Formation of a new thunderstorm cell, cumulonimbus calvus, having indistinct and flattened upper parts without the sharp outlines associated with the capillatus species. It was producing showers over the eastern side of Malta and was part of a Mesoscale Convective System (MCS) having fused V-shaped thunderstorm clouds producing very heavy rainfall offshore with the highest rainfall of 27.8mm occuring in Gozo. Otherwise, the sky was covered with altostratus clouds. The first thumbnail shows a slight lowering of the cumulonimbus cloud which became known as ‘murus’ by the WMO. The weather sounding showed lots of instability and wind shear on the day which are both ingredients of an MCS being defined as long-lived thunderstorms larger than individual thunderstorms but smaller than extra-tropical cyclones and the wind shear causes thunderstorms to self-heal or auto-generate as happened in this case. The photographed thunderstorm seemed to have become super-cellular. Some time later, as the MCS arrived, cumulonimbus arcus formed and the sky took a strange green colouring indicating very deep clouds and the potential for severe weather phenomena such as waterspouts and hail which very likely happened over our surrounding waters. The second thumbnail showing the weather sounding confirmed much instability and wind shear conductive for super-cellular storms. However, convective inhibition (CIN) was relatively strong.

However, looking at the surface pressure chart on the third thumbnail, showed that the nearby cold front and bad weather line seemed to have eroded this cap in the atmosphere allowing for the full release of the severe thunderstorm potential. The fourth and fifth thumbnails are the visible satellite image and the precipitation with thunderstorms mobile app respectively showing the V-shaped nature of the continuously re-generating thunderstorm cells. The sixth thumbnail is the rain radar showing very heavy rain with up to 100mm per hour on the waters to the north of gozo which then moved southeastwards and only marginally effected land. In the case of direct hit, this MCS had the potential to produce over 100mm or 4 inches of rain given that it lasted for several hours. The seventh thumbnail shows a thunderstorm line taken at around 0600 CET showing that the MCS was originally a frontal system which had broken down and which part of it became such storm due to the availability of the required ingredients. Click here to follow how the storm had developed into cumulonimbus arcus.