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Understanding Weather – not predicting – 03 May 2013

What happened?
The strangle hold of the Atlantic high pressure cell in the upper air above 35000 feet has grown in spread as well as in depth.
A marked anti-cyclonic circulation was prevalent over most of southern Africa for most of the week. This created an area of somewhat lower pressure on the Kaoko coastline but the impact was mostly confined to the areas adjacent to the coast.
It also lead to a rare weather anomaly, at least for this time of the year. It created a diabatic effect over a large part of the central plateau and the north eastern third of the country.
The results was unseasonally warm afternoons as evidence by the very warm weather at the beginning of the week.
The presence of a strong high pressure cell to the south west, however, dispelled this effect by night, hence the relatively cool night temperatures.
This is autumn and the southern hemisphere have to do with much reduced sunlight hours during the day, so a gradual decline in temperatures are to be expected.
The oosweer at the coast continued intermittently as the airflow from the north east drives relentlessly towards the lower lying areas at the coast.
As the air descends, it compresses, releasing energy and the coastal residents have their wonderfully quiet and warm evenings.
If conditions so far this season are anything to go buy, this winter will be marked by two-day oosweer spells, almost every week.
There is no rain in sight on our weather horizon, in fact the dry upper air control is so dominant, all moisture as far north as Zambia and the DRC has been dispelled.
What’s coming?
As the Atlantic high on the surface slips around the Cape, it produces a strong airflow from the south west on Saturday. This carries right up to the northern half of Namibia.
For the first time this year, night temperatures on the central plateau will go below 10oC. In the southern half it will be much cooler with night temperatures barely reaching 5 degrees. This is accompanied by very high barometric pressures, reminiscent of winter, and it continues through Sunday.
Higher surface pressures are always an indication of denser air, hence higher air pressure. By Monday, milder autumn conditions returns and stay for most of the week, much like a repeat of this week.
By the end of next week, however, another major high pressure intrusion is expected and temperatures will fall again. Daytime temperatures throughout the forecast period should remain pleasant reaching about 22 degrees in the south and 27 degrees further north.

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