Formation of large cumulonimbus cloud with lots of cumulus clouds ahead of this cloud system due to further cooler air arriving from the NW as shown in the surface pressure chart. This was caused by the eastward movement of a low pressure system bringing its coolest side towards the Maltese Islands. The current weather view taken on the day showed huge temperature contrasts between Malta and nearby areas. The visible satellite image showed isolated thunderstorm clusters forming very close to the Maltese Islands as the cold front developed nearby. The first thumbnail shows the thick cloud cover contrasting with white cumulus below it taken only few moments before a very localized hail storm in the vicinity. A noticeable feature was the large hail that this cloud had produced (6th thumbnail below). More details can be found here.