What does a 'red flag warning' mean, exactly?

Red flag warning: They're some of the most dreaded words in fire-prone California.

By now, many locals understand the term to basically mean, "Watch out for fires. It's about to get real."

LAist has been demystifying some of the jargon around fires, so when Betsy Lawlor from Diamond Bar asked us to break down exactly what a red flag warning is, we were more than happy to explain.

Who decides to issue a red flag warning? What goes into that decision? And what should you do in response? Here's everything you need to know.

The basics

Red flag warnings are put out by the National Weather Service when local weather conditions are in place that could cause fires to spread rapidly.

According to the agency's online glossary of weather-related terms, a red flag warning involves the following conditions:

1. Sustained wind speeds of 15 mph or more
2. Relative humidity of 25% or less
3. Temperatures above 75 degrees Fahrenheit

However, NWS meteorologist Lisa Phillips said crossing those thresholds doesn't automatically trigger a red flag warning -- and a warning can be called even when some of those criteria are not met.

"It is case-by-case," Phillips said. "We might not meet every single criteria, but we know from experience that, yes, we're definitely going to need to put a red flag warning out."

Phillips said the decision depends on variables like the dryness of local vegetation, the time of year and the history of burns in the area.

Wind whips trees as firefighters battle the Saddleridge fire in Porter Ranch, Calif., on Friday, Oct. 11, 2019. (Noah Berger/AP)

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The details

Red flag warnings are confined to specific areas, often outlined on maps put out by public safety departments. In many cases, flat coastal areas may be outside the warning zone, while nearby hilly neighborhoods will be included.

"Topography plays a really large role," Phillips said.

Areas at higher elevations tend to have gustier winds, while valleys with slower wind speeds may fall outside the red flag warning zone.

Phillips said NWS forecasters do their best to exclude areas that don't need to be under a warning, but, "you can't pinpoint every single region and say, yes for you, no for you," she said. "We do the best we can in terms of sectioning everything off."

Red flag warnings often follow a less severe alert called a "fire weather watch," which indicates conditions could potentially worsen within the next three days.

Once the NWS decides to issue a red flag warning, it often includes an estimated time for when the warning will be lifted. NWS meteorologist Mark Jackson said those estimates are based on weather forecasts.

"When we decide on an end time, what we're seeing is that you won't have that combination of strong winds with low relative humidity," Jackson said.

That doesn't mean fires will be extinguished or even contained by that time, he said. It just means the conditions that make fires much worse are coming back down to normal.

What a red flag warning means for you

Red flag warnings tell other agencies, like fire and transportation departments, that it's time to staff up and prepare to respond to fast-moving fires.

But they're also intended to put the public on high alert.

You may have heard the phrase "Ready, Set, Go" for fire preparedness. "Ready" involves steps like clearing defensible space around your home and developing an evacuation plan with your family.

Jackson said a red flag warning is your cue to get "Set" to leave your home, even if you haven't yet been told to "Go" through an evacuation order.

"Don't wait until you've been told to evacuate to get your stuff," Jackson said. "There may not be a fire. But if there is, you're set to go. You've got your prescriptions, your photos, pets, personal papers and plastics like credit cards."

When a red flag warning is in effect, you should also take a few extra steps to avoid causing a massively destructive fire yourself.

Don't use any outdoor machinery like lawn mowers that can create sparks.

• If you're out camping, fully extinguish any outdoor fires you've started.
• Don't idle or park your car over grass.
• Don't throw lit cigarettes or matches out of a moving car. (Honestly, just don't do this ever. Come on, people.)

Fire resources and tips

• If you have to evacuate

  • Cheat sheet: Your very short guide to getting packed up right now to evacuate
  • Cheat sheet: A very short guide for how to leave your house if you need to evacuate

• If you have more time:

  • Every day is now fire season. Let's get you ready for the next wildfire burning near you

  Things to consider:

  • Evacuation terms can be confusing. Here’s what they mean and how to sign up for alerts
  • This is why fire officials don't want you to stay and defend your home

  Navigating fire conditions

  • Cheat sheet: Your very short guide to driving in high winds and fire danger
  • High winds and fires mean power outages. Here's how to prep

  How to help yourself and others

  • Resources for SoCal fire victims, evacuees and first responders
  • What to do — and not do — when you get home after a wildfire
  • If you want to help fire victims, resist the urge to volunteer
  • These are the steps fire victims need to take to make an insurance claim

  Understanding how it got this bad

  • Yes, wildfires are getting worse. The history that got us to this moment