Guest Contributor | Aug 30, 2019 | 0
Weekly weather overview to 23 June and short-term outlook to Wednesday 28 June 2019
Visual: Synoptic may for southern Africa at 06:00 on Friday 23 June.
Source: South African Weather Service, www.weathersa.co.za/images/data/specialised/ma_sy.gif.
Two important weather features were present for most of the week. The interplay between these two larger systems produced some amazing local phenomena which were not entirely out of season, but perhaps slightly late. It also created the anomaly of very high temperatures on the coastal plain while further inland, surface temperatures were considerably cooler.
The first feature was the strong continental high at 1028 mB over two thirds of the sub-continent. This high is marked by the large red circle in the visual. It brought cold conditions to South Africa, Botswana and Zimbabwe. In Namibia is was noticed by cooler surface temperatures along the Botswana border but that is more or less where its impact ended. For instance, Grootfontein showed a regular early-summer temperature profile but only 300 km to the east, it was much cooler, especially in the early morning hours.
On the northern rim of the large continental high, the airflow was easterly, shifting to north-easterly over Namibian territory. At the 500 mB plane their was some instability, manifesting as alto level cirrus cloud over Western Zambia and the Caprivi section of the Zambezi region. The dominant airflow on the high’s northern rim is indicated by the pink arrows
The second feature was the well-demarcated trough just offshore the northern and central Namib. This trough was evident at both surface and mid-level indicating that it had a reasonably strong influence on the weather at the coast as well as deeper inland over the coastal plain. It is indicated by the blue arrow. The resultant pressure differential between Botswana and the Namib produced Oosweer conditions over the coastal plain which started intermittently on Tuesday but became stronger every day as the trough migrated southward hugging the coastline. This was amplified by the weak low-pressure system forming just offshore the central Namib, shown by the small blue circle.
By Friday the Oosweer reached full intensity with surface temperatures reaching the upper thirties even before midday. The offshore dust bleed was so pronounced that it showed up on satellite images.
The visual also shows the South Atlantic high pressure cell roughly 600 km offshore Oranjemund. The difference in strength between the continental high (1028 mB) and the South Atlantic high (1024 mB) indicates that Oosweer conditions will remain for a day or two.
On the Radar
Oosweer continues over the weekend but not as intense as on Friday. The trough starts breaking up in the north during Sunday but the airflow remains east. It will be less windy and with somewhat lower temperatures but it will still be Oosweer. Sunday evening should see the last of it.
Inland, the continental high stays in situ but weakens to about 1020 mB. This reduces the pressure differential between Botswana and the Namib, also contributing to a gradual decline in the intensity of the Oosweer.
By Monday, conditions have settled and the continental high is departing. Quiet,calm, hot days are expected over the interior with the dominant airflow from the north, helping to raise daytime temperatures to around 30°C in the north and the upper twenties over the central interior.
The South Atlantic high reaches the Cape during Monday night, bringing down temperatures in the Karas region on Tuesday, and spilling over to the Hardap and Omaheke regions on Wednesday morning.
With a strong south-westerly airflow along the coastline, no Oosweer is expected next week.