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Understanding Weather – Not Predicting – 9 March 2012

What happened?
Apart from the north, this week was a dry period.
Favourable patterns were the persistence of active, moist layers across much of Angola with a weak low pressure area just of the Kunene mouth.An equally weak but persistent anticyclonic rotation over Botswana helped feeding some (limited) moisture into the Kaokoveld and Owambo. The combined presence would bring unstable air further south as soon as the opportunity emerged: they call it waiting in the wings and it is, overall, a pattern not unknown on our weather map.
The unfavourable features were the tropical cyclone Irina (the 9th so far) with a persistent vortex away to the south close to the ice.
Weather is generally mobile but it needs a trigger. Irina was just this, sucking aloft warmer Mozambique Channel waters and then driven by a higher pressure area south of the cyclone’s core.
A cyclone works like a huge vacuum cleaner. Being an intense low pressure area over many hundreds of kilometres over the ocean, it sucks up moisture and sends this into the upper layers above 30,000 feet. All that is then required is an anti-cyclonic circulation acting as the engine that drives this high level moisture over the continent and eventually as far west as Namibia. This was provides by a strong southerly airstream some 15000 feet deep across the eastern continent, supporting the descending air mass there, but ensuring that the ability to tap the moist band on its northern edge was not dissipated.
Meanwhile, the cold front’s meeting with Irina also initiated a move in the Southern Ocean. The ridge pushing the cold front along had a more southerly reach so forming secondary vortex developments and weakening and occluding the main vortex.
So with this improvement why was the rainy weather still absent?
Good question, but to get all these factors moving does take time to build, but as one can see today that build-up has extended quite deep into Namibia, simultaneously a trough-line (ITCZ) became identified with the broad cloud band reaching from much of Angola across the continent to the Indian Ocean.
What’s coming?
As this week unfolds, matters improve as two waves are predicted along the Angolan border (not lasting long) with advection into central Namibia where slack winds prevail. A weak upper trough  forms during the weekend above the northern coastline, inviting further input from the Angolan air-mass, helped by the flow around the weak anticyclone over Botswana. This should build showers over the north-western, and the northern half of the country as well as over the central interior.
A warm water patch enables an ageing Irina to be pushed west by the now more intense anticyclone, but Irina dies out (Monday) over the southern ocean. A strong flow on its western side curves round to become an easterly wind (for us) with some depth into our eastern airspace, but it does not upset the prevailing pattern. The cloud band persists over Angola and eastward. The oceanic Inter-Tropical Convergence Zone hosts a new cyclone, Koji, but it stays over mid-ocean.
Two cold fronts form in the western Atlantic but track south-east and weaken south of the Cape.
Thundery weather should reach much of the interior in the course of this weekend but with limited range, mainly the northern half, into early week. With the tropical influence, heavy falls, 25mm per hour or more, are possible later next week, from Tuesday onwards. Scattered rainfall will remain the order of the day.

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