Unpredicatable. Variable.
Mediteranean.
Four seasons in one day.
Rainy season is winter, ie May to September (Southern Hemisphere, remember). Expect overcast conditions with lots of rain. Once or twice a year there may be a few small hailstones in with the rain, but never snow or frost. A windstorm may blow over a few trees and stuff.
Cape Town weather being what it is, we had a heatwave in July 2000.
Summer is hot and dry. Daytime temperatures regularly exceed 30 centigrade, and it's possible that there will be no rain at all through December and January.
Cape Town weather being what it is, we had torential downpours on 1 January 2000.