A Red Flag Warning is a warning issued by the National Weather Service (NWS) in the United States to alert community organizations, firefighters, and citizens to the fact that conditions are ripe for the emergence of severe wildfires in the next 24 hours. For conditions to qualify for a Red Flag Warning, there must be evidence that a critical state will be reached within the next day, and indexes like humidity and predicted average winds may be used to determine whether or not such a warning should be issued.
In a step down from a Red Flag Warning, the NWS may call for a fire weather watch, which means that the weather could potentially promote the emergence and spread of fires, but the situation is not critical. However, a fire weather watch can be upgraded to a Red Flag Warning on the basis of new information and emerging events, so people are often encouraged to stay alert when such a weather watch is issued.
A classic set of circumstances which would led to a Red Flag Warning would be a period of drought in a region with abundant fuel for fires, accompanied by predictions of dry lightning and high winds. These conditions could result in catastrophic fires, as the lighting could start small fires which would spread quickly due to dry conditions and high winds. By being alert to danger, the hope is that such wildfires could be headed off before the situation became critical.
A Red Flag Warning could be called anywhere, and it is usually a signal to fire departments and firefighting organizations to go on full alert. Additional staff may be called in to ensure that the area is covered, and fire watchers may be reminded to stay especially aware. Community officials may also be on alert, prepared to issue evacuation orders and to support the efforts of their fire departments.
In areas where Red Flag Warnings are common, homeowners are often encouraged to use commonsense measures to protect themselves from fires. For example, it's a good idea to keep lawns mowed around structures to reduce the proliferation of fuel for fires, and trees and shrubs should be trimmed well back from structures. Local building codes may also include measures to make structures more fire resistant, such as attic vents which expel sparks and debris.