This is one method of doing it, pretty messy both technically and legally, as well as a very high risk of accidentally jamming public safety. Or CHP and CalFire can stick someone in the studio of a local FM radio station, sort of "ask" to take over the station, and broadcast official instructions based on official intel from their guys on the ground. Not a daily thing for sure, but in the case of another Paradise situation, for Shingletown or Placerville or Foresthill or Downieville, it should be something in the back pocket of the PIO's for CalFire and CHP or local Sheriff.
Isnt this the purpose of the Emergency Alert System? That takes over Radio, TV, and most media.
The Emergency Alert System (EAS) is a national public warning system that requires broadcasters, cable television systems, wireless cable systems, satellite digital audio radio service (SDARS) providers, and direct broadcast satellite (DBS) providers to provide the communications capability to the President to address the American public during a national emergency. The system also may be used by state and local authorities to deliver important emergency information, such as AMBER alerts and weather information targeted to specific areas.