Emergency Alert System

The Emergency Alert System (EAS) is a national warning system in the United States put into place on January 1, 1997 (approved by Federal Communications Commission (FCC) in November 1994), 
It pops up on your TV, radio, or phone for example if a tornado comes it will be on the news, radio, or phone and when it is over the National Weather Service will send you another alert when you can come out. The most serious alert that can be given out over the EAS is an Emergency Action Notification or a Presidential Alert. When an EAN is issued, a voice will say the following whilst watching TV or listening to the radio; "We interrupt our programming. This is a national emergency." After the alert tone, the text to speech or a real human voice will give instructions on the emergency and provide instructions on what to do to stay safe, or if the world is coming to an end due to warfare or a global pandemic. There also may be additional messages given out from a representative of the President. An Emergency Action Termination message will be issued when the threat is over. Additionally, there will be a nationwide test of the EAS that takes place in the US once a year along with a new presidential alert system that works very similar to weather warnings that are issued on a cell phone. 
Outside US.
Emergency Alert Systems are also used worldwide in countries like Japan, Canada, Australia and New Zealand and is ready to alert citizens on world ending events such as a nuclear war, a new global threat etc. 
Most recently, the United Kingdom government have been trialing mobile phone alerts with a service called Emergency Alerts that uses a similar sound to the WEA from the US which is the original attention signal used for the US Emergency Alert signaling a pattern.