With a heat wave hanging over the region, it took only a weekend for a wildfire raging near California’s northern border to swell into the state’s largest blaze this year. The McKinney Fire has burned more than 51,000 acres since it was reported Friday afternoon in the eastern reaches of the Klamath National Forest, fueled by “above normal temperatures and low relative humidity,” according to fire officials.  The fire was completely uncontained as of Sunday evening, with a fire watch in effect through Monday “for abundant lightning on dry fuels” from thunderstorms that were forecast to hit the area. “Probability of ignition,” fire officials forecast, was “100%.”  About 2,500 residents have been ordered to evacuate in rural Siskiyou County, said Courtney Kreider, a spokeswoman for the sheriff’s office. The police department in Etna, Calif., evacuated Pacific Crest Trail hikers by bus to Oregon on Sunday afternoon.