Understanding weather – not predicting 20 september 2013
Weather’s complexity is a persistent aspect of synoptic patterns. Coupled to the variability of local weather, it presents a multidimensional range of possible outcomes.
Despite this highly complex system, the weather as we experience it on the ground, if often rather simple and we only relate to it in sensory terms: warm, cold, wet, dry, windy, etc.
This past week saw a fairly settled pattern prevail. Increasing heat by day with generally mild overnights and an absence of windy conditions.
Some activity brewed over the Atlantic but its impact on Namibia was limited. However, some circulation from central Africa did happen and very small amounts of rain were noted in a band from Ruancana southeastward to the Botswana border.
An independent system originating from the Cape, also lead to small amounts of late winter rainfall over the Karas Region.
This activity came to light during the weekend and built during the week with both higher pressure to the east and a deepening trough in approach from the west.
Such a synoptic situation is utterly normal, but the key is the other set of accompanying characterizations. The world of weather revolves around identifying these, assessing their cause(s), their development(s) and their result(s).
Wednesday saw both the moist advection and the deepening trough intensify as the areas of activity converge with changing patterns tapping cold airmasses away to south. It was capable of turning this cloud band from a mere moister layer into one within which thundery conditions built. Such activity caused local fires in the Witvlei area for instance. A more widespread range of local showers have been reported from the Opuwo area to the Witvlei area: Otjiwarongo 0.5, Sitrusdal 0.3, Tsumeb 0.5 as well as Okaukuejo and Khorixas also with some rain.
A new cold airmass is identifiable at the 850hPa level over the southern Atlantic Ocean. The approach of a dominant high pressure system leads to a colder Friday night but this is quickly dispelled during Saturday by the increasing daytime warmth, and by airflow from the northeast.
The cold air advance seems limited to our south during Friday, warming as an easterly flow returns, countrywide during the weekend.
Another frontal potential is expected by Wednesday closer to the south. Its weather-making potential is less certain at this distance but several forecasts expect light rainfall over the central and eastern parts on Tuesday, possible extending into Wednesday. Remember, the first spring rains always coincide with the Windhoek Show.