The NOAA system has a name: SAME, or Specific Area Message Encoding[1]. As the name implies, those three bursts actually contain a whole packet of information regarding the incoming message, including areas of effect, hazard type, duration, source of the alert, etc. There are pieces of software out there that let you decode the packets, and some let you encode your own, like minimodem[2]. Do be aware that transmitting valid SAME packets over broadcast media tends to be very poorly viewed by the FCC because it actually triggers mechanisms in the broadcast system, as GP noted.

1: https://en.wikipedia.org/wiki/Specific_Area_Message_Encoding 2: http://www.whence.com/minimodem/