(a) a way of showing up the rivalry between Roherum and the weatherman
(b) a way of showing the misuse of magic (when Brankaresh changes the weather artificially)
(c) That there is always a price to pay for misusing magic (not only is Brankaresh punished, eventually, but he is left very weak and tired after performing this particular spell).
The weather forecasts are based on the format we use. I don't exaggerate the forecasts either. So far I can't justify going into a lot of detail on them as it would only slow the story down but may put up here what I think would be good posts here.