By Chris Kuo From Los Angeles Times LOS ANGELES—Wildfires continue to devastate the northern forests of California as scorching heat and severe drought are creating ideal conditions for conflagrations. The River fire, which broke out about 20 miles southwest of Yosemite National Park on Sunday afternoon, exploded to 2,500 acres in just a few hours and had spread to 4,000 acres by Monday morning, officials with the California Department of Forestry and Fire Protection, or Cal Fire, said. More than 450 personnel—including some using air tankers—were battling the blaze, which was 5 percent contained. Mandatory evacuation orders were in place in parts of Madera and Mariposa counties. The cause of the fire is under investigation, Cal Fire said. Dangerous heat, persistent drought and low humidities are priming California for extreme fire behavior, experts say. North of the River fire, the lightning-induced Beckwourth Complex fire, which includes the massive Sugar fire, …