The National Weather Service on Tuesday extended its red flag warning for Southern California until Thursday evening.

Red flag criteria is generally when relative humidity is at 15% or less, combined with sustained winds and/or gusts of 25 mph or greater. Both of these conditions must occur simultaneously for at least 3 hours in a 12 hour period, according to the NWS.

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.