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Understanding Weather – not predicting – 30 November 2012

What happened?
In daily weather terms, summer means an ability for moister air to penetrate into both upper air and lower level circulation, coupling with daytime heat to give convection and increased instability a chance to produce precipitation.
The result is bigger and better thunderstorms closest to the moist air mass while further away cloud development advanced the southern and western weather boundaries into the areas where upper air anticyclonic control has been present. The extension has seen both a weakening of the extent of this control and a thinning of its vertical extent, so giving the convection a more favourable airmass to thrust through.
Weather moves at a much slower pace than we mortals may desire, but the approach saw, variously, substantial (25mm or more) falls occur where, at this juncture in time they are to be expected. The Okavango catchment and adjacent countryside has benefited accordingly. In the more distant areas, the excessive heat formed cloud build-up, so limiting the insolation and its direct heat.
What we a re seeing, at this stage of the rainfall season 2013, is both the confirmation of the expected pattern coupled with an optimistic influence offering a more favourable aspect for the immediate future than these parts of Namibia would expect at this pre-Christmas period.
Now why?
The active trough departed the Cape, bringing cooler air northward while its eastward movement quickly established an anticyclonic core between 30oS and 40oS, ridging into the Mozambique Channel so lower level moist air surged toward the inland low pressure area. One way and another this pattern was to hold sway for the the duration of the week while feeding the evolving low pressure core appearing above mid-Angola and western Zambia. It might be early to identify this as being the southern presence of the Inter-Tropical Convergence Zone, but it is confirmed by similar developments in the rest of the southern hemisphere. Our synoptics are showing an increased rainfall probability, on a daily basis, across mid and southern Angola and western Zambia.
What’s coming?
As Thursday’s skies foretold, with increasing and thickening Cirro-Stratus and intermittent Cirro-Cumulus, moist upper influx is building. Rainy weather for Friday will be scattered across northern Namibia. Airflow patterns indicate a gradual departure apart from the northeast and extreme east. The further outlook maintains a generally northwest bias throughout the week,while active synoptics far to the south see recurrent ridges reappearing and advecting their lower level moisture into the sub continent.
A low pressure core remains over Angola for the period. This holds the promise of bringing moist air south over the central parts of Namibia. However, the South Atlantic high ridges in again by Monday leading to clear skies over the central and southern parts for most of next week.

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