It's been a long, hot stretch, and rainfall seems but a distant memory. All of a sudden, your local National Weather Service office has issued a "Red Flag Warning" for your area.
What does that mean?
It's an indication that conditions are ripe for extreme fire behavior over the next 24 hours. Think of it as a Fire Weather Warning. (In fact, here at FOX Weather, we will refer to Red Flag Warnings as "Fire Weather Warnings" for better clarity.)
The first ingredient is an ample supply of dry fuels to burn -- easy to find in many areas after a long hot and dry stretch. Then if the current or near-future weather pattern features gusty winds, low humidity, warm temperatures, and/or a chance of lightning, Red Flag Warnings could be issued.
Each region has different criteria for triggering a warning.
For example, in the Chicago and northern Illinois area, winds of 20 mph or higher with an afternoon relative humidity of 25% and 10-hour fuel moisture at 8% or less for one day will trigger a Red Flag Warning.
In Western Washington, dry fuels need to combine with wind greater than 10 mph at 20 feet high and humidity levels under 35% at night and under 30% during the day. Having lightning in the area or enough instability in the atmosphere to potentially trigger thunderstorms could also warrant a Red Flag Warning.
In Oklahoma and west Texas, humidity levels have to be at 15% or less with the winds and dry fuels to trigger the warning.
On rare occasions where the situation is particularly dire, forecasters will issue an Extreme Red Flag Warning. The Los Angeles area was under such a warning in Oct. 2019 during a historic Santa Ana wind event that was promising 80 mph winds for more than a day.
When Red Flag Warnings are issued, Fire Weather forecasters urge extreme caution with any fire because any spark could end up generating a significant wildfire.
Firefighter managers with Cal Fire say they will place additional firefighters on duty and staff more fire engines to respond quickly to any new fires.
What about a Fire Weather Watch?
A Fire Weather Watch is issued in the longer term when forecasters believe conditions could be ripe for extreme fire danger within the next 12-72 hours.
A Fire Weather Watch can be converted into a Red Flag Warning ("Fire Weather Warning") when conditions become more imminent.