Namibia Economist (Windhoek)

22 February 2013

Namibia: Understanding Weather - Not Predicting - 22 February 2013

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What happened?

The complexity of weather gained some attention and, just to underscore it, we have been exposed to more of its intricacies. The focus is on the rather distant Mozambique Channel where anomalous warmer sea surface temperatures have held sway for some weeks. Tropical Cyclones tend to develop over such warmer water.

Assisting this eventuality, the past few weeks there have been limited anticyclonic cores passing far enough south of the Cape to first provide days of Southeasters at the Cape and second to establish a core to the south of the Channel ensuring a cooler airflow above this stretch of water while advecting a deep air flow into the sub-continent towards the persistent heat low.

The prevailing pattern is much more representative of a winter scenario, it is inconsistent with the patterns evident above both the Indian and Pacific Oceans.

This complex variation is not readily explained, one effect was the rapid development of Tropical Cyclone Haruna above mid-Channel, with another complexity: its shallowness.

Such storm cores dominate the airmasses from surface to levels as high as 200 or 150 hPa (40 to 45000 feet), but not this one!

Closed isobars should prevail throughout the Troposphere but the charts of the past few days barely indicate a cyclonic pattern above 400hPa, 25000 feet, yet the surface influence drew enough flow to build and maintain the cyclonic core. The upward extent would normally provide a tower of ascending air which spreads out, restricting adjacent upper convection for some hundreds of kilometres. To a limited degree this has also been evident.

This lack of synoptic cohesion remains evident where our upper air patterns are also in disarray compared with those evident above both the Indian and Pacific Oceans.

What we see is a promising middle layer assisting surface development only to experience dissipation as the up-thrust enters the drier, 10000 ft thick, range immediately above. It is of some credit to the turbulence level that occasional 10 or 20mm falls occurred.

For much of the country, quite widespread falls of limited intensity were the best this week could provide.

There is now a deteriorating aspect looming, virtually countrywide.

What's coming?

A major trough and cold front passing to the south manage to slip past the weakening Haruna, colder air following assists its collapse. The ridge building behind the front establishes a core between 35 and 40oS by Monday and for the remainder of the week is, at this range, expected to provide a deeper inflow, noted on the 850 and 700hPa charts (surface to 10000ft), into and across the sub-continent.

The similar time-range upper air analyses though show scant variation in the weak but dominant upper air control through the 500 to 400hPa (18 to some 28000ft) levels.

The ensuing rain outlooks are not optimistic, though the adjacent moist band persists over Angola and Zambia.

For the past week (at least) a curious fact has emerged: distant days of "better" rainfall prospects are consistently kept those few days away. The forecast remains patchy for the coming week.

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