2009 saw the beginning of Vortex 2, the largest tornado
April has been notably dry for what is often considered the first month of Autumn, at least for the Cape. On average Cape Town experiences 8 days of rain, and around 40mm in the month of April, more than double the average of March. However this year, Cape Town International Airport has recorded less than 2mm of rain. The effects of this dry spell has been seen in the drying up of certain vlei areas around the Western Cape, which by this time of year are usually beginning to refill. Despite a couple of days of rain on the last few days of the month, rainfall totals for most areas were minimal, only rarely surpassing the 1mm mark.
The primary reason for the lack of rain has been a dominant high pressure located over the western parts of South Africa. Ridges of high pressure act to block cold frontal systems from progressing northward and having more effect on land. In the summer months, high pressure systems dominate a lot of the synoptic charts, ensuring any cold frontal systems to the south cross to the east, well south of South Africa. In the winter times, these ridges weaken and change position, this allows for the cold frontal systems with their associated low pressures, to move further north and in turn bring more rain to the country.
The images above (GFS Model) show on the left a dominant high pressure, which can be noted as preventing the rainfall to the south from impacting the country. On the right you can see the difference, with the high pressure being located to the far west, and the rainfall associated with the cold front to the south being unrestricted with its progression towards the Western Cape.
Looking ahead to May, the long term models seem to indicate that in the next couple of weeks we could see the April trend change and the high pressure systems break down a bit to allow for more rainfall events for the Cape. Despite the increased likelihood of experiencing cold fronts, the models seem to still indicate that at least for the first two weeks of may, rainfall totals will remain fairly low, with no significant impact. So while it seems likely that the Cape will finally be experiencing some rainfall, it seems quite likely that it will primarily be through weak cold fronts bringing light rain, at least for the next week to 14 days. The overall model pattern for then first two weeks of May looks similar to what one would expect in early April. However, it should be noted how quickly things can change and turn around, the question at the moment lies with how the jet stream will behave and whether it will remain to the south or move into a more climatological position.
It’s still early days for the Cape as far as winter rainfall is concerned, and only in June does the Cape reach its rainfall peak in terms of average amount of monthly accumulation.
You can find a discussion about this event on the forums: Dry Spell SW-Cape