A Fire Warning is a non-weather emergency message that the National Weather Service (NWS) relays on behalf of federal, state, tribal or local officials when a spreading wildfire or structural fire threatens a populated area. The officials supply the information, and the NWS sends it out over its own alerting systems, including NOAA Weather Radio All Hazards.
Who sends it
Non-weather emergency messages are originated by local, state or federal civil authorities, and the NWS relays them at their request. NWS Instruction 10-518 describes them as time-critical, life- or property-saving information from federal, state, tribal and local officials.
The forecast office may take the content from the requesting authority and format it for transmission. The authority, not the NWS, sets when the message expires.
What it says
The NWS description is a warning of a spreading wildfire or structural fire that threatens a populated area. It adds that authorized officials may recommend evacuation of areas in the fire's path according to state law or local ordinance.
Because the content comes from officials, the details vary from one message to the next. Read the text for the areas named and any instructions included.
How it reaches people
Relayed messages go out over NWS systems, including the NOAA Weather Wire Service and NOAA Weather Radio All Hazards, and they list the counties or zones affected. On weather radio and in the Emergency Alert System (EAS) the event code is FRW.
Fire Warning is one of the state and local codes, and it is optional for broadcasters to activate EAS for it. The NWS cannot send Wireless Emergency Alerts (WEA) to phones for these relayed messages, because it is not authorized to pass them to the Federal Emergency Management Agency's Integrated Public Alert and Warning System (IPAWS), the sole route to WEA.
How it differs from a Red Flag Warning
A Red Flag Warning is an NWS weather forecast: it is issued when the combination of fuels and weather supports extreme fire danger or fire behavior. A Fire Warning is about a specific fire that is already spreading toward people.
Local agencies may also run their own alert systems by phone call, text or email. Evacuation decisions belong to those officials, so follow their instructions whatever channel they use.