The Week’s Weather up to Friday 10 November. Five-day outlook to Wednesday 15 November 2017
Synoptic Map of southern Africa at 06:00 CAT on Friday 10 November 2017
Source: South African Weather Service. Link: http://www.1stweather.com/maps_fx.php?wid=77107&prov=WC&metric=true&language=en&map=SYNOP#tabs-12
The early Friday morning synoptic map of the SA Weather Service shows a rather unconventional picture. The leading bulge of the South Atlantic high sits directly south of the continent on its way to become the southern Indian high. Its core, however, is displaced much further south than usual, straddling the 45°S latitude. This is very far south for any high pressure cell’s core and then to remain at a strength of 1024 mB is also extraordinary. It is indicative of warmer sea surface temperatures south of the continent and it may just have a big impact later in summer to break down the South Atlantic highs as they migrate past the southern Cape. At this latitude a core strength of 1032 mB would have been expected.
Note also the core of the next approaching South Atlantic high in the bottom left of the map. It too is southward displaced, sitting across the 40°S latitude. It is also relatively small. Once this core reaches the Cape in about five days, it will also move further southward.
The most important feature is the strong low pressure system south-south-east of Madagascar. This system started gaining strength earlier in the week as a normal depression ahead of the South Atlantic high. Once it passed the continent, it increased in rotational speed and the pressure dropped to 1000 mB. This is not extraordinary for low pressure systems below the high pressure belt but it is the exception for this time of the season.
What the low pressure system does is to bleed away the high. Normally, the high would be blowing up the Mozambican Channel and recurving over the continent, making landfall on the Mozambican coastline. This is the major driver of moist air from central Africa into Namibia. But when there is a strong low pressure system south of Madagascar, the high is siphoned into the vortex, losing its northward mobility, and the result is this week’s weather.
While Namibia saw ample cloud cover and the odd isolated 5-minute shower, the conditions were not nearly as positive as last week, all due to the weakening effect of the low pressure system.
The synoptic map shows further that the southern Indian high on the right is relatively weaker than the South Atlantic high. Although it sits at the normal latitude, its lesser strength reduces the operation of the so-called Indian Ocean Transport, the main climate mechanism for advecting moisture from the Indian Ocean into central Africa and from there into Namibia, Botswana and the South African interior. This is another reason for the disappointing rainfall this week.
The lower pressure over the central areas of the sub-continent is also not very well developed. This can be expected to improve as the season progresses. This low pressure originates from continental heat and it is the third element required to bring moisture into Namibia.
It is evident from the map that all three mechanisms needed to drive rain to Namibia, were this week not functioning optimally. For the remainder of the season, it will be interesting to watch how the interplay between these three drivers develop.
All in all, conditions were not very conducive, indicating that full-blown summer conditions are not here yet.
Although there is some instability in the middle layers of the atmosphere at around 18,000 feet, these conditions are not replicated on the surface.
During the weekend, lower pressure are present over the interior with two areas of very low pressure. These are in the north near the Angola border and in the Kalahari in the south-east. General airflow is from north to south but the pressures on either side of the continental low, are slightly higher. This should lead to an onshore airflow at the coast but windy conditions are not expected. The pressure differential is too slight.
The next South Atlantic high approaches by Monday but with little impact other than a quickening of the wind at Lüderitz. Airflow over the interior remains north to south.
By Tuesday evening the high’s core is within 100 km of the coastline and it should have a major impact on wind and temperature in the Karas region, particularly the southern Namib, and a limited impact in Hardap, but mostly below the escarpment.
By Wednesday, early morning temperatures in Karas and Hardap will be considerably lower with a fresh wind from the east. These conditions are however restricted to the areas south of Rehoboth. The northern half of the country will remain hot to very hot, particularly in Ohangwena, the two Kavangos, Bwabwata and Zambezi.