Weather 12 June 2015
The overall synoptic pattern has now settled into a steady progression of very typical mid-winter elements. At the start of the week, a medium-strength high pressure cell was stationary over the eastern half of South Africa. Offshore Namibia lay the core of the South Atlantic high pressure cell, also of moderate strength and only slightly displaced to the north.
Between these two highs, an extensive area of low pressure was forming, eventually culminating in a wide southward flowing trough that covered most of Namibia and extended to the south all the way to the Western Cape.
By Tuesday, the trough has become more prominent and it lead to windy conditions over the interior. A weak vortex was forming just off the Cape Peninsula with a marked cyclonic circulation. In the upper air, this vortex was leaning to the north bringing considerable wind to Oranjemund and Lüderitz on the surface, and promising cloudiness in the upper air.
The presence of extensive cloud over the Karas Region may have lead to some isolated showers, however, none was reported by the Namibia Meteorological Service. Unfortunately, these southern areas are so sparsely populated that it is quite possible that some precipitation may have occurred in the southern Namib along the Orange River Valley, and deeper inland up to the Kalahari Gemsbok Park, without a soul witnessing these events.
This broad trough slowly migrated to the east, and it brought significant rain to the Western Cape and to the Northern Cape but the core just missed Namibia.
From midweek onwards, the South Atlantic high pressure cell made landfall bringing cold conditions to the south, and as the high shifted to the east, so the areas along the Botswana border became progressively colder by night.
However, the extant South Atlantic high covered most of South Africa by week’s end, stretching in a long loop back to the west and covering almost half of the distance between the continent and South America. This is a area of more than 5000 km in diametre measured from east to west. What it implies is that cold (typical winter) conditions will prevail but this elongated, very extensive high pressure cell, although cold and dense, forms a very effective barrier between the continent and the strong low pressure vortices at the roaring forties. For Namibians, it means cold nights, since it is winter, but an absence of frost. When it does go below zero, it is usually only during the half an hour before sunrise so black frost conditions do not occur.
Given its extent, high pressure conditions remain in control over the southern half of Namibia, while the northern half enjoys mild to warm days. Barometric pressures from Windhoek further north, read significantly lower than in the south.
The high continues its eastward migration over the weekend meaning cold nights for the Karas and Hardap Regions. As a result of its considerable extension over the ocean, it controls conditions for the next four days. But the only visible cold front passes some 600 km south of the Cape, so the northward extension is limited.
As the high slips over the eastern part of southern Africa, it extends further to the north over the South African highlands. On its northern fringe, the result is a prominent airflow from east to west, i.e. the reverse of the direction over the southern ocean. This takes the cold air over Botswana and it enters Namibia from the east. This is also very typical for winter, the cold coming from the “wrong” direction.
So while, Namibia is isolated to some extent against the severe cold from the southern oceans, the high itself is cold and dense but not below zero. From Monday onwards it splits the country neatly into two halves. The eastern half from the Botswana border up to the Windhoek longitude will be considerably colder by night, than the western half. Towards Wednesday next week, as the signature low pressure band migrates down the coast from the Kunene to the Kuiseb, the pressure differential may lead to strong Oosweer on Wednesday and Thursday.