Rikus Grobler | Oct 18, 2017 | 0
Weather 05 June 2015
The strong low pressure combination of a week ago shifted to the north as it approached the continent, reaching the Western Cape at the beginning of this week. This system developed into a proper vortex, increasing in intensity as the week progressed.
It gradually shifted towards the east taking its wet windy conditions away from the Cape Peninsula across the southern Cape and eventually to the Eastern Cape and KwaZulu Natal by Friday.
This vortex became so intense, by Thursday its core pressure only measure 992 mB, indeed very low and indicative of very turbulent surface conditions south-east of the continent.
Meanwhile, on either side of the vortex lay the South Atlantic high pressure cell (west) and the Southern Indian high pressure cell (east). The vortex lay between these two, driven from the west by the Atlantic high but it was the absence of high pressure between the two cells which created the channel for northward advection of very cold air. This reached Namibia at the beginning of the week, first bringing cold foggy conditions to the southern Namib, then gradually shifting towards the east, bringing intense cold to the Karas and Hardap regions, with accompanying cloudiness over the Orange River valley.
As the vortex moved to the east, it huge conveyor belt action continued to bring in cold air from the south, covering most of South Africa, then moving across Botswana, and by the end of the week, as is typical for Namibia, taking cooler air all the way north to Owamboland, the Kavangos and even the Zambezi.
By the end of the week, conditions have more or less settled. The southern Namib has cleared but remained very cold at night. Airflow over the Karas Region stayed south to south-west but for the rest of the country, airflow reverted to east along the Botswana border and prominently north-east over the northern half. This lead to slightly warmer daytime temperatures with both Rundu and Katima reaching the upper twenties in the afternoon.
The impact of the vortex, however, continued for the whole week and nighttime temperatures, even in the northern regions, consistently went below 10oC.
The weekend starts with a very strong and expansive South Atlantic high stretching across an area of almost 6000 km in diametre. Its western edge lies midway between Africa and South America, and its leading eastern edge slowly creeps around the southern limits of the continent. It drives a major cold front (between the vortex and the high), continuing up along the eastern half of southern Africa, taking its cold air even into southern Zimbabwe. The core is strong, reading about 1030 mB and elongated, covering a distance of more than 2000 km.
The Southern Indian high is comparatively weaker reading 1024 mb at its core. The northward push of the vortex remains prominent but as it crosses south of Madagascar, its immediate continental impact wanes, and conditions over Namibia during the weekend should be warmer with pleasant afternoons.
Temperature wise, the country is split along the Ruacana – Mata Mata line. South-west of this line, nighttime temperatures will continue to go below 10oC, even as low as two or three degrees, while north-east of this line, nighttime temperatures will remain above 10oC.
As the South Atlantic high shifts towards the east, the airflow in the Karas and western Hardap Regions will be prominently south to south-east. By the beginning of next week, the southern Namib, again can expect windy, cold conditions while the remainder of the country will be mild.
In general, Monday, Tuesday and Wednesday should bring us warmer days still, but by Wednesday evening, another cold intrusion will come from the east across Botswana. This follows the northern, east to west, circulation of the passing high but the effect is not expected to be as severe as this week’s cold.