Weather 15 may 2015

What Happened
As the South Atlantic high pressure cell slipped around the continent’s southern coastline towards the east, it morphed into the Southern Indian Ocean high, but it also shifted southward thereby limiting its anti-cyclonic impact over the continent.

Despite some influence in the upper levels, witnessed by high pressure control over the entire southern Africa, this only applies to the upper levels over eastern Africa. Over the western half of the subcontinent, especially on the surface, low pressure conditions reigned.
In Namibia, the southern and south-eastern quadrants experienced increasingly colder nights. These lower temperatures originate not from the south-west or south as would be expected, but rather are the result of colder Antarctic air that is circulated across the subcontinent, warming up slightly as it moves over land, but still remaining cold. This vast channel of colder air then recurves back over the continent entering Namibian airspace first from the south-east, and later in the week, from the east. This lead to night temperatures along the Botswana border to plummet with Gobabis posting its first reading below 10oC on Wednesday night. This is a typical winter pattern. It is somewhat anomalous that it is occurring so early but it portends a more intensive winter. This fact is also corroborated by the substantial precipitation over the eastern escarpment in South Africa, the Drakensberg massive, with early snow recorded in several places on its eastern and south-eastern slopes.
The big exception this week is that the departing high pressure cell is not succeeded by the next South Atlantic high. Towards week’s end, the South Atlantic high pressure cell was only a very distant promise, hugging the South American coastline. It will take about a week before it reaches the African coast. In this lull, much space is created for low pressure systems to develop and this is exactly what happened. By Wednesday, marked low pressure conditions have set in at the Cape and this gradually extended towards the north and the north-east. It brought much cloudiness to the Cape with a significant spill-over to Namibia where the first clouds were noticed on Thursday, first at Lüderitz, and then building up towards the east, covering most of the Karas Region.
In the meantime, a strong vortex has developed over the South Atlantic some 3000 km out to sea. This eventually developed into a very significant depression, forming a so-called cut-off low which lasted for about two days. It brought rainy conditions to the Cape and the cloudiness to southern Namibia. It also caused some strong westerly winds at Oranjemund and Lüderitz, spreading inland across the coastal plain.
Over the northern half, surface and middle level circulation remained north-easterly meaning the days warmed up quickly.
What’s Coming
The cut-off low weakens as it moves south and east and makes landfall on Friday. It drives a prominent cold front which should see continued wet, cloudy conditions over the Western Cape. As the weekend begins, a prominent trough develops from Angola, across Namibia and into the South African interior. This trough, bolstered by the low pressure control over the ocean, brings more cloudiness to the Karas Region. This grows in intensity as the weekend progresses advecting much moisture into the middle and upper levels above 18,000 feet. This intrusion migrates towards the east but it also grows in intensity with the result that by Monday, much cloud will develop over the Karas and the Hardap Regions and it may extend as far north as Windhoek. There certainly is a small chance of light rain over the south-eastern quadrant along the South African border and along the Orange River.
The absence of a high over the South Atlantic creates space for the continental anti-cyclonic circulation to continue. The upper level high pressure control over eastern Africa drives this airflow. By Tuesday next week, as the coastal low pressure trough moves southward along the coastal plain, the pressure differential between the interior and the coastline should lead to increasingly strong Oosweer conditions along the northern coast. This will reach down to the Kuiseb River.