Weather 02 April 2015
A very complex synoptic picture emerged as the weekend’s weather dominated by the continental low pressure system slowly morphed into a more typical Namibian south-west north-east split.
In the bigger frame, eastern Africa remained under the dominance of the high pressure influence that originated south of the continent and migrated up the Mozambican Channel, again visible almost up to the equator.
At the beginning of the week, the interior was still dominated by low pressure. Meanwhile, the high pressure system over eastern Africa has acquired some strength leading to a noticeable drop in temperatures on Monday evening that persisted during Tuesday and Wednesday. The intrusion came from the south-east and was visible over the entire Namibia south of Etosha as thin Cirrus clouds at a cloud base of about 15,000 feet. Its prominence during nighttime is an indication of its high pressure origin. When the sun sets, warming of the land surface stops. This is the reason for daytime barometric pressure to decrease and it is a typical summer phenomenon. Solar heating starts the moment the sun rises and as it warms the air close to the surface, this air takes up energy, expands and rises. The result of this process is a reduction in air pressure. When the sun sets, the source of the energy is gone, the heating stops and when there is a high pressure system in close proximity, it exerts its influence again.
This see-saw movement between high and low pressure at night and during the day was beautifully demonstrated by the local weather this week. During the day, the airflow was northerly to north-westerly but early mornings it has backed to south-easterly indicating the presence of the high pressure system south-east of Namibia over South Africa and the Mozambican Channel.
By Wednesday, the next South Atlantic high pressure cell, has moved to its customary location some 2000 km offshore and its core in line with Saldanha Bay in South Africa. Its outer rime, the 1016 mB line, has just made landfall at Cape Town, bringing fresh conditions to the peninsula. The southern Indian high pressure cell also was situated in its customary position a few thousand kilometres south-east of Madagascar, but this one’s core was much further south. In between lay a prominent cold front driven by the South Atlantic high but not quite reaching the continent. Its presence was more marked in the upper layers above 30,000 feet. In front of this cold front, the airflow was prominently north to south, creating a wide and very long (low pressure) trough from the Angola Zambia border across Namibia, across South Africa, and deep into the southern ocean.
This trough acted as the conveyor of tropical moisture, bringing in clouds over large parts of Namibia’s eastern half with an extension towards the west on Wednesday. The South Atlantic high was now approaching but it was opposed, this time not by a low pressure system, but by the high over eastern Africa. It created a continuation of the existing conditions meaning the low pressure intrusion over the western half of the country remained, and it was again expected to penetrate far south on Thursday.
The South Atlantic high pressure cell slips around the Cape, gaining some strength to about 1024 mB. High pressure control remains over eastern Africa. Namibian weather is effectively split into a western half and an eastern half. In the west, the low pressure system crawls down the coastline, driven by the outer rim of the high over eastern Africa. Over eastern Namibia, high pressure conditions gain control.
During Good Friday, Saturday and Sunday, the low pressure system is expected to gain strength, stretching from Kunene across Namibia to Karas. Rainfall is indicated from the escarpment towards the Botswana border except for the Zambezi Region which will be dominated by the eastern African high pressure. The cloud base could descend to as low as 9000 feet during day meaning there is still much potential for rainfall albeit only scattered thunder showers. As the system migrates towards the east, the west will start clearing during the long weekend. For Easter Monday and the rest of next week, rainfall is expected only over the eastern half lasting until Tuesday, but thereafter the entire country is expected to clear.