Red Flag Warnings Explained: What Fire Weather Means for You

Wildfires spread fastest when three things line up: dry fuel, strong wind and low humidity. When forecasters see that combination coming, they issue fire weather alerts so people can avoid starting fires and be ready to act.

Fire Weather Watch vs. Red Flag Warning

In the United States, the National Weather Service uses two levels of alert:

• Fire Weather Watch — critical fire weather is possible in the next 12 to 72 hours.
• Red Flag Warning — critical fire weather is happening now or is expected very soon, usually within 24 hours.

The exact thresholds vary by region, because what counts as dangerous in a desert differs from a humid forest. Most combine strong winds with very low relative humidity and dry vegetation, and dry thunderstorms — lightning with little rain — can trigger a warning on their own.

The weather behind fast-moving fires

Wind

Wind pushes flames forward, feeds them oxygen and carries embers far ahead of the fire front. Downslope wind events such as California's Santa Ana and Diablo winds are behind many of the most destructive fires on record.

Low humidity

Dry air pulls moisture out of grass, leaves and dead wood, making them much easier to ignite. Fine fuels like grass can dry out within hours.

Heat and drought

High temperatures and long dry spells dry out heavier fuels such as logs and shrubs, so fires burn hotter and are harder to put out.

What to do during a Red Flag Warning

• Don't burn debris, use fire pits or set off fireworks.
• Avoid mowers, chainsaws and grinders near dry grass — a single spark can start a fire.
• Check that trailer chains don't drag, and never park on dry grass.
• Put cigarettes out fully and dispose of them in a closed container.
• Bring outdoor cushions and doormats inside, and check your go bag.
• Keep your phone charged and watch for evacuation alerts.

Fire danger ratings outside the United States

Other countries use their own systems. Canada developed the Fire Weather Index, which many countries now use; Australia uses the Australian Fire Danger Rating System; and in Europe, the European Forest Fire Information System (EFFIS) publishes daily fire danger forecasts. Whatever the name, a high rating means the same thing: fires will start easily and spread fast.