Coen Welsh | Nov 14, 2017 | 0
Weather 13 May 2016
The week started with a weak low pressure system offshore from Cape Town. This system developed somewhat in intensity while the core of the cyclonic circulation shifted slightly northward. By Thursday, the core of the vortex, still rather weak, lay about 200 km offshore between Saldanha Bay and Port Nolloth with a wide extension to the north. This had a marked impact on local weather in the Karas and Hardap regions especially on the inland plateau above the escarpment.
On Wednesday the Namibian Meteorological Service issued its first warning of cold and wet conditions during Thursday and Friday for the Hardap and Karas regions, followed up on Thursday with another warning. However, eventually, it was only along the Botswana border and in the Karasburg district that some very isolated light showers occurred. Nevertheless, extensive cloudiness and windy conditions ranged from Leonardsville to the Botswana border and further south to Ariamsvlei and the Orange River. Beyond our borders, it followed a well-demarcated convergence line with most of the unstable activity in the Northern Cape just south of Upington.
Over the northern half of the country, conditions were more typical late summer with day temperatures still reaching the lower thirties north and west of Etosha.
On the bigger scale, it was noteworthy that the southern Indian high pressure cell remained considerably stronger than the South Atlantic high. The former sat relatively close to Madagascar with its outer rim bringing cool to cold conditions to the eastern half of the sub-continent. The clouds in the Hardap and Karas region followed a very interesting route. As is usual, the source of the moisture was central Africa, advected towards the south along a reasonably well-developed mid-level trough from southern Angola, across Botswana and into the Northern Cape. But due to the adjacent cyclonic circulation (the cut-off low over the ocean), this trough recurved over the Orange River valley, re-entering Namibian airspace from the south-east, and appearing as clouds in the Kalahari and Karasburg district.
The cut-off low, pushed along by the approaching but weak South Atlantic high rapidly migrated across southern Africa, and by Friday only a cold spot remained over southern Botswana and the Northern Cape.
Sea surface temperatures in the equatorial Pacific Ocean are fast receding with only a 1°C anomaly remaining over the western Pacific near Indonesia. But it is the subsurface temperature anomalies that must be watched closely. Currently, the anomaly is as much as -3°C and once this colder water reaches the surface, weather analysts in the United States expect a very quick reversal to a strong La Nina.
By all indications, this rapid transition will lead to a stormy winter for the southern hemisphere with rapid oscillation between colder and warmer periods.
During the weekend, the Karas region is expected to remain cold but the system moves fast and by next week, a more typical pattern emerges. The colder weather remains over southern Botswana while a well-defined low pressure system develops along the Namibian coastline, originating in Angola and spreading to the south, covering first the northern Namib, and then by Tuesday, reaching almost to Lüderitz. This creates the typical pressure differential between the inland plateau and the coastal plain with the resultant Oosweer. The further north along the coast, the more windy it will be.
By Tuesday, the country is split neatly into a western half and an eastern half with cooler stable conditions in the east and warmer, windier conditions in the west.