Yep, there are variables. I am thinking about out in the wide open in west Texas or Oklahoma, where you can see it coming in the daytime. Nighttime or obstructed view pretty much negates the early visual warning.

Static precip is going to be hard for the layman to discern from background interference. AM radios can be susceptible to computer noise, monitor noise (if its old CRT that is), DSL noise, phone noise, in fact a lot of different radiation sources can simulate what you might mistake for an incoming tornado, plus the fact that most folks probably don't get the chance to really listen to such an event that often.

All in all, I would prefer a good doppler radar system as a suitable EWD. Sooner or later there's going to be a real time internet link for folks to dial into that will have a doppler feed and graphics and even alert cues telling you what and how the system is developing. I would subscribe. Wouldn't it be nice to find out what the storm is doing 30 seconds before the general alarm is activated?