Weather Overview and short-term outlook to Wednesday 13 May 2020
Visual: Animated extract of daily airflow forecasts from Saturday 09 to Wednesday 13 May
Source: COLA (Center for Ocean Land Atmosphere) studies at George Mason University, US. http://www.wxmaps.org/index.php
As local weather transitions from late summer to early winter, the influence of the South Atlantic high pressure cell becomes more prominent. Both its presence and its absence have a bearing on surface conditions in Namibia.
As the week started, a very regular late summer synoptic ensemble was present. The South Atlantic high has almost completed its migration around the sub-continent, followed in its departure by lower pressures at the coast. On Monday this could be seen on the synoptic map as an offshore surface-level trough but still very close to the coastline. It stretched along most of the coastline from north to south. This trough reached south into the South African west coast area but gradually receded northwards as the week progressed.
Meanwhile, the South Atlantic high started morphing into the southern Indian high but not before forming a continental high over the high ground in South Africa and Lesotho. Once this high is in place, it controls weather directly in most of South Africa, but in a roundabout way in Zimbabwe, Botswana and Namibia.
Depending on how long it stays, it produces an area of higher pressure that this week covered, in addition to South Africa, also southern Zimbabwe, the entire Botswana and two thirds of Namibia. At its core over Lesotho the barometric reading was 1024 mB, a typically mid-winter reading indicating that eastern South Africa and Lesotho were both quite cold for May.
This extended high, also quiet at its core, has a distinct anti cyclonic circulation, more or less between the 1020 mB and 1016 mB isobars. It means that the air flows in a very large circle, starting in the Mozambican Channel then traversing Zimbabwe and Botswana before reaching Namibia.
For most of the week, the barometric reading over Botswana and Namibia above the escarpment was 1020 mB in the early morning. This is indeed a relatively high reading for this time of the season, and it was also exceptional that such high inland pressures were recorded so far west (up to the Namibian escarpment.)
Given the presence of the offshore trough, and the high pressure inland, a large pressure differential is created between the interior above the escarpment and the immediate inshore ocean. Following normal rules of physics, the atmosphere balances this difference with the air flowing from the high pressure to the low pressure.
The geographical area where this correction takes place is the Namibian coastal plane, i.e. the piece of land known as the Namib Desert between the western edge of the escarpment and the coast, roughly 140 km away. Depending on solar influence, this leads to strong winds and dust storms in the desert, and as it comes closer to the ocean, to what we know as Oosweer.
While the winds blow from east to west (because of the pressure differential) they also cross the escarpment meaning that they travel from a high elevation (1600 metres above sea level) down to about 300 metres above sea level closer to the escarpment and then down to zero at the ocean. In effect the air column within which the wind occurs comes down in height, compressing as it does so. Known as adiabatic compression it raises the temperature by roughly 1°C for every 1000 feet that the air descends (very roughly about 3°C for every 1000 metres).
This phenomenon produced very hot conditions along the coast this week. Temperature rose to between 37°C and 40°C depending on locality.
On the Radar
The visual shows expected surface conditions starting Saturday, 09 May, and then as they develop up to Wednesday 13 May. The first frame shows the prominent east to west airflow while the next approaching South Atlantic high is still far out to sea. The subsequent frames show how these conditions persist during Sunday but how the approaching South Atlantic high reduced the pressure differential and how this stops the Oosweer, roughly by Monday.
Monday evening, Oosweer should be present only along the northern Namib in the Damaraland / Kaokoveld area of the coastal plain in the Kunene region.
By Tuesday, conditions will revert to an early winter pattern with a fresh south-westerly blowing across the southern and central Namib with quite cold conditions in the Karas region.
By Wednesday, as is usual, the high’s core will be south of the continent with the colder weather then coming from the east. It will affect the south-western quadrant as far north as Otjiwarongo.