Guest Contributor | May 20, 2019 | 0
Overview for the decad ending Sunday 03 June 2018 and 5-day outlook to Saturday 09 June
Visual: Infrared satellite image of southern Africa on Monday afternoon, 04 June
The weather discussion for last week was delayed deliberately for a proper assessment of the prominent frontal system that crossed the Western Cape on Friday evening, 01 June.
At the beginning of last week the daily synoptic map produced by the South African Weather Service still showed a weather scene in transition. Both the southern Indian and the South Atlantic high pressure cells were in their antecedent positions east and west of the sub-continent. Squeezed between the two cells was a strong trough running from Western Zambia southward across southern Africa, feeding into a powerful low pressure system about 600 kilometres south-east of Port Elizabeth.
It was noticeable that the cores of both highs were displaced northward with the South Atlantic high more so than its southern Indian counterpart. Backward ridging over South Africa from the southern Indian high did not happen in any strength leaving much room for warmer air from Angola and Zambia to penetrate deep into the South African interior. This lead to a flurry of rainy weather but all the action was far removed from Namibia.
As the week progressed, the South Atlantic high which was relatively close to Lüderitz and on the same latitude, made its presence felt as it first impacted conditions in the Karas region and then incrementally further north until, by the end of the week, cooler weather has set in as far north as the Etosha southern boundary.
Over the past weekend, the South Atlantic high slipped southward as it crossed over land, bringing winter cold to the southern half of South Africa, however the brunt of the cold was limited to the area south of the Orange River. The frontal system that was very evident on Friday decayed over the weekend and basically fizzled out once it reached the eastern side of the sub-continent but it did produce some winter rain on the east coast.
By Monday this week, the high has covered most of the sub-continent with a northward outreach that went as far as Tanzania.
On the visual, the remnant of the frontal system can be seen south of Madagascar with very little instability over southern Africa. The light grey areas are indicative of colder temperatures showing that, as can be expected, the South African highveld (east and south-east of the sub-continent) is considerably colder than the Botswana, Namibia and Angola interior.
The visual also shows that conditions over most of southern Africa was fairly calm but sunny and cold. The presence of the high over land prevented any direct intrusion from the south. Still, the high pressure cell is by definition cold and stable so the local cold experienced in Namibia was not the result of a blast behind the frontal system, but the presence of a very large column of sedentary cold, but not freezing (sub-zero) air.
Over the weekend, the highs core shifted to the south by a good 600 km. This is in part due to the warming effect of land despite the reduced number of sunshine hours, and the anti-cyclonic air circulation over the sub-continent which is a standard feature of the region’s weather.
As from Tuesday, the high ridges northward covering the entire South Africa, Botswana, Zimbabwe and even eastern Zambia.
This implies cool, stable conditions for most of southern Africa. There is some activity on Namibia’s coastline since the pressure differential between the interior (Botswana) and the coastal plain is substantial. This creates conditions for Oosweer, almost every day of the week. Windy and dusty conditions should be present over the entire coastline, with the strongest winds on Thursday along the northern Namib.
The outlook is for a week that gets progressively colder. Extensive ridging is expected over Namibia meaning very cold nights. The airflow will be easterly indicating that, as is very typical during winter, the cold will come from the east and not the south. Consequently, the areas along the Botswana border and the Gemsbok Park will be the coldest. By Thursday, even such unlikely places as southern Angola, Western Zambia, Zimbabwe and Mozambique will learn the meaning of winter for the first time in many years.
Frost, however, is not expected due to the prominent easterly airflow.
The next approaching South Atlantic high follows quickly. By Saturday it is close to Cape Town. The implication is that the entire sub-continent will feel a cooling effect this week and before there is opportunity for a slight warming up, the next cold spell will arrive.