Weather 07 August 2015

What Happened
The inshore South Atlantic high pressure cell collapsed in a spectacular fashion early in the week.

This created lower pressure conditions off the coast, in the order of about 1016mB. At the same time, the migrating remnant of the continental high pressure cell over the eastern subcontinent, remained in situ over the South African highveld with a strong anti-cyclonic circulation on its northern rim.
This high, although not very strong, at a nighttime reading of about 1024mB, but covering such a vast expanse of the eastern subcontinent, was still powerful enough to create a strong easterly airflow. The pressure differential between the lower pressure along the coast and the higher pressure over the interior lead to strong and continued bergwind conditions (Oosweer) along the central and northern Namib. Over the southern Namib, conditions were not as windy but the katabatic effect of air descending from the inland plateau to the coastal plain, still chased daytime temperatures to the high twenties.
The reigning winds along the central and southern Namib are mostly southwesterly to southerly. This is determined by the location and the strength of the South Atlantic high. But when it collapses, as started happening on Tuesday, it loses its inshore strength, creating space for the anti-cyclonic circulation over the continent to bring a band of lower pressure from the Kunene mouth southward along the coastline, usually up to about the Kuiseb river, but this week, much further south, into the southern Namib.
The collapse of the South Atlantic high was in no small measure due to the presence of a very pronounced vortex some 1500km south of Cape Agulhas. As it rotates in a marked cyclonic way, it weakens the eastern rim of the South Atlantic high, causing conditions that whisk the cold front away to the east, losing much of its density, and preventing sub-zero surface temperatures over the sub-continent.
The reciprocating interaction between successive high and low pressure systems leads to a strong airflow from north to south. As the western rim of the continental high departs the sub-continent, the eastern rim of the next approaching South Atlantic high switches this advection mechanism around, and the airflow turns south to north. This has happened repeatedly this winter with three to four mild days, followed by two to three cold days.
When the airflow is predominantly north to south, cloudiness appears, usually at about 18,000 feet as was witnessed on Wednesday.
In the upper levels, above 30,000 feet, the airflow is zonal flowing from west to east, quickly shifting the systems to the east, thereby reducing the cold spells to two days or three at most.
What’s Coming
The collapse of the South Atlantic high pressure cell means lower pressures will remain offshore for the entire length of the coastline from Oranjemund to the Kunene. During the weekend, airflow over the Zambezi, the Kavangos, Owamboland and the Kunene province will remain easterly, but the wind speed will subside as the pressure differential balances out. This is due to the high pressure cell over South Africa leaving the sub-continent.
Over the interior, the airflow will be north-easterly, backing to northerly, the further south one goes. Both Hardap and Karas should see a mild to warm weekend with the wind predominantly from the north.
Over the weekend, the next approaching South Atlantic high is far out to sea, almost 3000km south-west of Lüderitz. It drives a very wide cold front which should reach the Western Cape by Tuesday night next week. However, another vortex south of this system does exactly the same as happened this week, and the cold front moves quickly from west to east. Again, the local impact is limited to the Karas region, and a day later to the Kalahari and the areas adjacent to the Botswana border.
The South Atlantic high is displaced significantly to the south, by almost 1000 km which is another factor why the cold intrusion barely touches the Karas region.