At 11 a..m. (PST) today, radio broadcasters and TV will take part in the first-ever nationwide emergency alert system test. Although emergency alert system tests have been done at the local level, we've never had one performed at the national level. This type of alert should come on handy when, say, slimy aliens attack (because they will), nuclear warfare erupts, a huge comet comes barreling toward planet Earth (you must see Melancholia; it's an actual masterpiece), or Harold Camping's premonitions of apocalyptic doom come true.
Expect to hear a 30-second "eeeeeeee" and familiar "this is only a test" messages during your TV or radio broadcasts.
Again, this is only a test. Don't panic. Not yet, anyway.