What is a red flag warning? Understanding different wildfire alerts

In Oklahoma, where dry winds and scorching temperatures are common, the risk of wildfires is a serious concern, especially during certain times of the year.

The National Weather Service regularly issues red flag warnings to alert residents to heightened fire dangers, advising people to be extra cautious about fire safety.

Whether it’s a small spark from a cigarette thrown out of a moving vehicle or a large prescribed burn on federal lands, these warnings indicate that conditions are ripe for fires to quickly spread.

From late winter to early spring and again in the fall, Oklahomans should stay alert to these critical advisories to help prevent wildfires and ensure their safety.

What is a red flag warning?

Red flag warnings are issued when forecasts indicate that a combination of high temperatures, very low humidity and strong winds in a given location could create an environment that is conducive to dangerous fires, according to the National Weather Service. The agency sets these warnings most often between February 1 and April 30, and again from October 1 to December 15, when typical weather conditions are consistent with that criteria.

More specific criteria established by the National Weather Service, in coordination with government land management agencies, uses thresholds to determine whether a red flag warning should be issued, when it should be issued and for how long. The criteria considers fuel conditions — a parameter that refers to the amount of water held by "small vegetation," like grass, leaves and mulch, which indicates how the land could respond to either wet or dry environmental changes — when they have reached a critical point that could be favorable to the growth and spread of large fires, according to the agency. 

Criteria also stipulates that a red flag warning is issued when relative humidity in an area drops to about 25% while surface winds or frequent gusts simultaneously climb to at least 25 miles per hour. Both conditions must occur simultaneously for at least three hours over a 12-hour period. Widely scattered, dry thunderstorms could trigger a red flag warning as well, with the National Weather Service deeming a thunderstorm "dry" if it produces less than 1/10 of an inch of rainfall.

Other environmental factors that contribute to a red flag warning include: lightning, when it is significant and occurs for the first time after a hot and dry period; dry cold fronts forecasted to cause strong and gusty winds and abrupt wind shifts, but little or no rainfall, especially in areas where there are ongoing wildfires or prescribed burns; and any weather and fuel conditions that could otherwise elevate the risks of fires when taken together, like unusually high temperatures and long-term drought. In California, for example, where massive and devastating wildfires are one consequence of a lengthy and severe drought in the American West, some experts have suggested that parts of the state are now experiencing a "wildfire season" that lasts year-round.

Red flag warning vs. fire weather watch

A red flag warning is technically not the same as a fire weather watch, whose threat is more explicit, although both kinds of advisories rely on the same fundamental criteria. Fire weather watches can develop into red flag warnings if environmental conditions progress to a point of serious urgency. 

The National Weather Service generally issues a fire weather watch to alert officials and the public to "the possible development of red flag conditions in the near future," often between 24 and 72 hours in advance of a forecasted weather event. Then, if necessary, it will issue a red flag warning to advise "of occurring or imminent red flag conditions, generally within the next 12 to 24 hours."