PBS Warn is a service of the Public Broadcasting System and local public television stations. It serves as a backup of the WEA (Wireless Emergency Alerts) messages that are received by your cell phones.
From the PBS Warn website :
PBS and your local public television station play a crucial role in protecting communities by ensuring uninterrupted distribution of Wireless Emergency Alerts (WEAs), including severe weather warnings and AMBER alerts. This service is part of public television’s commitment to strengthening the safety of all our communities.