Western Cape rainfall explained in anticipation of the next cold front


Photo by Lynette van Schalkwyk

The next cold front is heading for South Africa and is expected to make landfall around midnight tonight. The northwesterly winds will start to pick up tonight along the west coast, turning westerly as the front reaches the south coast on Thursday morning. The rain is expected along the south west coast at first, before spreading to the Western Cape interior whereafter moving to the Eastern Cape by the afternoon. High seas with wave heights of 6 to 9m are also expected along the south west coast from early on Thursday morning, spreading to the south coast by the afternoon. Cold fronts are responsible for most of the rainfall received in the winter rainfall region of the Western Cape.

The province is divided into three rainfall regions. A late-summer rainfall region, a year-round rainfall region and a winter rainfall region. The late-summer rainfall area includes the region around Beaufort West which receives most of its rainfall from late summer thunderstorms that occur between March and May. The year-round rainfall region includes most of the south coast and it receives rain when an onshore wind pushes moist air inland and up against the mountains in summer and from cold fronts in winter. This region also commonly experiences thunderstorms that develop over the interior and then move towards the coast. Lastly, there is the winter rainfall region, which covers the western half of the Province, including Cape Town and the west coast. Cold fronts are the major rain-producing system in this region.

Ahead of a cold front one often find areas of high fire danger and berg wind conditions in the Western Cape. This is as a result of the strong northwesterly wind ahead of the front that causes an offshore flow along the south coast. The cold fronts are usually preceded by a shallow low pressure system, referred to as a  coastal low, which plays an important role when it comes to the berg winds by enhancing the offshore flow. Berg winds originate over the warm interior and then flow towards the coast. As the air flows down the escarpment it heats up. When the warm and dry air reaches the coastal areas, it creates escalating temperatures. The dry and windy conditions associated with berg winds create the perfect environment for runaway fires. The recent and devastating Knysna fires is an example of the destruction that can be caused by this.

Rainfall occurs with the passage of a cold front, with showers setting in behind the front. A cold front represents the boundary between cold and warm air and therefore a definite drop in temperature is felt as a front moves past. They are relatively fast moving weather systems that cause rainfall for a few hours. However, post frontal showers can last much longer and persist for several hours. The rain associated with a cold front mainly occurs along the coast and adjacent interior.