Powerful winds fueling fast-spreading wildfires across Southern California this week are expected to gain strength Thursday, worsening conditions for firefighters already struggling with limited visibility to save lives, CNN reported.

The National Weather Service has issued a red flag warning until 6 p.m. Thursday local time, 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 San Francisco Bay Area.

All schools in Ventura County, northwest of Los Angeles, are closed through Friday because of the fires.

Earlier this week, forecasters warned that conditions looked alarmingly similar to those that caused “some of the worst fires in Southern California history.”