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Understanding Weather – not predicting – 18 January 2013

What happened?
Weather condition across the region were rather static for most of the week.
A well-developed and active equatorial low pressure core present above south-eastern Angola and western Zambia marked the southern limit of the Inter-Tropical Convergence Zone causing widespread rains across several hundred kilometres in the belt south of the Angola border.
It is the anticipated pattern of such low pressure cores to drift westward, being given a gentle nudge by the adjacent, equally persistent, Trade Wind flow and the rotation of the planet itself. Unusually, this equatorial core remains stationary. The associated rainfall belt remains within the convergent airmass and resultant falls are considerable.
This situation can be likened to our experience in 2011.
The seemingly complementary pattern is the cohesive upper air anticyclonic core holding sway between the middle layers and higher upper levels on the edge of the jet-stream that twists and turns from west to east above the surface core of the sub-Tropical high pressure belt.
Stationary, “blocking”, highs have made their mark on many weather maps. Their duration is usually limited to a few weeks, but the current example has survived from November. Although distant, the one from the other, there would appear to be a semblance of relationship, one waiting for the other to proceed or disperse.
In the Pacific Ocean, sea surface temperatures across the equatorial belt are now marginally below normal, indicating a return to a very weak La Nina while the area closer to the Asian continent remains slightly above normal.
Across most of Namibia, except the North and North East, intermittent inflows brought thundery weather into and across the country, but dry layers absorbed some of the build-up limiting the rainy intensities.
What’s coming?
The whole pattern seems poised for activity. Forecasts offer a more widespread spread of both moist air advection and resultant shower intensities, though the predictions limit an identifiable move of the equatorial pattern. They are more positive with an increased and maintained anticyclonic disposition both south of and around the Cape, so enabling increased lower level moist advection into and across the Namibian interior. The upper air picture is less responsive.
The evolving synopsis offers a continuance of intermittent thundery weather across much of Namibia. Distribution remains variable, but intensities, although also variable, are given an improved potential with a more widespread coverage.
The arrival of the main rain season for the central and northern half is still not strongly indicated.

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