Understanding weather – not predicting 6 september 2013
Winter 2013 departed, Spring is here.
Arid climates like Namibia are known by their “Variability”; a description unique in the climate world. Coupled with our unique geography, this variability leads to a divergent range of weather characteristics when measured from south to north, or from the coastal plains to the high interior plateau.
“Variability” in both rainfall and temperature is the norm in local weather. Our experience is that while we have an identifiable rainfall season during summer and autumn, its trademark is still a high degree of variability across both time and place.
For instance, the Karas Region receives some winter rainfall as the winter rainfall systems that regularly cross the Cape of Good Hope push northward, but these falls are erratic and mostly patchy. Overall, this winter has fallen short where rain is concerned across our south. Elsewhere these three months contribute less than a mere percent of the overall; Spring doesn’t do much better, apart from November the input is some 3%. Effective rain further north than the Karas Region, is as good as absent in these 5 months.
Temperature: – cooled by the considerable elevation limiting most of the country to below 40oC maxima, the winter also can provide astonishing ranges on just a single day. This week gave such an example. While southerly daytime maxima struggled to reach the mid-teens, across our northern areas night-time minima were, on the same day, a degree or two above that level. This is also typical of the Namibian Spring and there is a touch of the global warming influence here.
Rain petered out by Sunday: Rosh Pinah Exxaro: 8 and 2mm: the 3 consecutive days of 1mm or more matches their previous best. Karasburg 2 and 0.1. Reports from as far north as Grünau told of 13mm. Witputs district had some 41mm, the result of orographic influence exemplified!
Successive troughs crossed the western Cape barely reaching our far south. Increasing daytime solar heat gave midday warmth for much of the interior.
The anticyclonic control saw a middle layer, 700 to 500hPa levels, flow tapping air from some 10oS. by Wednesday and thickened by Thursday with a thin Altocumulus layer as its trademark.
Currently, outlooks offer the same synoptics close to, but not entering, our air space. Progressive anticyclones push low level ridges eastward reinforcing the persistent Mascarene anticyclone: another situation most desirable those few months ahead!
The easterly surface flow persists, one way or another, for most of the interior. To our west is a perceived lack of coordination between surface level patterns and those of lower level upper air charts, 700hPa and above. Anticyclonic control to our south at surface level fades by 850hPa while an upper trough emerges by mid-week: persistent northwesterly flow invites this but this new upper flow is dry, with no absorption of maritime moisture. A shallow cloud layer is its best offering.