As Edouard notes, this is a problem with the SiliconLabs driver - it's locking up when you try to connect, which means you have to force quit WeatherCat. It also sounds like you have the monitor and relaunch WeatherCat option ticked in its preferences, which is why it's relaunching itself when you force quit it.
To turn this off, boot WeatherCat into safe mode (hold down the ctrl key whilst launching it from the Finder) - it'll open just the Backup manager and won't try to connect. From there you can open the Preferences, go to the Units/Misc 1 tab and turn off the monitor option.
You can also go to the Tools menu and hit 'Configure Station Comms' - this will cause the Comms window to open the next time you launch.
If you don't have the latest driver installed, download it and install it - if it still isn't working you may want to try connecting through a USB 2 hub - some people who have issues with the driver on 10.11 have reported this working.