Weather 14 August 2014

What Happened
This week’s synoptic progression gradually morphed into a typical mid-winter pattern. Early in the week, the last vestiges of the high pressure cell over the eastern sub-continent departed to the east and the South Atlantic high pressure cell moved closer inshore.

At the same time the core also shifted to the north, lying by midweek some 1000 km due west of Oranjemund.
By Wednesday, the outer rim of the South Atlantic high has made landfall, bringing temperatures in Karas and Hardap down sharply. By Thursday the colder air has moved as far north as Otjiwarongo with a marked extension along the escarpment in the west, and a deep penetration up to the Grootfontein district in the east. The central plateau, while cold at night, did not go below 5o Celsius as did the west, south and east.
In the second half of the week, the high pressure influence over the southern half remained dominant but in the meantime, the southern Indian high pressure cell has regrouped and formed with some strength south-east of Madagascar. This gave us the very familiar winter pattern i.e. a relatively strong South Atlantic high with its core some distance out to sea, an area of lower pressure over the inland plateau and a very strong Southern Indian high pressure cell east of the continent. This is a pattern that has repeated itself frequently for the duration of this winter, having started at the end of May. But the unique feature of the past season, is the regular appearance of a vortex south of the continent. At some points, it came close to Cape Agulhas, but in most of the sequences, it moved along at a latitude some 1500 km south of the continent.
This week a similar vortex formed with its core very far south of the continent. By Thursday the core lay some 3000 km south of Cape Agulhas, very much within the ambit of the Antarctic circle. From this core northward to the continent’s southern edge, a series of cold front have formed, linking up to create a contiguous front stretching from the Antarctic circle to the Southern Cape.
Ahead of this front, the airflow is prominently north to south while behind it the airflow is from south to north. This advection of cold air from south to north is driven by the southern rim of the South Atlantic high which is the reason the cold intrusion reached as far as Otjiwarongo and the interior of Botswana. Since the high is by definition cold and dense, the colder conditions replaced the prominent east to west airflow of the previous week, leading to mild and windless days.
Still, we are almost at the middle of August and the sunshine hours per day have now increased to 11 hours and almost 30 minutes depending on how far south or north one looks. So there is a daily dose of energy infusion leading to pleasant afternoons, but the moment the sun sets, high pressure control takes over and cooling is rapid.
What’s Coming
The streaks of strato-nimbus clouds that covered large areas of the interior on Wednesday and Thursday are only the precursor for much activity to follow. This cloudiness indicates the intrusion of moisture from the north-west but because of the high pressure presence on the surface, the cloud base is pushed aloft to the 500 mB level (around 18,000 feet). It is also noticeable that the entire system moves rapidly towards the east, driven by the zonal flow from west to east in the upper levels.
While Friday is still cool in the Karas region, the impact of the high pressure cell recedes as it moves over land. However, the South Atlantic high remains strong and stays more or less static over the ocean so the southern Namib will experience fresh, cold wind from the south-west, and the entire Karas region will have a cool weekend.
By Sunday the high pressure cell has again moved towards the east, covering much of eastern Africa and reducing in strength to some 1020 mB. This sets the stage for the signature anti-cyclonic circulation over the sub-continent, bringing in an area of lower pressure from the north. As is typical for Namibia, this lower pressure band enters the country from the Kunene mouth, then migrates southward along the coastline, covering most of the coastal plain. This effect spreads to the south and the east bringing warm conditions to the interior. By Tuesday, the entire country is below 1016 mB indicating warm days and mild nights.