Weather 08 May 2015

What Happened
For the whole week, high pressure control reigned on the surface and in the mid levels of the atmosphere.

The major driver of local weather, the South Atlantic high pressure cell had its core over the ocean midway between Cape Town and Oranjemund. Its local impact could be felt from the weekend and throughout the week with lower nighttime temperatures. Over the interior, early mornings have become chilly with temperatures around 10oC. The high pressure control is so dominant that by Thursday, the high covered most of South Africa, Botswana, southern Mozambique, southern Zimbabwe and the southern half of Namibia. This indicates that the high pressure cell is very broad in extent with its core more than 2000km across.
Over eastern South Africa and Mozambique, it brought a few hours of light early winter rain to the Eastern Cape, Kwazulu Natal and Mozambique south of the Zambezi. However, the source of this moisture is the leading edge of the high as it migrates east and it is only the difference in elevation between sea level and the central plateau that leads to small amounts of precipitation as the airflow ascends up the eastern face of the escarpment.
This has zero effect on weather conditions in Namibia but it must be taken note of because the early winter rain so far east certainly suggests a not insignificant anomaly and it may herald a colder than normal winter for us.
By the end of the week the high was still very much in situ although its eastwards movement has created the space on its western side for a weak mid-level trough to develop from Aiona in Angola, hugging the Namibian coastal plain all the way south to the Orange River. In the north, the pressure is at its lowest, around 1012 mB while it gets progressively stronger the further south one goes. In the southern Namib, the barometric pressure is around 1016 mB which is also and indication of the high pressure impact from the south. The result is Oosweer conditions over the northern Namib while the southern Namib experiences strong southerly winds.
This high pressure dominance creates a strong zonal airflow in the upper air from west to east, indicating zero probability for late rains. In the mid level between 15,000 and 30,000 feet, the airflow is in exactly the opposite direction, being driven by the northern fringe of the high over South Africa. This created weak conditions for rainfall over the Zambezi although the actual precipitation was very disappointing.
With such a strong high pressure influence from the south, the local weather picture remains largely static. Over the southwestern quadrant, airflow is from the south while over the north-eastern quadrant, airflow remains north-easterly. This indicates that higher surface temperatures will continue over Namibia’s northern half.
What’s Coming
The high pressure cell slowly moves east departing the subcontinent over the weekend. Contrary to developments a week ago, the departing high is not immediately succeeded by the next South Atlantic high. By Sunday, a low pressure system has developed west of Lüderitz and it will lead to windy and wet conditions over the central coast. This low pressure system is driven by the next South Atlantic high which will be located about 2000 km southwest of Saldanha Bay over the weekend.
However, the approaching South Atlantic high is much weaker than the high of this week, reading only about 1020 mB at its core so there is much room for a trough to develop over Namibia from the Kunene all the way south into the Northern Cape. Although not a single forecast indicates any rainfall, that trough may just jump a surprise on us from next week Tuesday onwards.