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23 May 2014

What Happened?
A brief cold front passed from west to east during Monday night and Tuesday. Typical for the airflow ahead of a cold front, the wind was mostly north to north-west but this rapidly turned around during Monday night, first coming directly from the south, then veering to the east as Wednesday progressed. It is also typical for airflow behind a cold front to be from the south, but as the front moves towards the east, the wind direction changes, first to east, then to north-east.
The rapid deterioration in night temperature was a result of a prominent ridge that extended from the core of the high pressure cell as it covered most of South Africa. A ridge is a high pressure extension marked by areas of lower barometric pressure on either side. This ridge covered a large part of southern Botswana on Tuesday and Wednesday and vestiges still lingered on Thursday although by then it has partially dissipated due to solar heat and moved to the east. Its departure allowed conditions to return to normal as was noticed by the improvement in night temperature later in the week.
By this Friday (23 May) a prominent low pressure cell has developed south-east of Lüderitz about 1000 km out to sea. It is relatively early in the winter season to have low pressure cells this far north, but it is probably an indication that the Cape will have a good rain season. It is also an early indicator that the coastline from around Sandwich Harbour to midway between Swakopmund and Hentiesbaai, can expect to receive spells of very heavy fog during June and July.

Low pressure cells are by the nature of the vertical airflow within their cores, the conveyors of moisture. If they develop over warmer oceans like the Indian, they bring in huge volumes of moisture that are then transported on the outer-most fringes of high pressure cells. However, when they develop over a cold ocean like the Southern Atlantic, their moisture is restricted but they do tend to have a significant impact on the coastal plains at elevations of less than 600m above sea level.
This is where the Cape winter rain comes from, the spill-over of which provides Namibia with its scant but ecologically valuable winter precipitation in the South. It is also the source of the heavy winter fog typical of Walvis and Swakop.
This low pressure area off our coastline also sets the stage for our so-called Oosweer which is determined by the differential in barometric pressure between the low pressure cell over the ocean, and the high pressure cell over the interior of the sub-continent.

What’s Coming?
If this week’s synoptic pattern was a case of stuck between two high pressure cells, the approaching pattern is the exact opposite. Stuck between two low pressure cells, temperatures across the interior should remain fairly stable with less divergence between day and night. This coming weekend, both Saturday and Sunday offer conditions that are conducive to mild Oosweer while the interior experiences colder airflow from the east.
By Monday, a weak high pressure area covers most of the subcontinent, running ahead of another prominent low pressure cell which will still be over the Atlantic by then. Being rather weak, the high has only a limited influence on night temperatures but it is the roaring low pressure area over the ocean that demands attention. Depending on how far north it may rotate, do not be surprised when by Tuesday next week, we have some pretty rough seas with a very strong west wind, on Tuesday and Wednesday.

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