Weather 04 March 2016
The very delicate interplay between high pressure and low pressure systems was vividly demonstrated this week with the presence of a moderate high pressure cell immediately south of the southern Cape. This cell, reading only 1020 mB across its core, had an influence from the surface right up to the upper levels at around 45,000 feet. As the week progressed, the cell’s upper level influence waxed and it slowly crawled around the continent and into the Mozambican channel.
From the north came a sustained onslaught in the form a well-demarcated trough that ran from the surface, also across all levels into the upper air, stretching from central Angola, in a wide curve across the Kunene Region, through Namibia from north to south, and finding an abrupt end at the Orange River.
This highly complex synoptic pattern is rare and it produced much rain across Namibia for the entire week. At the beginning of the week, the bias was in the north and towards the west. As the week progressed, the bias of activity remained in the western half, but the strength and vertical extent of the trough relentlessly kept on advecting moist air from north to south. This shifted the core of the system towards the south where widespread and substantial rains were reported over the Hardap and Karas Regions, from the escarpment to the Botswana border.
What made this system so unique and intense? It is the presence of the moderate high pressure cell just south of the continent. This cell acted as a very effective barrier, damming the trough in the south, thereby containing the moisture in a growing low pressure reservoir that covered the southern half of Namibia. Without the presence and the vertical reach of this high pressure cell, the trough would have reverted to its customary position in the mid-levels and it would have flowed into the Northern Cape and then onwards towards the eastern Cape. This is a very typical pattern for southern Africa but it was upset by the high south of the continent. In a sense, the high is a barrier that arrested the southward flow of the trough, damming it up at the Orange river, enabling the natural instability inside the low pressure system to come to full fruition.
By Friday remnants of the system still lingered over the south with a slight intrusion into the southern Namib, another rare occurrence, but over the weekend, the bias shifts from west to east, and the trough gets pushed into Botswana.
The obvious consideration for a water-hungry Namibia is how long the system will remain.
The weekend begins still with lower pressure over the western half. In the meantime, the high south of the continent starts moving into eastern South Africa and extending northwards. These conditions carry on through Saturday and Sunday.
By Sunday evening, the core of the low pressure system has shifted to southern Botswana and the adjacent areas of Namibia and South Africa. The outer rim of the next South Atlantic high pressure cell makes landfall during Sunday night leading to lower night temperatures in the south and windy conditions along the escarpment.
By Monday, Namibia’s weather scene has reverted to a more conventional picture with high pressure dominating from the south-west and low pressure conditions over the Kavangos and along the Botswana border (northern section). Along the interface of these two opposing systems a strong convergence zone develops but is limited in vertical extent. This leads to continued advection of moist air from Angola, but at a higher elevation, i.e. above 12,000 feet. The convergence zone has the potential to produce scattered falls of low intensity but only in the eastern half. Rainfall prospects for the Kavangos and the Zambezi remain very positive.