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18 April 2014

What Happened?
A prominent trough developed over the previus weekend leading to the by-now signature airflow from southern Angola to the interior of South Africa. With Namibia sitting smack in the middle of this passage, the effects of the low pressure band was seen far and wide. Widespread rain of limited intensity fell from the north-west (Damaraland) to the south-east (Kalahari) with only the coastal belt (Namib), excluded.
The week was also marked by a noticeable drop in temperatures. Only the north and north-east managed to reach 30o C during the day while the central plateau remained in the low twenties. Further south, daytime temperature barely reached the lower twenties while night temperatures went as low as 12 degrees.
Airflow in the trough (low pressure band) was from north-west to south-east, more typical of a late January synoptic pattern, but the rainfall was limited. The main restricting factor is the influence in the upper air (above 45,000 feet) of a strong high-pressure cell situated some 1000 kilometres south of Madagascar over a large patch of southern ocean that is much colder than it should be. The source of the moisture that gave us this week’s late summer rains, however, is the Indian Ocean north of Madagascar where sea surface temperature in turn, are a few degrees warmer than usual.
This close proximity of two substantial bodies of colder and warmer water has a well-defined influence on the general weather of southern Africa. For the past six weeks we have seen the monotonous repetition of a synoptic pattern that advects cold air from very far south, push it up the Mozambican channel where it meets its northern neighbour, the low pressure area over the western Indian Ocean, collects vast amounts of moisture, and conveys this back over the continent in a wide arch all the way from Mozambique, across Zambia and the DRC into Angola. From, there it is advected back south across Namibia as the sub-tropical trough from on the rim of the departing high pressure cell.

The repetitiveness of this pattern has given us an above-average rainfall season so far. Towards the end of this week just before Good Friday, the South Atlantic High pressure cell has cleared away most of the moisture that was present earlier in the week. On cloud maps, it shows a clear southern Africa all the way north to the DRC and Tanzania. But as the high pressure cell moves around the southern Cape, it again creates surface conditions that allow another trough to form over southern Angola, and start creeping south slowly. The further development of this massive rotation, will see this low pressure area cover most of the Kaokoland next week.
As the direct impact of the high pressure cell shifted to the east, so did the trough, and temperatures became warmer again for the latter part of the week. The rain-bearing trough is all but a memory, now situated over eastern South Africa.

What’s Coming?
The next cold front is about four days away, following on the rim of the high pressure cell that is currently (Thursday) just crossing Cape Agulhas. Good Friday and the rest of the Easter weekend will see pleasantly warm days but there is a small (less than 40%) chance of soft rain, originating from a tenacious influx of moisture in the middle levels (15,000 to 30,000 feet) from Zambia.
These clouds are visible as wide blankets covering very large areas but their tops are flat. No strong convection meaning no cumulonimbus (CBs) hence nice, clear and dense skies for pilots.
However, everything changes immediately after the Easter weekend when, on Tuesday, temperatures will fall rather dramatically. The early winter’s first approach of the so-called 540 isohyet, where it is zero degrees Celcius on the surface, lies only a hundred kilometres south of Cape Town by Tuesday, and the effect of the cold will be felt across the entire Namibia.

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