It works here too, on occasion, but then again I do my own forecasting and have my own radar feed so I don't watch TWC..
Basically, the "Local on the 8's" feed on cable is provided by a computer called WeatherStar that is installed at the head end serving each area. When TWC drops to LOT8's, the WeatherStar computer takes over and overlays the customized local forecast and local radar into the video mux on top of the National forecast. Sometimes if the WeatherStar goes out of sync and either starts late or ends early, you can see the national forecast underneath it. There is still no HD version of the WeatherStar, which is why the TWC HD only shows the National forecast even when a local forecast is available on the SD channel.
Dish and DirecTV are a different animal, because their broadcast centers serve the whole nation (even large cable companies like Cox, Comcast, and Time-Warner are broken down into small local service areas). So they have to do LOT8's via a zipcode based app that runs on the receiver itself. It's buggy and doesn't always catch the signal to do the overlay, hence you see the national forecast.