Red flag warning

Red flag warnings are associated with drought conditions, very low humidity, high or erratic winds, and the possibility of lightning.

The weather criteria for fire weather watches and red flag warnings are determined locally by each Weather Forecast Office (WFO) in coordination with land management agencies, and vary with the WFO's warning area based on the local vegetation type and topography. They usually include the daily vegetation moisture content calculations, expected afternoon high temperature, afternoon minimum relative humidity, and daytime wind speed.

Firefighting agencies often respond to such a warning by adjusting staffing and equipment resources through interagency dispatch protocols, including requesting additional personnel, equipment, and aircraft. To the public, the warning means high fire danger with an increased probability of a quickly spreading vegetation fire in the area within the next 48 hours. Outdoor burning bans may also be proclaimed by local law and fire agencies based on red flag warnings, for both the general public and land management officials. A fire weather warning gives extra guidance to land management officials to refrain from controlled burns.

In October 2019, the National Weather Service introduced a variant to denote even more dangerous conditions: extreme red flag warning. Analogous to the particularly dangerous situation (PDS) wording on a high-end severe weather warning or watch, this means that conditions for fire growth and behavior are extremely dangerous due to a combination of strong winds, very low humidity, long duration, and very dry fuels. It was used for the first time on October 29, 2019.

A related but less imminent forecast may include a fire weather watch, which is issued when there is a high potential for Red Flag conditions to develop, as distinct from a red flag warning, which denotes high confidence that Red Flag criteria will occur within 48 hours or less. The watch is issued 18 to 96 hours in advance of the expected onset of Red Flag criteria, intended to alert fire and land management agencies at least a day ahead for resource allocation and firefighter safety.