper levels by a weaker high pressure cell situated south of Madagascar. On the western side, the South Atlantic high pressure cell was subdued and largely sedentary, creating room for the zonal flow from the east to penetrate Namibia from the Okavango across the north, and to the south, for the entire length of the country. Notable was however that the moisture content was highest in the lower to middle levels, with a continued high pressure presence in the alto levels, originating from the core of the cell south of Madagascar. Tropical cyclone Guito in the Mozambican channel petered out and drifted south, as expected but was still faintly visible by Thursday morning on satellite images that focus on the levels close to the earth’s surface.

This anticyclonic circulation provided the funnel for advection of air from far south, even some 2000km south of the continent. The full effect was largely neutralised by the presence of the high pressure cell, slightly south of the cyclonic core, but generally in the same vicinity. Confusing as it sounds, it must be remembered that the low pressure cyclonic circulation, as it weakened, remained surface bound, while the high pressure cell, the remnant of the ever-migrating South Atlantic High, slipped over it creating a typical inversion layer. This caps the moisture at a rather marked elevation and to some extent prevents full development of wet conditions right up to the end of the troposphere at about 56,000 feet. An anti-cyclonic upper air circulation dominated the northern half of Namibia. As it shifted slightly to the east, over the Namibia Botswana border, it advected massive amounts of moisture over the western half of the country as evidenced by the intensity of the falls on Tuesday and Wednesday. The source of this moisture is Angola and Zambia. But the signature convergence zone, the result of the clash between the South Atlantic High and the Mozambican channel low, was present for most of the week, so penetration south of Mariental and west of the escarpment was limited, again reflected in the official rainfall data. It did move somewhat further south than expected on Thursday with the triangle Windhoek Gobabis Mariental, forming the core of the intense trough that formed just north of the convergence line.
See rainfall data on page 3.

What’s Coming?
The South Atlantic high pressure cell approaches slowly and takes it time to slip past the Cape of Good Hope, indicating that conditions further north will remain unchanged over the weekend and into the first two days of next week. However, as the high pressure area moves beyond the Eastern Cape, it creates the typical booster that clears the eastern sub-continent but recurves to circulate over Namibia from the north and north east. This penetration follows the convergence line south to the Kalahari and into Botswana and South Africa’s North West Province. Rainfall indications for the whole country remain positive for the weekend, but the emphasis shifts to the eastern half from Tuesday onwards. The South Atlantic high makes its mark as far north as Luanda in Angola.

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