Understanding Weather – Not Predicting – 16 March 2012
While major weather patterns were evident around the hemisphere, Namibia experienced a mixed bag of lesser, but locally active, patterns.
The moist airmass across Angola developed minor circulations which were tapped by some weak mid-level disturbances so bringing moist patches, typified by visible cloud presence, across the country. The favourable circulation over the adjacent ocean drifted north. But the ability to have vortex-type circulation form above and adjacent to the coastline was another helpful development.
Meanwhile the remains of Irina grew above a warm water patch but, caught up by the push of an intensifying anticyclone to its east, was edged into a colder water area, fading within some 36 hours. A cold front passed the Cape and its driver, another active anticyclone pushed rather than squeezed around the Cape, forming a ridge northwards, clearing up the vortex-prone area in the Mozambique Channel. There did follow some maritime input across the land from this high pressure core, while the remains of Irina’s influence, a band of upper air (10,000 to 20,000 ft) anticyclonic influence edged its way eastward slowly migrating across Namibia.
After all this, what did Namibia get? -Some, but not much, rain across a quite widespread area and on different days. But for March, this week saw only a slow recovery from the less favourable rainfall which marked the first week of the month. During March, most of Namibia is suppose to get a considerable portion of the season’s rainfall.
The broad pattern shows several anomalies. A complex set of synoptic patterns around the far south of South America gave some 24 hours of easterly winds as intense as 700mb (10000 feet) at Port Stanley, Falkland Islands (60oS), while spawning yet another intense vortex: down to the 940 millibar core pressure, which passed our area some 48 hours later. The Indian Ocean kept the Inter-Tropical Convergence Zone active but the next Cyclone Joli-Koni tracked south, collapsing. La Nina’s support, the consistent Trades and the persistent 40oS anticyclonic cores, maintained their control with their north-south orientation.
Today, Friday has a cold front cross the Cape with a brief northbound wind- flow penetrating our south, at least, but mobility has the upper hand as the whole pattern sweeps eastward. Quite quickly, a weak middle layer anticyclone takes its place above the Orange River valley, slowly advancing east, while allowing an easterly flow, tapping Congo air of course, across the northern half, at least, for just about the full period (mid-week and more). The cloud band persists over Angola and eastward, linking with the ITCZ which has a new cyclone, Lui, form just off the Australian continent.
Above southern Angola, a weak cyclonic circulation draws away the surface moisture over Damaraland, only to spew it out in the upper layers where the anti-cyclonic rotation with its core above Botswana, puts it back into circulation over Namibia.
The South Atlantic High collapses, slipping past the Cape, and in so doing, allowing moisture from over Angola and the Congo, to be drawn southward across Namibia. This system, however, lacks depth, and rainfall will remain isolated, scattered, and of lower intensities.