Update (Thurs 28th). The model runs have converged nicely (though
For the past week, memebers of Storm Chasing SA have been following the development and forecasts of a cut-off low that was forecast to make landfall in Cape Town today Sunday the 11 October 2009. For the past 5 days the forecasts have been extremely mixed and went from Thundershowers to rain, to thundershowers back to rain and eventually thunderstorms did develop. The biggest hint at a positive result was the GFS models forecast of conditions on Saturday which showed CAPE values in excess of 400 for Cape Town with a Lifted Index of -2 to -4 which is positive for thunderstorm development and indicates instability in the atmosphere.
At 10am on Sunday the 11 October 2009 the first few mammatus clouds were visible and then continued to exist on and off throughout the day. Later in the day there was an extremely large cumulonimbus cell which also produced mammatus clouds near sunset.