SAN DIEGO (FOX 5/KUSI) -- A Red Flag Warning that went into effect Monday for San Diego County mountains and valleys has been extended due to strong gusty winds and low relative humidity. The Red Flag Warning, which was originally set to expire Tuesday at 10 p.m., will now last until 8 p.m. Thursday, the National Weather Service San Diego said on its website Tuesday. The San Diego County Office of Education announced the closures of the following school districts amid the Red Flag Warning: Mountain Empire Unified School District Warner Unified School District  "The safety of students and school staff is of the utmost importance to San Diego County school districts," SDCOE said. Evacuations lifted for Lilac Fire in North County The strongest winds could be felt Wednesday night, with winds increasing northeast 20 to 30 mph. Residents should expect gusts from 40 to 50 mph, while isolated gusts may reach up to 75 mph for the San Diego County and Santa Ana mountains, according to NWS. For relative humidity, NWS says it will be a "very poor recovery Tuesday night andWednesday morning of 10 to 20 percent in wind prone areas." Wednesday and Thursday's relative humidity will be as low as 4 to 10 percent. "If fire ignition occurs, conditions are favorable for rapid fire spread and extreme fire behavior," NWS said. NWS defines a Red Flag Warning is defined critical fire weather conditions that are either occurring now or will shortly from a combination of strong winds, low relative humidity and warm temperatures. Thousands of San Diego Gas & Electric customers are experiencing outages during another round of public safety power shutoffs that went into effect Monday at 6 a.m. An 85-acre fire that prompted evacuations also occurred during a Red Flag Warning Tuesday just south of Pala Mesa overnight into Tuesday. Evacuations have been lifted, and that fire is currently 50% contained.