The story of where this myth came from is interesting. Its about moving the saint's bones to Winchester Cathedral from up here in northern Hampshire. It rained all day and its believed that the saint was not amused at being disturbed . Probably some more knowledgeable catter will correct or provide more detail. It rained here so dig out the raingear.
As a former employee of the Met Office, I am certain that there is no scientific evidence to support the myth, but the met Office have been wrong before (Hurricane Mr Fish)