Weather 20 May 2016

What Happened
The local weather has shifted from an unstable late-summer pattern to a more predictable, more typical winter pattern. This week saw the swift migration of the South Atlantic high pressure cell past Cape Agulhas to a sedentary position due south of the Mozambican Channel where it morphed into the next southern Indian high pressure cell. In the process, the high’s core shifted south, finding itself some 2000 km south of Madagascar, even a few degrees south of the 40°S latitude. This is a significant southward displacement of the infant southern Indian high, mirrored by the rapid strengthening of the core as it moved to much colder water.
At the beginning of the week, while still west of the Mozambican Channel, this high had a modest core reading of 1022 mB. As it moved east and south, it grew in intensity and by Thursday measured a very impressive 1036 mB at its core. The South Atlantic high, at that point, measured a typical wintery 1024 mB but the core was not well-defined, and displaced far to the north. By Friday, it is covering the entire length of the south-west African coastline from Cape Agulhas to the Kunene river mouth.
Between these two highs, a strong trough developed, not so well defined at the surface level but very prominent in the mid-level between 15,000 and 30,000 feet. This trough showed a rapid rate of advection, taking warm tropical air to a point in the ocean south-east of East London, and just west of the strong southern Indian high.
When the southern Indian high is in close proximity to the African continent and the South Atlantic high is offshore Africa’s south-western coast, a remarkable pressure differential is generated in the so-called col between the two. This is what created the strong trough and the commensurate north south advection.
Alas, for Namibia, we are to close to the source of the advected air (Angola and the DRC) to see its full effect but as the system develops on its journey towards the south-east, it grows in strength with much instability and vertical activity, but unfortunately, very far from our territory. Still it is one and the same system.
All the indicators on the progressive synoptic maps show that the weather is now in winter mode. It is only the relative weakness of the South Atlantic high that prevented much colder weather, although the eastern Cape and the South African highveld experienced proper winter conditions. This was amplified by the strength of the southern Indian high which had an effect as far north as Tanzania.
What’s Coming
The strong southern Indian high remains in place for another day but by Saturday it is displaced by the South Atlantic high which then covers most of South Africa and a large part of Botswana. For the duration of the weekend, local weather remains under high pressure control through all levels from the surface aloft to the top of the troposphere. It brings cold but stable and sunny conditions across most of South Africa and Botswana from where it penetrates our realm. By Saturday evening, the cold coming from the east, covers most of Namibia’s eastern half.
Sunday morning will be very cold but not yet frosty. This cold will affect the entire country from north to south with the only exception the coastal plain below the escarpment.
The pressure differential between the interior and the coastal plain is expected to be mild, indicating an absence of Oosweer. The high will remain in control until at least Tuesday by which time only a sliver of lower pressure will be present over the northern Namib.
The next approaching South Atlantic high arrives on Wednesday, repeating in a similar fashion, the conditions of this week.

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