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Understanding weather: not predicting.

What happened?

September marks the divide between the remnants of winter's influence and the onset of summer. Terms like "Spring" and " Autumn" while often used are, more appropriately, descriptive of shades of either summer or winter.

The weather this week

was a result of the interplay between the prominent high pressure cell in the south Atlantic and a slightly weaker high pressure core in the Mozambique channel. The latter advected moister air from Angolan airspace across north west Namibia while the former is the engine that drove the movement of this moisture actross the central plateau and towards the Kalahari. The result was three consecutive days of moderate rainfall in a broad swathe from the Kaoko region to the southern stretch of the Botwana border.

Increasing day temperatures saw a weak surface trough appear between two anticyclonic cells. The anticyclonic circulation enabled a deeper air-flow to advance southward. Relatively high daytime temperatures provided convection which saw moisture turn into cloud.

Daytime heat at this time of year has meant values into the 30[degrees]C+ range for some 6 hours or so. Global Warming has given a gentle upward nudge which saw maxima above 37[degrees]C, even touching the 40[degrees]C mark. Such values are, by historical record, usually the preserve of October. The encroachment of the tropical air-mass is now part of the weather pattern.

Limited moisture in the lowest layers ensured the convective thrust could only tap middle layer moisture. But thundery weather ensued with quite considerable measures unofficially noted (unofficially implies no measured value available).

Historically the occurrence of early rain has a doubtful reputation. It is usually associated with the onset of El Nino conditions meaning that the main rain season the fizzles and disappoints.

But this new season is becoming more and more the preserve of La Nina with enhanced north-south-north circulation at surface and adjacent upward levels.

Sea surface temperatures across much of the Pacific Ocean are still below normal and have cooled by almost a degree in the last two month indicating La Nina is back.

What's coming?

The belt of thunderstorms slowly drifts towards the southeast but as it moves, the moisture is depleted and rainfall prospects over the Kalahari are rather limited.

Yet, the promising pattern of airflow between a distant anticyclonic core to the east and the south Atlantic anticyclone shows little sign of departure. Although prospects, based on computer predictions, offer a decline in thundery activity, the persistence of a static pattern presents various options. The key will be which level can dominate and draw in favourable moister air.

As the front end of the south Atlantic high pushes closer to the continent, it draws cold air from far south and pushes it across the Cape and further north over Namibia. Wednesday should see a marked drop in temperature. The further south, the bigger the effect.

But the influence of the anti-cyclonic circulation over Botswana and Zimbabwe remains and towards the end of next week, enough moisture will have entered the Kavango and Caprivi regions for a reasonable expectation of rain. After the influx from the south, general wind direction will return to east, then swing around to north and eventually north west.
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Author:Olszewski, John
Publication:Namibia Economist (Windhoek, Namibia)
Date:Sep 30, 2011
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