Understanding Weather – not predicting – 28 June 2012
Weather’s international dominance of the headlines took a step or two back during the past few days while there was a return to more normal patterns. For Namibians, more “normal weather” means a return to milder daytime temperatures for much of the country. Being winter, night-time radiation ensures temperatures in the single figure range but quite rapid warming ees the 20 degree Celsius levels are maintained for the high sun hours at least. Six hours of unremitting sunshine does much to maintain that warmer aspect. The normal feature is that within the sub-Tropical High Pressure belt clear skies and duration of sunshine will predominate.
The other description of such a climate is arid. Rainfall is limited and within the harsher geographical ranges close to non-existent for a considerable portion of any calendar year. From a variety of angles though, our climate and its norms vary from those given in text books.
The anticipated weekend activity came and went with only limited impact for us. The far south, closer to the active zone, saw cloud and some rain but the impact was limited by the rapid departure of the whole system and while the cold air drive was there, its northward flow was limited by this rapid movement. The colder stretch during Friday and its frosty night was already being swept aside. Any further northward advance was restricted to the eastern sub-continent. The weather potential subsided as the upper air patterns were taken over by an anticyclonic influence extending above the middle layers.
This is not an unusual set of events by our normal winter standards, but it displaced the robust activities of the previous week and thrust aside the prevalent range of synoptics to be found earlier, extending into the current time period visible across the remainder of the southern hemisphere.
Such a controlling pattern sees a reversion to the zonal flows associated with winter allowing only a restricted intrusion of the otherwise prevalent north-south-north flow to be found elsewhere in the hemisphere. While the La Nina orientation took a back seat, the prospect of its opposite number:the El Nino Southern Oscillation is but a muffled throb on the meteorological radar. Persistent zonal flows restrict the ability to advect moist maritime air of any depth into the interior. The upper air control serves to limit convective activity created by surface heating.
For the weekend, the main driving force finds itself restricted to an area far south of the continent. Barometric pressures over the interior are normal for winter, with a mid-level trough remaining a feature for most of the forecast period. With the core of the high pressure system rooted over the Atlantic, daytime temperatures should seek the low twenties. There is is little relaxation of the northerly drift across much of the country. A more active vortex appears during Monday around Gough Island advancing east but weakens as a secondary forms (Tuesday) in the cold, unstable air to its west. When the secondary steepens, a southerly inflow arrives by Wednesday. A colder outlook prevails, but rapid movement, typical of secondary vortices, may restrict its northward push again. Daytime warmth prevails, but frost at night, especially after Wednesday, marks the signature of early July.