The weather situation of the day is explained here. This cumulonimbus cloud had developed as a result of instability as explained there. In fact, the visible satellite image (3rd thumbnail) showed that this low pressure was filled with such clouds. The first thumbnail clearly shows low-level cloud formation probably stratocumulus praecipitatio heavily affected by the very strong surface wind while the second thumbnail is a storm over Gozo as seen from Cirkewwa. This Mistral severe gale event is highlighted in the fourth thumbnail while the fifth thumbnail shows an undulating jet stream sending cold air over the Central Mediterranean.