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31 January 2014

What happened?
The week started with good prospects for rain. By Tuesday, these have fizzled out somewhat with only scattered, but widespread falls recorded,.  However, a strong airflow from the northeast advected substantial moisture in the surface and middle layers leading to improved conditions for the latter part of the week, and expected to continue over the weekend.
During Tuesday, a prominent low pressure area was situated between the continent and Madagascar over the northern half of the Mozambican channel.

Coupled with the strong anti-cyclonic gyration over southern Africa, it created a marked airflow from north to south.
On the western side of the sub-continents, a weak inshore low pressure area formed off Luderitz leading to cyclonic circulation over the southwest, driven by the approaching South Atlantic High.
A very typical convergence line formed almost directly from north to south dividing the country’s rainfall arena into two prominent halves.
West of the convergence line it was cooler and clear with almost zero precipitation while east of this line, it was hot and cloudy, with numerous thunderstorm from the Kavango in the north right up to the eastern half of the Karas region. Towards the end of the week, a very strong anti-cyclonic circulation has established itself over the entire sub-continent with its core more or less over the Sashi, the border between Botswana and Zimbabwe.
 This anti-cyclonic wheel is ideal for Namibia, and has been absent or out of position for most of the rainfall season so far.
Namibia sits on the western downward edge of this system and as long as it remains in situ, it advects moisture from Zambia and the DRC across Angola, into Namibia, and depending on the proximity of the South Atlantic high pressure cell, across most of the country east of the Namib.

What’s coming?
The core of the anti-cyclonic circulation remain over eastern Botswana for at least another three days.
 The position of this circulation is very important. If it lies too far west, it leads to excessively high temperatures over central and southern Namibia, with very little rainfall.
If it lies too far east, the circulation of moisture only reaches Botswana and the eastern fringe of Namibia, again preventing the inflow of moisture.
Very large volumes of moisture from the surface to the upper levels above 40,000 feet will continue to enter Namibian airspace during Saturday and Sunday.  Heavy precipitation is indicated for the north eastern and central eastern areas with scattered thundershowers basically expected across the whole country except the desert. By Tuesday next week, the system has drifted south and east, with rainfall expected south of Gobabis, across the Kalahari and into South Africa and Botswana.

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