The reason it was called a "thunder-gust" mill is because it only ran with full force after a heavy shower. An interesting little tidbit. I have several ancestors who owned mills (in Maryland and Virginia) but do not believe they were ever referred to as a "thunder-gust" mill. Just one little reminder to always search GoogleBooks or Genealogybank.com for references to phrases that don't make sense or seem to have fallen out of use. There are other sites as well that may contain printed references to terms that are now obscure.
Citation reminder: We are a strong believer in citing genealogical source material in the spirit of Evidence Explained. However, we choose not to include properly formatted citations in these blog posts. There's always enough information in the post to create a citation and full citations are included in my how-to newsletter Casefile Clues.