© Brandon Runnells© North Carolina Forest Service© South Carolina Forestry CommissionThe cause of the Carolina Forest Fire is under investigation. The blaze erupted amid wind gusts of up to 40 mph and extremely dry conditions, fire officials said.A fire in Horry County north of the Carolina Forest had scorched more than 300 acres by Sunday morning and was burning out of control, officials said.Red flag fire danger warnings were issued across South Carolina. The fire was 85% contained after burning nearly 300 acres, officials said.In Polk County, North Carolina, near the border with South Carolina, a fast-moving brush fire ignited just after 2 p.m. local time on Saturday and grew overnight, threatening the towns of Melrose, Tryon and Saluda, where mandatory evacuation orders were issued, according to the Saluda Fire and Rescue Department.The Polk County Fire started at about 1:30 p.m. on Saturday, caused by a downed power line near Highway 176, officials said. By 9 p.m. Saturday, the blaze had spread to 400 acres with no containment reported, according to fire officials.Dry and breezy conditions were prompting red flag warnings across much of the Southeast on Saturday and into Sunday. Most of the red flag warnings expired by Sunday morning as winds calmed down.But other areas in the Southwest were bracing for critical fire weather on Sunday and into Monday, including parts of New Mexico and Texas.