The main photo was taken around sunrise. The photographed cloud is altocumulus castellanus indicating that the cold front as per the SLP chart on the sixth thumbnail was getting closer following high clouds taken on the previous day (Click here). Althaugh the weather sounding on the fifth thumbnail indicated anti-CAPE or convective inhibition through the blue shaded area, it also showed lightning and the formation of altocumulus clouds at heights between 3km and 4km. Hence, it is being concluded that the photographed clouds formed due to rising warm airmass at those heights including the first thumbnail taken from another view showing the wide distribution of the same cloud species in the sky. The second thumbnail was a photo taken at the hills of Mellieha showing lower-level altostratus opacus clouds at heights of probably between 2.7km and 3.1km and more significantly trapped stratus clouds or fog in the distance which was observed during that morning. The third thumbnail shows a cloud mosaic of altocumulus stratiformis undulatus clouds perhaps virga included as strong mid-level wind shear was observed at that level increasing rapidly in speed from 61kmh at 2.5km altitude to 113kmh just 2km above that layer as Malta was influenced by a strong WSW polar jet stream. The fourth thumbnail shows the edge of the bad weather with undulations still observed due to the buffetting of wind. Thunder was heard so the base of the cloud was actually that of a cumulonimbus producing isolated rain showers in which hail was even reported as parts of the lower cloud joined with altocumulus above.
The jet stream chart in the eighth thumbnail explains what was going on in the weather. As hinted herewith, it was the polar jet stream effecting the Maltese Islands being unseasonably undulating and very strong containing wind speeds of above 120kmh. This pummelled an unseasonably cold polar airmass towards the Mediterranean causing a Genoa low pressure system to develop along with a cold front in which clouds formed. The cloud structure is collaborated via the infrared satellite image on the seventh thumbnail. Finally, the upper-level trough on the ninth thumbnail indicated instability through the red colour. A severe thunderstorm was limited by the cloud height and the fact that the seas were seasonably cool and not enough landmass to provide the necessary lifting of a Wintry airmass by the strong May sunshine.