Wireless Emergency Alerts are now up and running in Central California.
The technology sends a short text message to all cell phones in a threatened area warning of dangers like tornadoes and flash floods.
The service is completely free.
The alerts are one-way broadcasts to all devices communicating with a particular cell-phone tower. Users do not need to have GPS, register their location, or activate the service. All major cell phone companies in the U.S. have committed to the service and every cell user is automatically signed up.
You can opt-out of the messages except those of the highest-level emergency.
Phones built within the past two years are capable of receiving the messages. Many older phones are, too. To find out if your phone receives Wireless Emergency Alerts or learn how to disable the messages, we've listed the Wireless Emergency Alert websites of different carriers next to this story.