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02 May 2014

What Happened?
Surface level synoptics and alto level stream maps displayed a remarkable contradiction, but one that is not unknown for southern Africa this time of the season.
On the surface, more or less at 1000 mB, the airflow came from the east, backing to north-east, the further north one went. Early morning barometric pressure clearly indicated that the interior plateau and the Kalahari Basin to the east, were very much in the tight grip of a high pressure cell. This cell has washed slowly over the subcontinent pushing up from the south and eventually, as the week progressed, made its mark as far north as Tanzania. It cleared the lower levels of all moisture, but in its wake followed a moderate cold. At the northern end of this influence, zonal airflow from the Indian Ocean pushed the northward migration back over the continent, leading to easterly and north-easterly airflow over most of Namibia. The contradiction lies in the fact that at the alto levels, 40,000 feet and above, the airflow was in exactly the opposite direction. This created the observed contradictory weather pattern.
In short, closer to the surface, the airflow, and intrusion of moisture in the middle layers, still stuck to a late summer pattern while the upper air displayed all the signs of winter. The result is the end of the rain season with cool nights and pleasantly warm days as the sun’s energy get to work on the ground surface.

By mid-week, a strong low-pressure cell has developed some 2000 km west of Cape Town. This, of course, is the next approaching cold front. However, this low pressure areas formed further north than expected, and by the end of this weekend, the effect will be felt rather dramatically in the Western Cape, especially on the higher areas surrounding the coastal plains.
The cold front does not drive itself. This is done by the South Atlantic High Pressure cell behind it. But the low pressure cell is an area of clockwise circulation meaning it causes a north wind on our coastline, thereby bringing the Angolan low pressure area south along our coastline in a clearly defined trough of low pressure. It prevents substantial fog at Walvis Bay and Swakopmund, but it does create the ideal conditions for another round of Oosweer.
So while the cold front contains all the elements of a typical winter cold front, its circulation tends to drive the cold air south, away from Namibia. It is only once the high pressure cell behind it makes landfall, that the airflow swings around advecting very cold air from far south across the entire country.
Rainfall was very limited and only in a few spots in the north.

What’s Coming?
The low pressure area approaching Cape Town brings with it the expected cold, rainy conditions of the Cape. But the system has sufficient strength that all forecasts for next week indicate a possible spill-over to Namibia’s Karas Region. So do not be amazed, if by Tuesday, some clouds develop in the south, and that it actually produces precipitation.
It is still too early in the season to expect snow, but if the current pattern repeats itself a few times, like the summer pattern did, then it is not unreasonable to expect snow at Aus in July.
The synoptic pattern over the weekend, will be determined by the northern extent of the cold front. Expect a warm weekend, but get out the jerseys, by Tuesday next week, the daytime temperature will drop a few degrees.
A persistent north-easterly flow remains in the middle level with inflow from Zambia, so expect some scattered clouds in the sky towards the end of next week.

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