Understanding Weather – not predicting – 16 November 2012
Consistent daytime heat is an expected feature at this time of year, providing for daily development of a broad heat-inspired low pressure area over the mid and western sub-continent. The daily development of this low pressure area is what gives us our very typical hot and very hot early summer days. It usually lasts until around Christmas.
This low pressure area covers central, northern and north-eastern Namibia. Beyond our borders it extends over south-eastern Angola, western Zambia, western Zimbabwe, Botswana and the Northern Cape Province in South Africa. This area is vast hence its impact substantial. Its main feature is to create a consistent airflow crossing the subcontinent from the east to the west.
Geographically this inflow must ascend and penetrate a broad plateau with altitudes above 3000 feet, some 10oC cooler than sea-level and also with reduces barometric pressures. Generally, weather charts ignore this, giving us inland readings based on sea level conditions.
These descriptions clash with our recorded realities, yet match the descriptions offerred regarding our climate and weather. Our probable rainfall season begins about now. It is dogged by variability, yet supports a virile ecology. Persistent heat rarely exceeds the 40oC mark, even in our (lower altitude) south, yet the overall rain values (taken from a range of countries associated with drier climates) differ from those given and reflect our arid environment.
From another angle, we have continued to experience climatological divergence. The lack of cohesion between our lower level synoptics and the prevailing mid to upper air patterns has been dominant for yet another week, continuing at cross purposes with widespread descriptions of Trade Wind zone expectations: dry conditions prevailed throughout.
Surface anticyclonic development persist during the new week so providing ridges rounding the Cape and forming cores south-east of the subcontinent, but with only brief southeasterly wind patterns at the Cape itself. The resultant development sees limited inflow inland because there is limited cohesion between surface activity and upper air flows at, for an example, 10000 feet, 700 hPa. The higher ranges are dominated by and persist with an anticyclonic core extending far away westward to above mid-Atlantic. So much of the active convection is thwarted by this upper level control, too high for such convective currents to effectively reach, let alone penetrate.
So we have 3 weather zones prevailing: a dry west, a partly cloudy central and a thundery possibility for northeasterly parts. Any showery development must cope with this not so favourable upper air presence.
Meanwhile the water in the South Atlantic is slightly cooler than normal in a band stretching from Namibia to South America, but again, slightly warmer than normal in the area between South Africa and the South American continent. This strange occurrence is offset by the Indian Ocean where the water is substantially warmer than normal. This will lead in the next few weeks to more pronounced cyclonic activity especially above areas where the water temperature exceeds 29oC. It is an element to be watched as it may provide some respite to the current rainfall forecasts which indicate a below-normal season.