Guest Contributor | Jul 19, 2017 | 0
Weather 19 June 2015
The elongated potato-shaped extension of the South Atlantic high pressure cell gradually moved over southern Africa from west to east during the week. Yet the intense cold which was forecast for Wednesday never materialised. There are two distinct reasons for this.
The high pressure cell is of moderate strength reading about 1024 mB at its core. But it is not its relative strength that exerted the biggest influence, it is its immense sprawl. From beginning to end of week, it eventually covered an area reaching 5000 km across from farthest east to farthest west. Since high pressure cells are typically surface phenomena, it restricts the advection of cold polar air from the south. Highs are still quite cold but when not amplified on its leading (eastern) edge by a cold front, it affects mostly those parts of southern Africa south of the Orange River.
The second major mitigating factor was the presence of a fairly strong vortex some 2000 km south of Madagascar. The vortex rotates in the opposite direction as the high pressure cell and while the later tend to descend, the vortex reaches aloft. Therefore, it weakens the cold front and disperses it over a very wide horizon in the upper levels of the atmosphere, where it is by definition already cold. The vortex is a dissipating factor that conveys the cold front faster to the east, which is the reason why it missed Namibia. It crossed from west to east just south of the Orange River.
This was also witnessed by the substantial difference in temperatures between the Namibian inland plateau and the interior of the Northern Cape Province for instance. As the high shifted to the east, local airflow quickly changed from south to east, Wednesday to Thursday, and then to the north-east by Friday. The air coming from the east over Botswana is still cold, but it does not go below zero so it does not lead to frost.
Over the Kunene Region, the signature low pressure area became more pronounced as the week progressed. Lying on the north-western fringe of the high pressure cell, it brings in warmer air from the north, and it effectively splits Namibia into a western and an eastern half. The east is cold, especially in the mornings, while the west remains warm, reaching the upper twenties in the afternoon. This yin yan pattern is very typical for winter and is very typical for Namibia. This also creates a noticeable pressure differential between the interior and the northern Namib, which generates the regular Oosweer (berg wind) conditions north of the Kuiseb.
The South Atlantic high pressure cell covers the interior of southern Africa for the duration of the weekend. But the factors discussed above remain in place and Namibia, with the exception of the south-eastern quadrant, the Kalahari, will continue with cold nights but no frost, and pleasantly warm days. Due to the presence of the high, the days should be still with very little wind.
The Kalahari and the area along the Botswana border remain cold by night being exposed to the anti-cyclonic circulation along the northern ridge of the high.
By Monday, the high pressure cell over land splits off from the South Atlantic high, leading to a cold front approaching by Monday evening, making landfall during Tuesday, and drastically dropping the temperatures to zero and even to below zero by Wednesday.
However, another vortex has already formed south-east of the continent and the same mechanism that was at work this week, is repeated next week. The cold front, however, is far more intense and it must be expected that by Wednesday or Thursday next week, nighttime temperatures south of Otjiwarongo may be close to or below zero.
Airflow in the upper levels remain zonal meaning, the cold front will linger for three, perhaps four days, then it will be swept to the east.