This photo represents the back-side of a shelf cloud althaugh the shelf cloud itself was never observed in this particular storm. Hence, the back side of a mini cold front and the inside of the gust front cloud were actually being observed. In fact, the cloud base looked pretty similar to turbulent mammatus and cloud masses as the cold downdraft was rushing outward and an increase in wind was felt. In fact, a maximum wind gust of 68kmh (Force 8) was officially reported in Luqa at that time, some distance away from Xghajra where this photo was taken. Hence, the photographed cloud is known as cumulonimbus arcus or specifically a whale’s mouth in this instance. The first thumbnail is a panoramic photo of the same cloud taken some minutes later. The turbulent cloud mass was well in the background to the right whilst precipitation was observed on the background left. The second thumbnail is altocumulus stratiformis with some undulations following the squall line whilst the third thumbnail is a photo of accumulating cumulus clouds in the afternoon of the variety ‘radiatus’ as the centre of the low pressure observed in the visible satellite image was passing over Sicily.

This weather situation was caused by a very small low pressure system that passed over Sicily as interpreted by the seventh and eighth thumbnails showing the SLP and 500mb pressure charts respectively in a series of unusual bad weather systems that effected the Central Mediterranean on the third week of May 2019! In fact, the photographed clouds were part of a squall line that was moving from southwest to northeast except for the third thumbnail photo which was the cumulus clouds present around the small centre of the low pressure system that drifted southeastwards towards Sicily during the day. The weather sounding on the fourth thumbnail showed a turbulent atmosphere with a degree of instability which was to be followed by a change in wind direction towards the NW. The cloud height was estimated to be from around 425 metres up to 3600 metres and more whilst the prevailing wind was always from a SW to WSW direction starting from 17kmh at the surface up to 113kmh at the 6km altitude.

The forward flank of a cumulonimbus arcus cloud or shelf cloud would look similar to this photo ‘Cb arcus‘. I have another very similar example of such a cloud accessory or whale’s mouth here, the photo taken on 28 November 2016.