As we head into Autumn the large scale weather patterns are changing, more low pressure systems will move closer to the Western Cape, whether this be cold front with associated troughs or cut-off low pressure systems. Autumn typically has a very good history for thundershower activity in Cape Town, here's to hoping this year will be the same.

Looking at the GFS model for this week it appears as though Cape Town may (or may not) be in for some heavy rains on Wednesday through to Thursday (30 - 31 March 2011). The models are currently showing an area of low pressure moving towards South Africa from the west, though this low isn't very deep, but seems to be drawing a lot of moisture regardless. The models show this small area of low pressure behind held above a high pressure cell to the south.

The GFS model is showing 1.2 - 1.5 inches of rain on Wednesday and Thursday, this is equivalent to approximately 30-35mm of rain. Most of the rain will occur on the Wednesday night it seems, with as much as 20mm forecast to fall in a period of only 6 hours.

As with all these systems the forecast remains extremely touch and go with things changing with every run. The South African Weather Service has a forecast currently showing thundershowers on Wednesday for Cape Town which seems very possible. I'll bring updates as the new model runs come in and in case they continue to flip-flop.

Below is the 00z 27/03/2011 model run for a 6 hour period.

Update (17:22 - March 27) : The latest current model run shows quite a different scenario taking place in that instead of the small shallow low pressure system, it's back to following the old model runs in developing a fairly deep, large cut-off low, though in the end the rainfall totals are looking similar. The next models will be out shortly.