Weather 28 August 2015

What Happened
For the duration of this week, the dominant airflow over the entire country was from north to south.

This is the result of the stationary position of the southern Indian high pressure cell. Its core has remained stuck some 2000 km south-east of Madagascar with very little indication that it intends moving east.
The core of a high pressure cell is an area, often covering between 1000 and 2000 km where the air is dense, cold and stable. However, this whole system very slowly rotates, in the southern hemisphere, in an anti-clockwise rotation. Further away from the core, as can be explained by simple physics, the speed of rotation picks up due to the larger radius, the air moves faster and the barometric pressure drops. In winter, the cores of the high pressure cells in the southern hemisphere typically reads around 1032 mB decreasingly gradually towards the outer rim to about 1016 mB. This is regarded as the outer limit of the high pressure cell.
When the southern Indian high pressure cell is sedentary, as happened this week, its northern rim (1016 mB) has a marked impact on eastern Africa. Along the northern rim, air flows from east to west, entering the continent along the Mozambican and Tanzanian coastline. This air then moves across the continent, still driven by the southern Indian high, but slowly curving towards the south as it crosses Zambia, the southern DRC and Angola. Eventually, the airflow completes the circle and it enters Namibia from the north along the Angolan border, and from the north-east along the Zambezi. It is this anti-cyclonic circulation which advected moisture across a very large part of Namibia this week. Unfortunately, the cloud base sat at around 14,000 feet and the clouds themselves were very shallow with the cloud tops hardly exceeding 25,000 feet. But it did cause some very light rainfall over the interior and towards the south-east along the Botswana border.
Meanwhile, the core of the South Atlantic high pressure cell sat at the beginning of the week some 2000 km offshore, in line with Lüderitz. As the week progressed, the high approached the continent, encircling its southern coastline and gradually covering the Cape interior. However, early in the week, a so-called cut-off low has developed west of Saldanha Bay in South Africa, weakening to a very large extent, the impact of the South Atlantic high. This cut-off low controlled the weather in South Africa but with little impact in Namibia other than boosting the trough in the mid levels, in other words, boosting the north to south airflow across Namibia.
However, as the South Atlantic high sent out an extension across the southern Cape, another very strong vortex formed some 2000 km south of the Cape. This dissipated the approaching cold front, literally sucking it away and preventing a severe cold intrusion on the continent. But by Friday, the proximity of the South Atlantic high has pushed the entire system to the east, the trough having shifted to the Kavango East and Zambezi running south across the interior of Botswana and into South Africa.
What’s Coming
What was a menacing cold front four days ago has dwindled into a minor cold intrusion which should reach the Karas Region during Sunday evening. For the weekend, the airflow from the south is strongly opposing the airflow from the north. By Monday, the Karas Region will be substantially colder but the impact further north should be limited.
This intrusion migrates quickly to the east and by Tuesday, the cold will cover the south-eastern quadrant over most of the Kalahari along the Botswana border and further south up to the Orange River. On Monday and Tuesday, the wind will be fresh from the south, backing to south-east, and then due east by next Wednesday.
The southern Indian high is not expected to budge, remaining south-east of Madagascar opposing the impact of the approaching South Atlantic high. Along the western escarpment and over the interior from north to south, where the pressure differential is at its highest, it should lead to windy, dusty conditions on Tuesday and Wednesday.