Weather 05 August 2016
A well-developed frontal system crossed the sub-continent south of the Orange River. Ahead of the front, airflow was from north to south, bringing warmer days to Namibia’s interior. Behind the front, the airflow was south-west, returning us to cold nights from Wednesday onwards.
The synoptic map showed a typical winter pattern with a high pressure cell south of Madagascar, the South Atlantic high offshore west of the sub-continent, and a low pressure system in between. The two highs moved slowly but the low pressure system gradually gained strength, eventually forming a well-developed vortex over the ocean about 1000 km south of the Southern Cape. Both highs measured around 1028mB at their cores, indicating a balanced pressure system over the mainland.
By Thursday, the vortex lay south of Madagascar and the South Atlantic high has spread over a large part of the subcontinent, covering the South African interior, Botswana and Namibia south of Etosha.
However, during the week, the sunshine threshold has crossed the 11-hour a day mark for latitudes north of the Orange River. The daily solar irradiation plays a major role in the transition from winter to spring. Essentially, there are two opposing systems, – lower pressure with warm air from the north, countered by higher pressure and colder air from the south. These two systems typically meet over Namibia along a front called the convergence line. Most often this imaginary line runs from the Kunene region through the Namibian interior to the Karasburg district. In winter, the convergence line is often absent, or displaced towards the north, even into southern Angola, In summer, the line is usually in its typical position, tending to move towards the south as the summer progresses. This is the reason why the rain season in the south starts later than in the north.
This week was a very good example of the continuous struggle between the two systems. The cold intrusion was brief and as the high migrated to the east, it quickly moved from the Karas region to the Botswana border. Meanwhile, north of Etosha, the warmer central African low pressure system was in control bringing daytime temperatures, even in winter, to the lower thirties.
By Friday, the high has departed to the east with only a weak ridge over Botswana. This split Namibia nicely into a western half and an eastern half. Windy, warmer conditions are present in the west while colder, quiet conditions cover the east.
The global weather pattern has shifted to a neutral position in the transition from a warmer phase to a colder phase. The question that now bugs meteorologists is how far the pendulum will swing to the opposite end of the spectrum, i.e. how cold will the cooling phase become. This is just another way of describing the familiar oscillation between El Nino and La Nina conditions.Given that the southern hemisphere is approaching the transition from winter to spring, and that both solar duration and intensity increases incrementally each day, it must be expected that Namibia, generally will move to warmer conditions.
During the weekend, the Karas region remain cold at night with windy conditions over the southern Namib. The last remnant of the cold intrusion will continue to linger over the Botswana border with cold nights in Hardap and Omaheke.
By Sunday evening, the familiar low pressure system starts moving in from Angola, first in the Kunene region, and then spreading to the south and the east. This will bring windy conditions to the coastal plain of the northern Namib and hot days to the north-west, the north, and the north-east.
From Monday onwards, warmer conditions spread to the interior and the south with a weak mid-level trough developing from Angola to the Northern Cape. Expect some cloudiness over the interior from around Wednesday next week.