Unfortunately there isn't anything that can be done for this in WeatherCat - either the OS will release the driver or it won't - if it doesn't it's because it either thinks there's some other software also using the driver or an OS bug or the driver has crashed in such a state it cannot be unloaded. Over the years, various OS versions have broken this functionality, only to be restored in a subsequent OS release and/or Silicon Labs driver updates.
Something you could try, is going to the Tools menu in WeatherCat and select 'Configure Station Comms' - this will cause WeatherCat to forget the current connection information and quit. When you launch it again, you'll have to reconfigure the communications, picking up new connection information. I wouldn't be too hopeful this will affect it, but it might be worth a try.