Guest Contributor | Jul 29, 2020 | 0
Weekly weather overview and short-term outlook to Wednesday 22 January 2020
Visual: Animation of lightning over southern Africa on Friday 17 January for a one-hour window.
Source: AfricaWeather https://www.africaweather.com/storms . This material can be accessed on a non-subscription basis by selecting Storms in the drop-down menu on the africaweather.com homepage, then clicking on lightning and pressing the play button to activate the animation.
For most of the week overall weather conditions were rather static with little change from the beginning of the week up to Wednesday night. Moisture continued to enter Namibian airspace from Western Zambia and southern Angola but local conditions were not very conducive due to weak upper level high pressure control.
The result was widely scattered falls of brief duration and medium intensity across a very large part of Namibia but favouring the north-western quadrant, the central high ground and some isolated spots in the south. The general pattern everywhere was the same with falls up to 5mm in some areas and zero just five kilometres away. Convection remained depressed with very little lightning and where there was some thunder shower development, the storm did not last ten minutes.
As the week progressed, the South Atlantic high pressure cell grew in intensity, reaching a core reading of 1028 mB by Thursday. Fortunately the core was more or less in its regular position about 1000 km west of Saldanha Bay so the local impact was limited.
On the eastern side, the southern Indian high was fairly week, registering a core reading earlier in the week of only 1016 mB with the core somewhat further south than normal, just hugging the 40°S latitude.
The distance between the two highs’ cores was also relatively big allowing space over the sub-continent’s interior for lower pressure conditions to develop on the surface.
But a South Atlantic high of such strength does not go without local effect. By Friday the South Atlantic high’s core has built up to 1036 mB (a winter reading) and some strong winds, up to 30 knots, were recorded at the 1024 mB isobar, roughly 600 km west of Oranjemund.
This meant that the high pushed far northward along the Namibian coastline, recurving back onto the mainland more ore less at the Kunene river mouth. This led to clear skies over southern Angola and reduced moisture over Namibia’s northern regions.
In the meantime, the system that brought in moisture at the mid-levels (12,000 to 22,000 feet) earlier in the week, was pushed to the east by the Angolan intrusion of the South Atlantic high, running from Western Zambia through central Botswana in a due south direction, entering the northern Cape east of Ariamsvlei.
The combination of the strong south-to north wind along the coast and the fresh north-to-south flow through Botswana created a phenomenon that is not often observed. By Thursday night a weak cyclonic circulation has formed over southern Botswana, drawing in moisture from far north (Zambia) and circulating it into Namibian airspace from the south-east. As it moved into Namibian airspace from the east, it encountered the leading rim of the South Atlantic high (active on the surface) and a very large convergence zone formed.
This was best witnessed during Friday as shown by this week’s visual of the numerous lightning strikes recorded by satellite in only one hour.
This development, despite only favouring the southern half of the country up to the escarpment, is a positive sign for the rest of the season. It shows that as soon as mid and alto level high pressure control subsides, there is the potential for an active convergence zone to form, leading to enhanced convection, thunderstorms developing over large areas, and productive rainfall.
On the Radar
The South Atlantic high is expected to stay strong as it migrates around Cape Agulhas, however, it is not expected to move fast. In fact it is expected to remain south of the continent for the entire outlook period.
The high’s outer rim makes landfall somewhere during Friday night covering most of South Africa’s western coastline but the cyclonic circulation that started over southern Botswana keeps growing in strength, moving into the Northern Cape and from there move in a wide curve through the Karoo until it eventually reaches the ocean where it is expected to form a so-called cut-off low.
The high’s northward push will bring cloudy, wet conditions to Oranjemund on Saturday perhaps lingering into Sunday and pushing up the Orange River valley, possibly as far as Noordoewer. This system’s northern extension may even reach Lüderitz during Saturday.
For the rest of the country, rainfall prospects are not very favourable with all the outlooks predicting a relatively dry week for the Namibian interior.
The effect of the offshore cut-off low must however not be underestimated. Depending on its strength and its position relative to the mainland, it may draw in more Zambian moisture through Botswana, which has the potential to produce some startling surprises in the central east and south-eastern quadrant along the Botswana and South African borders.