Install sass using npm globally - npm install -g sass
Go to file watcher in phpstorm and write the following arguments and path in the right field and hit apply.
Arguments:$FileName$:$FileParentDir$/css/$FileNameWithoutExtension$.css
Output paths to
refresh: $FileParentDir$/css/$FileNameWithoutExtension$.css:$FileParentDir$/css/$FileNameWithoutExtension$.css.map
This should do the trick, if you change the folder structure, the structure of arguments as well as output paths to refresh will be changed.