"Rain-prone" is not an English expression, and it isn't common to make adjectives with "-prone". It's more common to use "prone to", such as: "She is prone to get angry over small matters." "Drunk drivers are more prone to traffic accidents." There is also the less common position of "prone" (face down), such as: "The victim was found lying in prone position."

Definitely, you can use it in many different sentences. Here are just a few examples: He was prone to anger. He was prone to getting excited. Young people are prone to fall into temptation. People who are prone to aggression can be dangerous. Source of examples: many things.org To answer your question, "rain-prone" is not really something that you would say. I hope this helps.