Formation of cumulonimbus due to remnants of cold air (left over from previous severe thunderstorm) above the warm sea. Again, the weather was unstable and a trough can be seen in the 500mb weather chart. This photo clearly shows an over-shooting top above the anvil which indicates a very strong updraft within the thunderstorm and hence which is able to penetrate even the stable layer of atmosphere (indicated as lfc-El in the weather sounding). The anvil itself forms when rising moist air finds a stable layer in the atmosphere hence continuing spreading horizontally. The first two thumbnails show the same cloud but with a wider angle. Notice how the over-shooting top developed between the 2 thumbnails taken just minutes apart.