Understanding Red Flag Fire Weather Warnings

Learn about red flag fire weather warnings: what they mean, how they’re issued, and essential steps for firefighters, residents, and communities to stay prepared.

Understanding the weather patterns that create a Red Flag Fire Warning is key to preparing for and responding to the dangers posed by wildfires.

What is a Red Flag Warning?

A Red Flag Warning means there is a high risk of a fast-spreading wildfire in the area within the next 24 hours. During this time, authorities might put a burn ban in place and ask people not to burn anything. This warning system has been in use since the 1960s and signifies that a combination of factors, such as low humidity, strong winds, and dry vegetation, has created a high risk for wildfires.¹

Criteria Used to Issue a Red Flag Warning

To issue a Red Flag Warning, several weather factors are evaluated:²

Ten-Hour Fuels Below 8% Moisture

This criterion measures the moisture level in small vegetation such as grass, leaves, and mulch, which typically respond to changes in humidity within ten hours. When the moisture content in these fuels drops below 8%, they become extremely dry and highly susceptible to ignition, increasing the risk of wildfires.

Relative Humidity Below 25%

Relative humidity measures the amount of moisture in the air compared to its temperature. When the humidity level drops below 25% for an extended period, the air becomes very dry, which accelerates the drying of vegetation.

Winds of 15 mph or More at 20 Feet Above Ground

Winds reaching speeds of at least 15 mph for several hours can drive flames across larger areas, making wildfires more intense and harder to control. Wind also helps carry embers, which can ignite new fires far from the original source.

Seasonal Timing of Red Flag Warnings

These warnings are generally issued during the peak fire seasons in spring and fall—specifically from February 15 to April 30 and from October 1 to December 15—when conditions are typically drier and more conducive to fire.

How are Red Flag Fire Warnings Issued?

A Red Flag Warning is issued by the National Weather Service to inform the public and fire management agencies of dangerous weather patterns that indicate a combination of high temperatures, low humidity, and strong winds that create dangerous conditions for wildfires.¹

What is the Impact of a Red Flag Fire Warning?

The impact of a Red Flag Fire Warning can be tremendous, which is why it’s crucial to take extra precautions. If burning is allowed in your area, ensure that burn barrels are covered with a secure, weighted metal lid with holes no larger than ¾ of an inch. Never toss cigarettes or matches out of a moving vehicle, as they can easily spark dry grass along the roadside. Always extinguish outdoor fires thoroughly—douse them with plenty of water and stir to ensure everything is completely cool. Most importantly, never leave a fire unattended, as even a small ember can quickly catch dry vegetation and spread into a wildfire.³

How to Respond to a Red Flag Warning

During a Red Flag Warning, it is crucial for everyone—fire agencies, community residents, and businesses—to follow specific guidelines to minimize the risk of wildfires and ensure safety.⁴

For Fire Agencies

• Maintain heightened vigilance: Be prepared to respond quickly to any fire incidents in your area. Have resources on standby and ensure proper staffing levels based on the forecasted fire risk.
• Monitor and update local fire alerts: Keep the public informed through social media, local news outlets, and emergency notifications about burn restrictions, burn bans, or any prescribed fire restrictions in effect.
• Coordinate with local agencies: Ensure communication with law enforcement, emergency management, and local government officials to execute a unified response plan in case of a wildfire.

For Community Residents

• Stay informed: Watch weather reports and sign up for local emergency alerts to stay updated. Follow the guidance of your local fire department and be prepared to take action if a wildfire develops.
• Clear defensible space: Maintain at least 100 feet of defensible space around your home by removing dry vegetation, trimming trees, and keeping grasses watered. Make sure your home’s roof and gutters are free from debris.
• Prepare for evacuation: Develop and practice your Personal Wildland Fire Action Plan, including identifying multiple exit routes from your area. Keep your “Go” kit packed with essential items like medications, important documents, first-aid supplies, and pet care necessities.
• Avoid risky activities: Refrain from actions that could spark a fire, such as using lawnmowers on dry grass or leaving outdoor fires unattended. Be cautious when driving—never throw cigarettes or matches from your vehicle, and avoid driving in dry tall grass.

For Businesses

• Monitor fire restrictions: Ensure that any equipment used in your business operations, such as lawnmowers or chainsaws, complies with local fire restrictions. Avoid using them during high-risk periods, especially near dry vegetation.
• Ensure fire safety measures: If your business is in a fire-prone area, make sure your property is equipped with fire safety tools and the access roads are clear and properly labeled.
• Employee preparedness: Educate employees on fire safety, proper handling of outdoor equipment, and the importance of reporting any fire hazards immediately to authorities.