A red flag warning is a critical emergency alert issued by the U.S. National Weather Service (NWS) to warn that conditions are highly favorable for wildfires to ignite and spread rapidly. These warnings are based on locally determined criteria per Weather Forecast Office, typically combining drought, very low humidity, strong winds, and the threat of lightning. When issued, fire agencies bolster their staffing and equipment, and local governments may enact outdoor burning bans, informing the public that a rapidly spreading fire is probable within the next 48 hours. In response to worsening conditions, the NWS introduced an extreme red flag warning in October 2019—first used on October 29 of that year—to signal especially dangerous fire behavior from strong winds, very low humidity, and dry fuels. A related, less imminent alert is the fire weather watch, which is issued 18 to 96 hours ahead when there is a high potential for these dangerous fire conditions to develop.