Understanding Weather – Not Predicting – 17 Feb 2012
Following the off-on-off rainfall pattern of the past few weeks, this week saw a triple-barrelled pattern take position in our skies. The predictability of thunderstorms held sway above the north-eastern parts, cloudiness appeared across a broad range covering northern, eastern and across the central high ground.
Such a weather base provides for light convective showers, at least. But for those areas away to the coast and across the south, the heat of summer prevailed.
In arid Namibia, the outsider can query this: at least there was some cloud and some rain.
In our case, the view from inside, February is a key month on our weather calendar. With a range of weeks where the rainfall prospects have been at best intermittent and with limited intensities being recorded more often than not, there is much doubt about the rest of the rainfall season .
Our climate range is dominated by the variability factor and that has taken centre stage during much of the current season.
We have also seen tropical cyclones active in latitudes from where moist maritime air should be advected towards the daytime heat of our south and west.
At the moment, despite favourable La Nina influence and Inter-Tropical Convergence Zone (ITCZ) presence very close to our northern border, tropical cyclone influence has kept an anticyclonic influence active in our middle altitudes, so restricting convective ability. With two contrasts on stage, extremes occur – reports note occasional tropical downpours while neighbours gain only the cold down draught.
Tropical Cyclone Giovanna has disturbed the desired pattern, but the ability to draw moist air recurs with the presence of the ITCZ latent.
As the week advanced, a significant vortex appeared over south-eastern Angola, well-marked from the surface up to the upper middle levels, above 20,000 feet, and remaining into the weekend. The inflow of moisture into the north-east was apparent by mid-week. From satellite imagery the convective aspect was prominent with subsequent widespread downpours across the northern, eastern and central interior.
There is mobility where the current patterns are concerned. Giovanna moves south rather than south-west while a developing anticyclone is in place, south-east of the continent, by the weekend. The equatorial wave weakens though as it drifts further west, but advected moist air penetrates into central Namibia. Meanwhile a weak surface layer inflow advects moister air towards our eastern, perhaps even south-eastern, parts and, more significantly, another wave appears over southern Angola with the expected ability to develop widespread thunderstorms across the northern interior, from the escarpment eastward. Rainy prospects remain positive note for the northern half of the country. As the high over the Kaokoveld slips south and then east, rainfall expectations for the Kunene Region improves.