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09 May 2014

What Happened?
Local weather’s most prominent feature for the week was a well-defined, broad trough (low pressure band) that stretched intra-continental from the middle of Angola to way beyond Cape Agulhas.
The South Atlantic High Pressure Cell kept its distance, remaining far from our shores, slightly east of the halfway mark between Africa and South America. One would assume that with this cell so many thousands of kilometres away from the continent, late-summer rainfall conditions would be positive, but alas, that did not happen.
The reason lies in the sea surface temperature of the Mozambican channel. While a prominent high pressure cell remained stationary over the eastern half of southern Africa, the sea surface temperatures in the Mozambican channel have now decreased hence the uptake of moisture at its original source is weak. Inflow from the other source of moisture, the Indian Ocean north of Madagascar, is pushed too far north by the high pressure cell to have much impact on the western side of the sub-continent. It is simply too far, and the driving forces are to weak to lead to a major influx of moisture from Angola across Namibia.
The so-called Inter-Tropical Convergence Zone which is the most important conveyor and container of moisture for us, is also now far removed to the north, only showing up on synoptic maps some 1000 km away from the Namibia Angola border.

As a result of this week’s trough, airflow was predominently from the north, shifting to northwest over Kunene and Erongo regions. This leads to a very typical condition in Walvis Bay, the so-called Northwind, popularly joked about as “the smell of money.” It is only when the Northwind blows that the older part of Walvis Bay gets the smell from the fishing factories, hence the joke about its value.
Although the trough as a conveyor of moisture was strong, the moisture itself was minimal. But as the system saturated towards the south, there were reports of widespread, light rains in the area between Keetmanshoop and Aussenkehr. Further north, the trough lead to light showers typically of less than 1mm witnessed by the rain in Windhoek early Thursday morning, and the isolated showers over Damaraland.

What’s Coming?
The cold front pushed along by the South Atlantic High Pressure Cell crosses the Cape over the weekend. Airflow across Namibia will first switch from north-west to due west before resuming its typical south-westerly flow.
The really cold air is mostly behind the leading edge of the cold front, so as thecold front migrates to the east and the wind turns to south-easterly by Sunday, the nighttime temperatures will see a marked drop. The approaching high pressure cell behind the cold front will control the weather for most of next week, leading to the first significant occurrence of fog at the coast, from the dunesea to north of Swakopmund. This system moving in from the Atlantic Ocean dispels the trough, leading to cold, damp conditions at the coast. No Oosweer for the next week.
Across the interior, a typical early winter pattern sets in with the wind backing from south-east to due south, to east during the course of the week. This wind from the east is also the source of the first winter cold that will move in over the Kalahari, Omaheke, and the eastern areas of Otjozondjupa.

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