Dark clouds ahead of us

This is a type of cloud I never saw before in my whole life. I think it is Cumulonimbus during a thunderstorm. I also think that the strange effect in the yellow top of the cloud is caused by an awful high upperwind speed ahead of the squall line.
Anyway: it was quite impresssive. We left the South African east-coast in the early morning hours and are now steering for another tactical waypoint to stay out of the unfavourable currents. So far with a lot of succes.