Understanding Weather – Not Predicting – 10 February 2012
The New Year’s greeting was accompanied by the percussion of thunder, the flash of lightning and the steady beat of myriad rain-drops upon the roof. The seemingly slow build up created by the heat of mid and late December across much of the south and western country was ripened to fruition as the prevailing area of activity, in effect the Inter Tropical Convergence Zone, extended its influence south and westward across Namibia. This development was assisted by a bend in the upper air isobars around a seemingly weak trough which formed west of the Cape during the last days of December.
This pattern did not appear with much exuberance on the various maps and charts but there was sufficient influence to bring about a broad flow of Congo air as well as a reasonable surface flow from the east to ensure active cloud-building and convection. There was very little influence to thwart this combination of airflows. Although intensities took some days to really have effect, enough reports indicate heavy falls to emphasize the tropical air impact which this combination would be expected to fulfil. Taking somewhat of a back seat, the upper air core of the Indian Ocean surface anticyclone remained present above the eastern sub-continent. The presence saw our airspace as being part-and-parcel of its northwestern area of influence, meaning that the air flow maintained an easterly to northerly stream across the more westerly area of the sub-continent. As one might have seen, some cloud developments were restricted, while others thrust into and through this layer, but at the same time the airflow was advecting moist air from its adjacent Congo or semi-Congo air.
There is the feel that, once again, our rainfall season, November to April, is coming more and more under the sway of the strengthening influence of climate change. As one may observe world-wide, this influence is alive and well and living on planet Earth.
As we have seen across the 3 previous seasons, this rainfall pattern is capable of making its presence very obvious on our rainfall stage. The local effect would seem to be very much in keeping with situation found during this last northern summer across North Africa, where ITCZ presence was markedly further northward of the normal range and, as far as one can establish, accompanied by similar rains as one would expect.
Away in the Pacific, what must be an event of considerable effect, the current La Nina maintains its control and would seem to be providing, as one would expect, rains commensurate. Australian floods and trough-lines preventing Trade Wind flows from pushing further north than the 10S latitude provide evidence of the local level of control.
While the ability to rain has been present across the north of Namibia, the last few days of December brought the active rainfall season home to much of the north-west and central parts while the New Year promptly saw an extension further south and southeastward.
For the next few days, at least, there is little change expected in the overall rain-friendly weather pattern.
Congo air persists throughout adjacent to our north, if not extending quite frequently and in depth well across central and southern parts.
Rainfall prospects loom in similar standards to those which we have seen across the past week. The still remote upper air high may mollify some cloud building so that heavier falls may be limited but the moistness of the air lends itself to patches of widespread, softer rain.
Only the far southwest seems beyond the rainy zone