I'm not aware of any commands or programs that do completely what you want.
But wicd does show you in percentages how strong your connection is and should give you a list of connections ranging from strongest to weakest signal. So I know it doesn't drop weak connections, but you'll only have to check the top of the list. Check the screenshot of the menu.
When installing wicd it will replace networkmanager. It's in the ubuntu repositories so you can install it immediately with
sudo apt-get install wicd