20 June 2014

What Happened?
The balmy days of the early week gradually shifted to colder conditions as the incumbent weak trough was pushed away by the intrusion of the South Atlantic high pressure cell. By midweek, Lüderitz was first to bear the brunt of the approaching cold front as the 35 knot south easterly winds buffeted the harbour town so notorious for its cold windy conditions.
Whereas the first half of the week was under the control of the low pressure area, conditions deteriorated during Wednesday, turning noticeably colder on Thursday, eventually displaying a prominent southern half northern half split. The most visible effects were the strong south westerly winds south of Walvis Bay, and the opposing, strong north wind north of Swakopmund. Due to strong zonal flows in the upper air moving east towards and crossing Botswana, the latter part of the week displayed a remarkable two-worlds weather – very cold with scattered rain in the south, with warm, even hot, weather in the north. At surface level, the cold intrusion from the south west eventually became the strongest sensory feature, bringing cold air as far north as the Kavango River. As a portend of what to expect over the weekend, the passing cold front was pushed ahead by the leading rim of the most intense South Atlantic high pressure cell so far this winter. Already, at a distance of several thousand kilometres away from the continent, the approaching high had core readings of up to 1028 mB. As the cell approached the coastline, it drifted slightly south, picking up in intensity as it moved over colder water. By Thursday, the core had a barometric pressure of 1036 mB, indeed a clear indication of its intensity. This strength is what enabled it to advect cold air so far north.
Slightly out of the ordinary, surface winds first came from the south, then south east and later due east while the airflow in the alto levels, above 35,000 feet remained prominently west to east, indicating a relatively strong control of the approaching high pressure cell over the upper air system.
This is another reason why the effect of the cold front will remain longer over the interior plateau of the entire sub-continent. It must be noted that two weeks ago, a similar pattern showered hikers in the Fish River Canon with such heavy rain, they had to abandon part of the trail.
What’s Coming?
The weekend is firmly in the grip of the high pressure cell. But as is typical, it now finds itself located over the eastern and southern part of the sub-continent meaning the airflow backs from south to east, then north-east. Expect a few blistering cold days with the wind over the central plateau coming from the east. This high pressure cell is not followed immediately in its wake by a prominent low pressure area, as one would expect, but is strung out, south of the continent, linking up with another, weaker South Atlantic high pressure extension. Although the extended high pressure area is not as intense as the core, the combined effect will lead to a full week of rather unpleasant, very windy, very cold conditions. It is only late next week that the severe cold, for most of Namibia, is expected to be replaced, slowly by somewhat warmer air. Starting either late Sunday or during Monday, the pressure differential between the coastal low north of Hentiesbaai becomes very noticeable. This weekend and the early part of next week will again be marked by a strong north wind over the northern half of the coastline. Deeper inland, windy conditions, even up to 40 knots may persist over a very large stretch of the escarpment. That means unpleasant conditions from Kunene all the way south to the mountains of the Karas Region. As the coastal low shifts south, strong Oosweer will be experienced north of Hentiesbaai, becoming milder as it moves southward, settling into the typical mid-winter calm and warm coastal conditions further south, later in the week.