A Red Flag Warning (RFW) is a forecast issued by the National Weather Service (NWS) when specific environmental conditions align, creating an environment highly conducive to the ignition and rapid spread of wildland fires. This warning acts as an alert for fire management agencies and the public that extreme fire behavior is either occurring or is imminent within the next 24 hours. The underlying principle is that a single factor alone, such as high wind, may not trigger the alert, but the simultaneous combination of adverse weather and fuel conditions dramatically increases the fire danger. The RFW is only issued when three specific factors—wind speed, relative humidity, and fuel moisture—are present at critical, predetermined thresholds.
Wind Speed and Direction
High wind speed is a primary meteorological factor that drives the severity of a wildfire. Wind directly influences a fire’s behavior by increasing the oxygen supply to the flames, which intensifies the heat and rate of combustion. This effect causes the fire to burn hotter and faster, increasing the difficulty of containment.
The sustained wind speed threshold for an RFW typically ranges between 15 and 25 miles per hour, though the specific criteria vary by region. Strong winds also carry burning embers and firebrands far ahead of the main fire front. This process, known as spotting, creates new, dangerous fires that jump over established fire breaks, making suppression tactics ineffective.
The direction of the wind is also integral to the forecast, as it allows fire behavior analysts to predict the path and rate of spread. An abrupt shift in wind direction, often associated with a weather front passage, can expose a fire’s flank, suddenly turning it into a massive, fast-moving head fire. This unpredictable element is a major concern for firefighter safety.
Low Relative Humidity
The second necessary factor for an RFW is a very low measure of relative humidity (RH). Low RH is a direct indicator of how much moisture the atmosphere is pulling from the surrounding environment. When the air is very dry, it acts like a sponge, drawing moisture out of vegetation and other potential fuels.
Red Flag Warning criteria typically require relative humidity to be at or below 25%, with some areas using a more stringent threshold of 15% or less, often sustained for several hours. As the air dries out, the vegetation loses its internal water content, making it significantly easier to ignite. This process reduces the energy required for a heat source to bring the fuel to its ignition temperature.
The lack of moisture in the air also prevents the dew point from being reached, minimizing overnight humidity recovery. Normally, cooler nighttime temperatures would cause RH to rise, adding moisture back into the fuels and making them less flammable. When RH remains low throughout the night, the fuels stay critically dry, setting the stage for extreme fire behavior the following day.
Fuel Moisture and Combined Risk
The third critical component is low fuel moisture, which represents the state of the vegetation available to burn. Fuel moisture is the water content within the vegetation, and it is separated into dead and live fuels. Dead fuels, such as dried grasses and small twigs, are highly responsive to changes in atmospheric humidity, often drying out completely within ten hours.
A common RFW threshold requires fine dead fuels, often referred to as 10-hour fuels, to have a moisture content of 8% or less. Live fuels, like shrubs and trees, respond more slowly but become a significant factor during long-term droughts. Moisture content below 60% is considered critical in some regions. When fuels are this dry, the fire does not waste energy evaporating water, allowing the entire heat output to drive the fire’s spread.
A Red Flag Warning is issued because the risk is exponential when all three factors align. Low relative humidity quickly dries the dead fuels to a critically low moisture content. Once a fire starts in this dry material, the high wind instantly supplies oxygen and rapidly spreads the intense heat, making the fire uncontrollable. This confluence of dry fuels, dry air, and high winds dramatically increases the probability of a small spark becoming a large wildfire.