Understanding Weather – not predicting – 01 March 2013
This week saw a continuation of the very dominant high pressure system covering most of the southern Atlantic Ocean and southern Africa. This synopsis has been in situ for many weeks and it is noticeable that leading meteorology stations across the globe, do not concur on the reasons. From primary as well as instrumental observation, it is clear that some promising built-up of cloudy conditions takes place in the lower levels between 10,000 and 20,000 feet with some cumulus convection reaching 25,000 but above this level, rain-making conditions cease to exist. It also severely restricts the rainfall potential of the cloud band beneath 25,000 feet. It it were a normal season, cloud convection should easily push into and above the 40,000 feet level. But indeed, this is not a normal year.
Weather station input provides both surface and upper air data. The set of charts produced tell the meteorologists the what and why of the prevailing pattern: the cause, occasion and effect are duly analysed and explanation is forthcoming. With an absence of data, those who need to know are left clueless. Such a situation is now in force. It is obvious that the conditions in the upper air prevent adequate rainfall but there is no consensus opinion why this is the case
Visual observation sees disappointing cloud development widespread across Namibia. The presence of a stubbornly persistent dry airmass in the middle to higher levels prevents fulfilling cloud development. Instrumental analysis of the air both above and adjacent to us is to all intents and purposes non-existent. A presumed cause lies in the realms of the high upper air, the stratosphere in effect, but with no detail to work from, this remains just a presumption.
A moister surface advection enabled localised convergence storms to develop during Tuesday. The narrow and shallow convergence belt reformed on Wednesday, but with only limited results.
The situation is now dire.
A cold front and trough pass south of the Cape and proceed eastward by the weekend with a semblance of a ridge behind it. But with limited upper and southward extension its ability provide a moist inflow is equally limited.
Each one of the charts show no weakening of the upper air core extending from the western Atlantic through to our middle layers. That there is a slender layer of middle level moisture with sufficient turbulence to inspire some thundery activity beneath this upper system seems destined to persist also. An ability to break up this extensive upper core has yet to appear.
Local thundery showers can develop, but their intensity and cohesive spread remains limited for just about the entire country.