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30 May 2014

What Happened?
Conditions across the sub-continent have been under the influence of two complex low pressure systems, the first of which announced its arrival over the weekend with windy conditions over the interior. In its wake followed two cooler days.
The second system crossed the Cape during Wednesday bringing a strong northerly airflow to Namibia but by Thursday this effect has dissipated and fresh south-westerly winds were experienced on the southern coast, and over the Karas and Hardap regions. The rest of the country remained under the control of weak east and north-east winds as a result of an early winter anti-cyclonic circulation with its core over Zimbabwe.
The signatory high pressure cells, both west and east of the continent were there but they were both rather weak, so the overall pattern was low pressure induced, marked by strong north-south-north airflow. These conditions advected the odd layer of cloud which is rather exceptional for late autumn / early winter.
The complexity of the series of low pressure systems and accompanying cold fronts can not be overstated. By Wednesday evening, no less than six different cold front were visible on the standard South African Weather Bureau synoptic map for 18:00. North of this procession of cold front only weak high pressure cells appeared, but at their expected latitudes, indicating a less aggressive expansion of the tropical low pressure belt. Scientists link this to budding El Nino conditions over the equatorial Pacific Ocean. Warmer sea surface temperatures mean more volatile equatorial low pressure cells with enhanced vertical velocity and less lateral effect. The result, which we experienced this week, is windy conditions, colder nights but pleasant days as the weaker high pressure cells (1016 to 1020 mB) are not as cold as the more intense cells (1024 to 1032 mB).
The successions of cold fronts south of the continent leads to opposing airflow over southern Africa.   The eastern half is dominated by airflow from the east and the north-east while the western half, particularly over Namibia, the winds from the north-east are opposed by the strong winds from the south-west. This creates a weak trough (low pressure band) flowing along the typical intra-continental conveyor lane from our north across the Kalahari into the north-western half of South Africa.

What’s Coming?
The low pressure cells remain the dominant feature during the weekend and into Monday and Tuesday.
Wind at the coast will be strong in the south, fresh around Walvis Bay, and weaker further north. At Oranjemund and Lüderitz a strong Southwester will blow for most of the weekend, veering south by Tuesday.
Continuing the pattern of this week, the airflow over the Kalahari, Omaheke, Caprivi, and Kavango will be north-easterly, leading to milder conditions.
On the western fringe of every low pressure cell is a cold front. As winter progresses, these typically shifts further north by the week. Next week Tuesday night, the first so-called 540 dam-line* will move as far north as Port Nolloth bringing in cold air leading to a marked drop in temperature. Colder conditions will remain over the entire country during Wednesday, Thursday, Friday and into the next weekend. * The 540 dam-line measures the vertical difference in barometric pressure, but it also doubles as an isohyet. At this line, the temperature on the surface is 0oC. North of this line it becomes progressively warmer while south of it, colder, i.e. below zero. The further north the line moves, the more severe the impact across Namibia.

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