Hi David,
You can open the data file in a (real) text editor, such as the free TextWrangler* and hand edit it - each entry is a single line so it's pretty easy to delete stuff (see the section WeatherCat and data in the manual), but if the file is corrupt you may have problems, particularly if there's cross linked sectors etc.
Before doing anything run Disk Utility across the disk, then take a copy of your WeatherCatData folder.
*http://www.barebones.com/products/textwrangler/