[She's half-walking, half-skipping to his table, drawing a chair for herself without even bothering to ask if she can sit down, because... well. She can, she has so decided.]
It's just the science of the weather! Once you got that down, something like predicting the weather is easy. [And, well, she's naturally talented -- she can predict the storms no one should be able to, so perhaps that is in play here, as well. Certainly the functioning of this world isn't a natural one, in any case.]