Formation of wave-like features within the cumulonimbus cloud moments before the occurrence of a severe thunderstorm in the area which caused damage. This cloud formed the way it did due to cooler air (downdraft) sinking and spreading out. This sinking activity had prevented further warm air from being pulled into the storm cloud updraft. The rolling effects are the result of wind acting on the storm cloud itself. The first thumbnail photo, obtained from Maltaweathersite Fb page, shows the very dark base of the cumulonimbus cloud which formed very quickly.