Evacuation Immediate is a non-weather emergency message that the National Weather Service (NWS) relays for government officials when they recommend or order immediate evacuation under state law or local ordinance. It is not limited to fire: the same product can carry an evacuation for an approaching tropical cyclone or a release of flammable or explosive gas, and its content comes from the officials, not from an NWS forecast.
Who decides
Non-weather emergency messages are originated by local, state or federal civil authorities, and the NWS relays them at their request. The NWS forecast office may take the content from the requesting authority, and that authority sets when the message expires.
The NWS description is a warning where immediate evacuation is recommended or ordered according to state law or local ordinance. Whether an evacuation is recommended or ordered depends on those rules, so the wording can differ from one place to the next.
Where it shows up
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 EVI.
Evacuation Immediate 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) for these relayed messages, so a phone alert, if there is one, would come from officials through the Federal Emergency Management Agency's Integrated Public Alert and Warning System (IPAWS).
How to use it
Treat it as the officials' instruction, not a forecast. Read the areas named and any directions included, then act on what local officials say.
Local agencies may also reach people through their own sign-up systems by phone call, text or email. Those channels can carry route and shelter details that a short broadcast message leaves out.
What it does not tell you
A county or zone list is coarser than the evacuation area officials may have drawn, and conditions can change after the message goes out. An area left off the message is not necessarily safe, so check with local officials if you are near the edge.