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16 May 2014

What Happened?
As the reigning cold front early in the week passed the Cape, it briefly brought nigh temperatures down but this was replaced by warmer weather on Tuesday and Wednesday, in the absence of a strong high pressure cell. Solar heating provided sufficient energy to make this a mild, very typical autumn week. But conditions are gradually changing to winter and this is evidenced by a succession of cold front, scheduled for the weekend and again later next week, and driven by the South Atlantic High Pressure cell that has also grown in intensity. As the cold front moved to the east, it gained considerable strength as its core settled some 2000 south east of Madagascar. As it grew intensity, it created a major south to north airflow behind it, amplified by the succeeding high pressure cell.

This force was so powerful that it carried colder, dry, antarctic air right up to the southern limits of Tanzania and the DRC. One might wonder what this has to do with Namibia, but the impact is not negligible. These strong airflows from the south, pushed the so-called Inter-tropical Convergence Zone so far north that its southern extremity barely reaches northern Angola. The result is that our usual source of summer moisture sits over 1500 km away from our northern borders, meaning absolutely zero advection of moisture. Needless to say, this is also the most discernible indicator of the end of summer and the transition to winter. Later in the week, the passing high pressure cell made its influence felt over eastern South Africa. With the next South Atlantic High Pressure cell fairly close to the coastline, it means Namibia was squeezed between two highs, both from the east and from the south-west. Again, this only contributed to clear sunny skies, for the entire country, but it did lead to colder nights, now also noticeable in the Omaheke region. It is a rather unique feature of Namibia’s weather that it is mostly influenced by the position of the successive high pressure cells during winter. As the new high makes landfall, it brings colder air from the south and as the departing high lingers over eastern South Africa, the airflow at its upper rim, brings in cold from the east, dearly felt in the Gobabis district and the Kalahari along the Botswana border. This weekend, we will have just such a tandem high position, so expect colder nights.

PS. See the summary of April and full season rainfall under weather, courtesy of the Namibia Meteorological Service.

What’s Coming?
The two high pressure systems dominate the weather. The approaching South Atlantic High makes landfall by Friday leading to a sudden drop in temperature and windy (south western) conditions on the southern half of the coastline. The preceding high remains over South Africa for the weekend slowly drifting to the east. Between the two very prominent highs is a weak low pressure area. This is indicated on synoptic maps as an extended cold front reaching up to the Orange River. However the cold front is not very strong, and except for the stretch of coastline Oranjemund to midway between Lüderitz and Walvis Bay, the effect will not be dramatic. For the rest of the country, i.e. the interior plateau, sunny day conditions with cooler, and even cold nights are expected from Sunday evening onwards for about two days. The cold front, does however, push along a trough (low pressure band), again reaching all the way from Angola to Agulhas. It is rather weak, and the source of moisture is depleted, so very little in terms of late-summer precipitation is expected. As the trough shifts east by Tuesday, normal autumn conditions return. No Oosweer in sight for a week.

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