Overview for the week and 5-day outlook to Wednesday 17 October 2018
Visual: Core position and isobars of the southern Indian high pressure cell as on Friday 12 October.
Source: Mauritius Meteorological Service, http://metservice.intnet.mu/synoptic-chart.php
Although local surface temperatures were hot to very hot over the interior above the escarpment, the broader weather stance still showed distinctly wintery features. This was most noticeable in the South Atlantic high pressure cell’s slow migration across the southern extremities of the sub-continent, bringing substantial rain to the Eastern Cape and Southern Kwazulu Natal.
The visual for this week is the daily synoptic map of the Mauritius Meteorological Service. This may seem an odd choice but at this time of the year, it is the region to which Namibian observers must look for clues about the progression of the rest of the season.
Note that the southern Indian high’s core is prominent at 1024 mB and roughly 1500 km south-east of Madagascar. As the season progresses, the core will shift to the south, perhaps by as much as 1000 km, but that is not certain at this stage. What one must pay attention to is the position of the Inter-Tropical Convergence Zone (ITCZ). It is obvious that there are no indication of the position of the ITCZ on the Mauritian map. The reason is because it is uncertain exactly where the convergence zone is now.
Closer to mid-summer, the Mauritian map will indicate the ITCZ’s position every day prominently.
On the opposite side of the continent, the South Atlantic high was at a similar latitude earlier in the week, but with a slightly higher barometric pressure at 1028 mB. As the week continued, the South Atlantic high slipped around Cape Agulhas, at the same time strengthening to 1032 mB but with the core displaced to a position a few hundred kilometres further south. Still, it was strong enough to cause a strong system to develop over south-eastern South Africa.
From a Namibian perspective, surface conditions were controlled by the spill-over from the South Atlantic high with a relatively high cloudbase at around 18,000 feet, hence little to no rain. Further north, in Angola and Zambia, similar conditions were present with only patchy clouds and no rain.
It is only in the very north of Angola bordering the Democratic Republic of the Congo, that tropical conditions were visible, and this brings the ITCZ again into the picture.
At this time of the year it is often difficult to indicate its precise location, or at least in our case, its southern boundary. Similar to oceanic conditions east and north of Mauritius, continental conditions in northern Angola and the southern DRC, can only be inferred from circumstantial evidence. At this point, it is safe to state that the ITCZ, were it more prominent would have shown at the Angola DRC border.
The implication for Namibia is that the seasonal development has been fairly regular to date, albeit somewhat delayed. During the rest of the year, the ITCZ will slowly migrate southward until, by about Christmas, it should sit right on the Namibia Angola border. It is from this source of moisture that Namibia receives it regular summer rains in cycles of eight to nine days when low pressure incursions come from the ITCZ and penetrate Namibian airspace.
Once the Mauritian map begins to show the ITCZ with more confidence, conditions on the western side of the continent will also be more conducive for rainfall.
Against all expectations, the weekend will be slightly cooler south-west of the weak convergence zone that runs more or less from the Kunene through Etosha south-eastward to Aminuis. The cooler air from the southwest is the result of a strong low pressure system, currently just offshore, but which will cross the Northern Cape during Saturday and Sunday.
This means that Windhoek falls within both zones with a cooler Saturday and Sunday night but late afternoons will still be hot.
The low pressure system fizzles out by Sunday evening as it warms up while traversing the sub-continent and by Monday afternoon, it should be a fairly regular early summer day.
The passing system clears the entire Namibia of cloud and the days, for most of the country excepting the Zambezi and the remote north-western corner of the Kunene, should be clear.
By Monday, some high level clouds will cover Namibia’s northern half, but very similar to this week, the rainmaking potential is almost nill. For the rest of the country and for most of next week, clouds will mostly be absent. By Wednesday, afternoon temperatures will reach about 35°C in the interior with a likely chance that the north and the north-west can go to 38°C.