Weather 27 May 2016

What Happened
A typical winter pattern presented itself on the synoptic map early in the week. Both the South Atlantic high pressure cell and the southern Indian high were in their customary position, of moderate, equal strength with their cores more or less on the same latitude, 33°S, in line with Saldanha Bay.
Towards the middle of the week, the two highs became conjoined creating a continuous ridge that crossed the subcontinent from west to east, straddling more or less the Orange river. South of the ridge, it was wintery cold while north of it, the days were mild but the nights cold. This ridge produced an extension over central Botswana by Wednesday which gave Namibia its typical early winter pattern: cold nights in the eastern half, windy conditions over the escarpment, and balmy evenings at the coast.
With the high dominating the eastern half, lower pressure was present along the entire length of the coastline. Although not definable as a low pressure system at around 1018 mB, it was still lower than the Botswana ridge at about 1022 mB. This slight pressure differential is sufficient to create airflow from the east and in the absence of strong convection over the interior, the wind from the east eventually crosses the whole country from east to west. From the ocean, a wind blows in the opposite direction during the night towards the escarpment. It is at this line where the two systems meet creating the windy conditions for which the the escarpment is known.
However, as the sun progresses during the day, the solar irradiation has a remarkable influence, enhancing the slight pressure differential, so it is often found, as was the case this week, that the wind on the coastal plains switches direction, blowing in from the east during the late afternoon and early evening.
By Thursday, the elongated ridge has migrated to the east and a strong frontal system was on its way driven by the next approaching South Atlantic high.
For the duration of the week, three different frontal systems lay south of the high pressure system. These would normally have a strong impact on the subcontinent but the high acts like a buffer, preventing the intensely cold air from reaching too far north. So, while general winter conditions reign, the intense mid-winter cold that leads to black frost, is prevented from reaching the continent. The cold nights are a result of the internal temperature of the high itself, and not of antarctic air advected from the south. This only comes later in winter.
For Namibia, a very typical early-winter weak with a cold south, cold Kalahari along the Botswana border and warmer temperatures only north and west of Otjiwarongo.
What’s Coming
By Friday, the high is fairly strong reading an expected 1024 mB covering most of the subcontinent. This colder conditions reach into Namibia from the east, dividing the country neatly in a colder eastern and a milder, more windy, western half.
The core of the next approaching South Atlantic high is still more than 2000 km out to sea. Between the continental high and the South Atlantic high, a strong trough is developing. This trough convects warmer air from north to south, but it is closely followed by a strong frontal system, driven along by the leading rim of the South Atlantic high.
During the weekend, the weather settles into a typical winter pattern with cold nights and warm, sunny days. As can be expected, the southern regions, Karas and Hardap, will be colder than the northern regions.
Overall the whole subcontinent is under high pressure control, meaning the absence of strong winds, sunny days and cold nights. It is only by about Wednesday next week that the frontal system approaches the coastline leading to strong south westerly winds at Lüderitz and very cold conditions over the Karas region.

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