Coen Welsh | Nov 14, 2017 | 0
Weather 23 January 2015
With the Kaiser’s birthday on Tuesday, folklore has it the rain season only starts now. There may be more truth in this more-than-a-century observation since it was noticed last week, the synoptic pattern has finally come to life and started responding again to conditions many thousands of kilometres away from us. The strong tropical cyclone which lingered for about 10 days east of Madagascar, has eventually moved sufficiently south to subside and dissipate. By the end of this week, a new low pressure depression is gradually gaining strength, this time situated about 1500km east of Madagascar. The tropical cyclone weakened the high pressure cell south of Madagascar considerably but it also removed the obstruction to the South Atlantic high pressure cell to move east. This cell was stationary for over a month since the end of November last year. It lead to the dry conditions in Namibia and the excessive heat experienced both in the North and in the South. The South Atlantic high is now mobile again, and this can be observed readily on the synoptic chart for Thursday. Perhaps the most significant change in conditions is the upper air anti-cyclonic circulation over the subcontinent. The epicentre of this gigantic rotating wheel has been stuck far north over western Zambia and eastern Angola for about two months. Intermittently, this circulation was upset, and even displaced by the strong airflow from the south at the alto levels, but it has now settled into a regular pattern, much more in line with rain. The anti-cyclonic wheel now has its axle more or less at the confluence of the Sashi and the Limpopo which is exactly where it should be, if the conveyor mechanism on its western edge, from Angola into Namibia, is to operate effectively and advect moisture in the middle layers (15,000 to 30,000 feet) into our airspace. The impact of this very high altitude circulation was witnessed almost every day this week, when the mornings were cool and the sky mostly clear. However, as the day progresses and the temperature rises, promising clouds started forming on a daily basis. When the engine that drives this circulation is working properly, then we experience substantial advection of moisture, covering the entire country, except for the Namib. This was seen on Wednesday and Thursday when good cloud development took place from Maltahöhe southward up to the Orange River. As this system drifted slowly towards the south-east, it also brought clouds across the Karas Region up to the Botwana and South African borders.
Rainfall since last week Friday
16 Jan: Mpacha 46; Rundu AP 27.2; Rundu 21.1; Whk HQ 3.4; Gobabis 0.6; Omaruru 0.4; Gfn 0.2; Opuwo 0.1;
17 Jan: Rundu 0.2; Oshikango 0.4; Eenhana 0.1; Omaruru 0.1;
18 Jan: Mpacha AP 18.6; Rundu 8; Oshikango 3.8; Rundu AP 2.4; Oshakati 1;
19 Jan: Rundu AP drops; Oshikango drops;
20 Jan: Bagani 65; Mpacha AP 7.6; Maltahöhe 2; Rundu AP 0.8; Tsumeb 0.2; Rundu 0.2;
21 Jan: Gobabis 1; Ondangwa 0.6; WvB AP 0.6; Otjiwarongo 0.4; Rundu AP 0.2; Oranjemund 0.2;
Conditions are such that rain can occur almost any day in any given spot except the Namib. However, the system is not strong yet, and rainfall remains sporadic, erratic and of limited intensity. These conditions continue throughout the weekend. By Monday, a more prominent trough has developed east of the convergence line, and it should provide three to four days of very favourable rainfall prospects for the entire country. As the convergence line slowly moves east, first the west, then the south and then the interior clear, with similarly positive conditions only returning during the first week of February. The so-called inter-tropical convergence zone (ITCZ) which has been mostly absent, or weakly defined for the better part of six months, is now showing the prominence which lagged for so long. The ITCZ is now very strong and well-defined across eastern Zimbabwe, Mozambique, the Mozambican channel, Madagascar, and into the Indian Ocean for several thousand kilometres. The western side is only building now, but already this week the first signs of it sitting just across the Angolan border, became visible. During the next week it is set to grow in strength and intensity, which will complete the bigger picture for enhanced advection from Angola during February.