Understanding Weather – Not Predicting – 19 January 2012
The severe heat of the previous three weeks was finally broken this week. The northerly flow attracted by the lower pressure, caused widespread rains over the central, eastern, northwestern, northern and northeastern parts. But except for Owambo, falls were generally lacking in intensity.
An equatorial low which would usually be found above Zambia became more evident in Angolan skies and, rather in keeping with the sequence of the past 4 years came much closer to our northern borders. The resultant vortex, more visible with its cloud patterns, steered an equally visible cloud band south across Namibia. Cloud bands of this nature invite thunderstorm development.
Cyclonic activity tended to fade and depart in the Mozambique Channel, but not for long, by Sunday a new vortex was deepening and attracting inflow from the ridge lying east of Durban, much of this moisture would normally have been fed in to the interior to feed, in the lower layers, the heat low. This was left to survive on what the equatorial vortex could attract and supply. But, as was becoming apparent, this equatorial low was still very much part of the ITCZ which became apparent, quite consistently, across southern Zambia, northern Mozambique and thence across the Indian Ocean to western Australia. Although this stretch covers the zone where one would expect to find it, its identity on both synoptic charts and satellite image was clearly obvious.
Another cold front approaches the Cape but there is only limited northward extension which, when the system arrives in line with the Tropical Cyclone, lacks the strength to draw the vortex toward the west wind flow and sweep it away. Meanwhile, the Inter-Tropical Convergence Zone and its active core survive into the weekend and continues the moisture advection southward. Although the core begins moving east during Sunday (along the Convergence trough-line) the moisture persists over the northern parts (except Kaoko) and cloudy, thundery weather remains the outlook for the week.
Further south though, cloudiness is to be expected with only limited prospects for rain; but bearing in mind the Congo air mass presence (although somewhat diminished), the chance or risk of heavier showers or storms is best borne in mind.
How does one assess a “heavy” fall of rain? Much of the definition work of the weather world was set in place from the stance of the Temperate Zone climate (where so much more “weather” occurs). By their standards Sub-Tropical heavier rains are phenomenal. In my analysis work, I have set the 25mm measure as being a reliable benchmark. Such falls or more are likely where and when such an air-mass is active. An absence of lower level moisture, in this instance, may see our version of the “phenomenal’ being avoided