Powerful winds that fueled fast-moving wildfires across Southern California this week are expected to pick up momentum on Thursday –– worsening conditions for firefighters who are already battling limited visibility to save lives.

The National Weather Service issued a Red Flag warning until 6 p.m. Thursday – which is used to describe “extreme and life-threatening fire behavior.” The warning is expected to affect 25 million people in Southern California and the greater San Francisco Bay area.