The US National Weather Service has created a tool that will allow you to create your very own weather radar using Google Earth and their kml generating tool!
Using the NWS radar tool, you can select any area within the US, single or multiple locations, and have the latest radar image displayed on Google Earth! (Looping imagery only available using single stations)
Once you open their web site, make your selections from the choices provided and click the submit button. That will create a compressed file ending in "kmz". You will be then asked to open or save the file. Selecting "open" will uncompress the file within the Google Earth application.
If your application supports the <refresh> tag, then:
the radar images will auto-update every 2 minutes
warning polygons have a 1 minute refresh
If you exit your application and save the settings, the KML that you create will load the next time you start the application without you having to re-create it.
Remember to save the .kml in your My Places folder so you can access it whenever you wish! I'd also recommend placing a shortcut to the .kml file on your desktop for even easier access!
In addition to specific radar sites, you can also select state or regional radar composites as well!