What are red flag warnings?

Firefighters try to save a home from a wildfire on Tigertail Road in Los Angeles on Oct. 28. A red flag warning is an alert issued by the National Weather Service when: • winds are strong, • the air is dry, and • moisture in vegetation is exceedingly low. A red flag warning means the National Weather Service has high confidence these dangerous fire conditions will actually come about. An “extreme red flag warning” was issued in October 2019 by the weather service’s Oxnard office. Just days earlier, the weather service office overseeing the Bay Area referred to its own red flag warning as exceptionally dangerous by calling it potentially “historic” and “extreme.” At the time, forecasters in the weather service’s office said the duration of the Diablo wind event, which persisted over two overnight periods, was making conditions particularly dangerous and potentially historic for the region.