What You Should Know About “Red Flag Warnings”

Today we will talk about what determines a Red Flag Warning and what the “PIG” is, the Probability of Ignition.When you see a Red Flag Warning pop up on your weather app or local news or in an email from us, it isn't just a gentle suggestion to be careful. Issued by the National Weather Service (NWS), it is the highest level of weather alert for fire danger.

Simply put, a Red Flag Warning means that conditions are perfectly aligned for a wildfire to ignite easily and spread rapidly. If a fire starts on a Red Flag Warning day, it will be incredibly difficult for firefighters to control. If you add not just high but the extreme winds we have had in December, that difficulty and the complexity of the incident get multiplied.  

Here is everything you need to know about what triggers these warnings and how you should respond to keep your community safe.

How is a Red Flag Warning Determined?

The National Weather Service doesn't issue these warnings on a hunch. Meteorologists work closely with local land management and fire agencies to monitor specific, measurable conditions. The Boulder County Sheriff’s Office uses fuel moisture measurements from Sugarloaf and the weather data that constitutes the Red Flag Warning among some other metrics to decide on what level fire ban gets enacted. 

While the exact numbers vary slightly by region—a warning in humid Florida requires different metrics than one in arid Colorado—the NWS is always looking for a specific, dangerous combination of three ingredients:

Condition	Why It Matters	Typical Warning Thresholds
Low Relative Humidity	Dry air literally sucks moisture out of the surrounding environment, turning living plants into highly flammable tinder.	Often drops below 15% to 20%.
Strong Winds	Wind supplies fires with oxygen, pushes the flames across the landscape, and carries burning embers miles ahead of the main fire, starting new spot fires.	Sustained winds of 15-20+ mph, or frequent higher gusts.
Dry Fuels	"Fuels" is the meteorological term for vegetation. If it has been hot and dry for weeks, grasses, brush, and timber are primed to burn.	Measured by how little moisture remains in dead vegetation.

A Red Flag Warning is issued when these three conditions are expected to combine within the next 24 hours. If these conditions are expected in the next 12 to 72 hours, the NWS will issue a Fire Weather Watch instead, which is a heads-up to get prepared. 

What Should You Do on a Red Flag Day?

During a Red Flag Warning, human activities cause the vast majority of wildfires. A single spark can escalate into an uncontrollable blaze in minutes. When a warning is in effect, you need to change your daily habits:

• Respect burn bans: Never burn yard waste, use fire pits, or light campfires.

• Park smart: Do not pull your vehicle over into tall, dry grass. The hot exhaust pipe or catalytic converter under your car can easily ignite the vegetation beneath it.

• Skip the yard work: Avoid using lawnmowers, weed whackers with metal blades, or chainsaws in dry brush or if you have a vehicle with steel track such as Skid Steers that some in our canyons have. Striking a single hidden rock can create a spark.

• Secure your vehicle chains: If you are towing a trailer, double-check your safety chains. Dragging chains on asphalt creates a shower of sparks that can ignite grass along the highway. 

• Dispose of cigarettes safely: Never toss cigarette butts out of a car window or onto the ground.

Be Ready to Move

Red Flag Days are the days you need to be hyper-aware of your surroundings. Keep your phone charged, ensure your emergency weather alerts are turned on, and park your car facing out of the driveway so you can leave quickly if an evacuation order is given.

What is the Probability of Ignition, and how do Firefighters use this metric?

In wildland fire science, the Probability of Ignition (PIG) is the estimated chance that a single firebrand (a glowing ember) will cause an ignition when it lands on receptive fine fuels, such as dry grass, pine needles, or dead leaves.

Typically expressed as a percentage, a PIG of 80% means that out of 100 embers landing on the fuel bed, 80 are expected to ignite a fire. Fire behavior analysts and firefighters use this metric to assess the risk of spot fires and to determine how aggressively a fire might spread. Every larger fire has a meteorologist and a fire behavior analyst on staff to make accurate predictions for the firefighters on the line. 

How is the PIG Determined?

The National Wildfire Coordinating Group (NWCG) uses specific tables to calculate the Probability of Ignition. These calculations rely on three primary environmental factors:

• Fine Dead Fuel Moisture (FDFM): This is the moisture content of "1-hour fuels" (dead vegetation less than 1/4 inch in diameter). Because these fuels are so thin, they react almost immediately to changes in humidity and temperature. FDFM is the single most critical factor in determining the PIG. As fuel moisture drops, the probability of ignition skyrockets.

• Dry Bulb Temperature: This is the ambient air temperature. Higher temperatures pre-heat the fuels, meaning an ember has to transfer less of its own heat to bring the fuel up to its ignition point.

• Shading (Cloud and Canopy Cover): Direct sunlight significantly raises the temperature of the fuels on the ground compared to the ambient air temperature. Fuel that is unshaded (exposed to direct sunlight) will have a much higher PIG than fuel located in the shade of dense trees or heavy cloud cover.

When firefighters are on the line or analyzing weather data, they cross-reference these three variables in the NWCG Fireline Handbook to predict how easily spot fires will start ahead of the main flaming front.

We hope this helps make some sense of some of the terminology that gets thrown around! 

Thank you for reading. 

Michael Schmitt

Chief, Sunshine Fire Protection District