Weather 30 January 2015

What Happened
The Inter-Tropical Convergence Zone is gradually developing to the western quadrant of the southern African subcontinent. Since the beginning of December last year, it has become very prominent over Mozambique extending out into the Indian Ocean across the northern half of Madagascar and for several thousand kilometres beyond that. But it has failed to develop over most of the continent except for brief intrusions that only reached as far west as Western Zambia. During the past two weeks however, it has extended westwards and became a prominent synoptic feature. This week started with a well-demarcated ITCZ stretching over almost 5000 kilometres from the Atlantic Ocean to the Indian Ocean. But it stubbornly remained just a whisker north of the Namibia Angola border, with only limited extensions southward. But with such a massive source of moisture on our northern border, it is only a matter of the right mix of conditions to see that moisture enter Namibian airspace. It has happened twice since the previous weekend, and it is witnessed by daily rainfall figures exceeding 60mm for several places in the Kunene Region. The effect became tangible when the Hoanib River came down in flood on Sunday, continuing on Monday, Tuesday and Wednesday. On Wednesday morning, Dr Flip Stander reported on the Desert Lion website that the Hoanib has filled the floodplain and broke through the dunes to the ocean. This momentous events was last witnessed in 1996 when all nine ephemeral rivers in Damaraland and the Kaokoveld reached the ocean. At the beginning of the week a fairly conventional synoptic pattern was present. The South Atlantic high pressure cell, although subdued, was in place with its core a few thousand kilometres offshore. South of Madagascar sat the secondary high with its customary south to north advection, driving airflow in the Mozambican Channel. The tropical cyclone in the Indian Ocean has died leaving the secondary high enough space to grow in intensity which it readily did, driving the zonal flow in the middle layers from Madagascar across the continent west ward to Angola. This helped move the ITCZ to its current position of prominence. Across the interior, low pressure conditions remained with a weakly defined convergence line visible from southern Angola, across Namibia and Botswana to the interior of South Africa. This is a fairly normal pattern for January and it brought widespread but limited rain to very large areas in Namibia above the escarpment. As a counterweight, the South Atlantic high pressure cell still tends to control surface conditions but its influence was only limited to the south-western quadrant. The powerful easterly airflow in the middle layers provided sufficient impetus to cover most of the country with moist air. Solar heating provided thae energy needed for convection with the resultant soft rain experienced in so many places. The lesser influence of the South Atlantic high is also shown by the cloud base which has come down from around 18,000 feet to just above or below 10,000 feet.

What’s Coming
The weekend starts with a prominent convergence line splitting the country into a south-western and a north-eastern half. This line runs more or less from Ruacana to Koes. West of this line it will be mostly clear skies and as the system moves eastward, cloudiness will also disappear over the interior and the eastern parts. By Monday and Tuesday only Kavango East and the Zambezi can expect some precipitation. The South Atlantic high pressure cell collapses to about 1016 mB at its core but this core is spread over a vast expanse of ocean. The surface temperature between Africa and South America is still elevated, leading to weaker high pressure systems but spread over much larger surface areas. Therefore, its size and not its strength leads to its local impact which is four days of reduced or zero precipitation, covering Saturday, Sunday, Monday and Tuesday. The effect of this surface high pressure control will be seen as far north as Opuwo in the west and Buitepos in the east. As the high pressure cell slips around the southern Cape, it becomes stronger, reading its customary 1024 mB when it is situated south of Madagascar. This helps drive the anti-cyclonic circulation over the subcontinent with much improved expectations for local rains towards the end of next week.