A rare roll cloud appeared over Portugal’s coast, surprising beachgoers as strong winds hit during an ongoing extreme heatwave.

Beachgoers in Portugal scrambled for cover over the weekend as the sweltering heat gave birth to a rare “tsunami roll,” or ominous wave of dark clouds over the water.

Check out this incredible roll cloud spotted off the coast of Portugal on Sunday. Roll clouds are most common with thunderstorms, but can also form when cooler air over the ocean sweeps under hot air over land,