Understanding Weather – not predicting – 08 February 2013

What happened?
February’s rainfall prospects remain influenced and restricted by an upper air pattern which varies considerably from the expected template for this time of year.
Our geography places us within the range of the sub-Tropical High Pressure belt, so the expectation of drier air, at least intermittently, is quite reasonable. During the summer months though, the combination of daytime heat coupled with an ability to tap the adjacent moist air mass to our north does much to pressure the upper levels of this dry air to relax its grip, enabling rainy weather patterns to develop.
Currently, these links are tenuous.
Each day the quite favourable surface pattern appears: anticyclonic cores progress south of the Cape, broadening with their arrival off the southeastern sub-continent, ensuring a moist inflow frequently as deep as the 850hPa level, some 5000 feet above sea level. Daytime heat ensures the surface low-pressure area forms above the western subcontinent.
As this airflow invades, its moisture assists convection, clouds form and further activity leads to thunderstorm development. Further activity requires a favourable pattern aloft in what are called the middle layers. Such a pattern is identified by a layer of air within which there is instability, i.e. vertical movement within the layer. In such air, convection becomes more volatile and thundercloud development is imminent.