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Weather 09 January 2015

What Happened
From before Christmas up to the middle of this week, the synoptic pattern was controlled by the extensive reach of the South Atlantic high pressure cell. The cell is not very strong but it has remained more or less in the same place for over a month. This creates higher pressure off the coast while solar irradiation over the interior creates a low pressure area on the surface. The result is the typical daily see-saw between heating and cooling. In the morning, the wind flows from the east or north-east then subsides, but as soon as the son sets, the pressure differential causes the wind to switch to west or south-west. It often leads to relatively cool nights, especially over the western half which is closer to the ocean from where the cooler air originates. Over the eastern half it is an entirely different story. From southern Angola through eastern Namibia into western Botswana, an area of low pressure develops on a daily basis. This is heat-induced and is also called a heat-low. With the reigning airflow from the east, this warm air is forced westward, and as it descends from the inland plateau to the lower lying areas, it has lead to the excruciating temperatures of last week and this week. It is commonly known as a heat wave. The broad swathe of warmer water between South America and Africa is still in situ and has grown in extent as the prevailing sea current in the far south Atlantic conveys this water from west to east. The result is that the cores of the South Atlantic high pressure cells are much reduced in strength, measuring only some 1020 mB, but these cores are displaced by at least 1000 km to the north, and the wider reach of the cell at 1016mB, over the past two weeks often reached right up to the equator. The week started with the core of the South Atlantic high some 2000 km due west of Walvis Bay. Under normal conditions this core should lie some 600 km south-west of Cape Town giving an indication of the displacement of this synoptic feature. A fairly well-developed low pressure area was situated just south of the Southern Cape. But by the end of the week, it has shifted towards the east, and a more conventional synoptic pattern presented itself on Thursday. This consisted of the high over the Atlantic, another high pressure cell just south of Madagascar, and two developing low-pressure vortices east and west of Madagascar. These latter two holds much promise for the next week.

Rainfall recorded on 07 Janurary 2015
Grootfontein 6.4; Naudabib Farm 5.0; Oshikango 4.6; Oshakati 3.8; Eenhana 3.0; Ondangwa 1.0; Outapi 0.8; Okahao 0.6; Tsumeb 0.3; Rundu Airport 0.2 (Source: Namibia Meteorological Service)

What’s Coming
The two growing vortices, either side of Madagascar, develop as the most important synoptic feature. At present, these two are only fairly well-defined low pressure areas but as the weekend progresses, they also grow in intensity. By Monday they reach tropical cyclone proportions and by Tuesday, they should be officially named, provided they grow into proper cyclones. What they do is to put copious amounts of moisture into the upper levels, up to 45,000 feet. Vortices act like funnels in reverse, scooping moisture from the surface by evaporation and advecting it aloft into the layers where moisture is required, that is at 700 mB (8000 feet) and higher, up to the 200 mB level (above 35,000 feet). Cyclones only develop over water when the temperature has reached 29oC. With warmer water continuing to move southward along the east African coastline, many cyclones may still develop between now and March, bringing some hope for a respite in the very dry conditions of the past month. Like all tropical cyclones do, they will slowly migrate southward, and may grow in intensity despite moving over cooler water. Limited rainfall is indicated for the southern half during the weekend, while the system slowly extends towards the north. The inter-play between high pressure and low pressure areas forms a shallow trough (low-pressure band) in the upper levels, stretching from southern Angola to the Northern Cape. By Monday, this system has grown in extent advecting moisture over the entire country. The most favourable conditions will however develop over the escarpment, particularly the Swartrand. Rainfall will however remain limited due to the proximity of the South Atlantic high pressure cell.

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