Why has no Red Flag warning been issued for San Diego?

There's no warning now, but that could change soon

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Often, when we experience hot, dry and windy conditions, you'll hear of the National Weather Service (NWS) issuing a Red Flag warning. So why aren't we seeing one now? 

The simple answer is that because of the recent rains in San Diego County, our fuel moisture levels do not meet the threshold that would trigger a warning.

The National Weather Service tells Team 10 the process is more involved than some may think.

By definition, a Red Flag warning is used to call attention to weather factors "that may result in extreme burning conditions." It is also issued "whenever a geographical area has been in a dry spell for a week or two."

National Weather Service meteorologist Philip Gonsalves says the rain this past weekend added to the moisture in the ground. 

"We didn't have very much rain, but it's relative. Relative to Orange County, and all these other counties just to our north, we did have more rain than they did," Gonsalves said.

The NWS consults others before issuing a warning, including Cal Fire, the U.S. Forest Service, and various fire departments with fire behavior analysts.

"Every morning, we have a conference call and we're part of the conference call and we coordinate on this conference call with some of these agencies," Gonsalves said. 

He says the Red Flag warning was first created for responding agencies, such as fire departments, but ultimately became a tool the public used as well.

"The absence of a Red Flag warning doesn't necessarily mean there's not an elevated danger, and certainly we'd like people to be cognizant of that fact," Gonsalves said.

Fires will always start much more easily under Santa Ana conditions – they're just not expected to be explosive and spread as fast as they would in summer when all the green brush has dried out and died.

How is fuel moisture determined?

Every two weeks, fuel moisture experts sample the fuels available throughout the county and enter that information into a database accessible to meteorologists, the NWS and other agencies.

Right now, San Diego's moisture levels are better than surrounding counties, such as Orange and Imperial, that are under a red flag warning. This is because the last few storms gave us more rain than other parts of Southern California.

Experts look at two things in determining moisture levels: Dead fuels and live fuels.

Dead fuels are material such as dead leaves that depend solely on the atmosphere for moisture—that is, they get wet if there's rain, high humidity, etc. Live fuels, by contrast, get their moisture from the plant's root system.

Dead fuels respond quickly to weather. If it rains, these materials soak up the moisture and won't burn as fast. If there's a drought or it's sunny and hot, they dry out fast. While the fuel moisture is up right now, that can change with multiple days of heat and hot winds, which could trigger a red flag warning for us.

Regardless of whether there's a warning or not, remember: There's a high fire danger anytime it's dry, hot and windy out of the east.

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