a young rain-maker must quickly master her abilities to save the kingdom from the Ruler of storms

I agree with all of the above. You are super close to having a solid logline -and I like the concept, too!

"Mastering her abilities" sounds like she must sit down and study what she knows she has. I think the best examples of character growth come from discovering inner values the character does not know they have.

agreed with all the above comments. I'll add that in this case, the setting and period sound important so it might be best to indicate where and when this story takes place.

As the others have said.

It's not clear what the main action is, what she is supposed to do.? If she's a rain-maker, is her goal to bring rain that the "Ruler of storms" has prevented from falling?? Or his her goal to prevent this "Ruler' from inflicting too much rains, causing destructive floods?? What's the principal dramatic problem she must resolve?