The National Weather Service provides several, self-updating radar map layers for Google Earth, the two most prominent being the Composite Reflectivity (current radar) and Storm Total Precipitation. The latter shows the estimated amount of precipitation that has fallen since the last one-hour break in precipitation and is a very valuable resource in trying to determine which river reaches are likely to run. However, because the map layer will 'reset' itself after an hour break in precipitation, it may not provide all of the information that you may be seeking after a series of storms has blown through your area. This is where the HADS and IFLOWS rain gages can be very helpful.
Download these map layers here. When you download a radar map layer from NOAA, a legend for the reflectivity radar images is provided. You can obtain a legend for the storm accumulation layer here and then drag it into the same folder as the actual map layer in Google Earth. Once you have downloaded these layers into Google Earth, you can adjust the layers' transparency as described here.