When Arthur returns from defending the honor of the young maiden, he notices a storm brewing directly above Camelot:
Arthur foresees a storm over Camelot upon his return. He later learns of the oath the knights have taken. Before this quest is taken, Arthur still believes that all his knight are upholding his pure ideals; after the knight that lived return from their quest, Arthur learns that Galahad is the only pure knight he has. Purity is the only way to see the Holy Grail, and Galahad is the only knight that faces no obstacles in seeing the Holy Grail. Sir Bors is imprisoned and Percivale must first become pure before seeing the Grail. Lancelot faces many obstacles before only seeing a veiled Grail. Gawain is distracted in his quest by a group of maidens and never completes the task. Arthur becomes cognizant of the fact the knights have not emulated their king in purity. The storm he sees shows the turmoil that is coming to his court. It is a storm that will break up the court and result in much devastation.
The above painting is The Temptation of Sir Percival (1894) by Arthur Hacker.