Actually, with today's technologies that are in place warning will be issued based on radar returns. If doppler shows any kind of rotation, it automatically causes a warning to be issued by NOAA's National Weather Service. Human eyes don't have to be in play. I'm guessing, though, that your saying "a tornado has been seen" encompasses that.
The upside is that there is more time given to the warnees. The flip side is that there are a lot more warnings. I hope people don't start taking it as crying wolf.
I have a friend who grew up in Tulsa. He said that sirens used to go off so much that most didn't heed them. He has a great picture of himself as a kid playing in the street with a tornado going by in the background. Crazy!
And yet that system still misses a few. They're so unpredictable!

