~/Library/Preferences/com.trixology.weathercat.plist (where ~ is your home directory).
I'd recommend renaming your existing file, rebooting then launching WeatherCat (whereupon a new file will be created with all settings back to defaults), this avoids the issue of restoring a potentially already corrupt file.
Before you do this, note any settings you don't immediately have to hand/know so you can reenter them when it boots with the fresh settings.