Can I Cast Thunder Wave?

Bart is now 50 feet up the cliff face, which means there's only 10 feet more to go for you. The people who would be caught in this would be sleek and mud. Are you sure it doesn't hit Kiteboard? No, Kiteboard's too far. And I guess actually it would also affect Gum Gum because he's up only 10 feet above you. All right. A wave of thunderous force emanates out from Bart. He takes a constitution saving throw - 19. That is a 13 for sleek.