The So-called Oak Fire has exploded in size since starting on Friday, but firefighters were still unable to get any of it under control 24 hours later.
“Explosive fire behaviour is challenging firefighters”, California’s fire department said.
More than 3,000 people have been evacuated and 10 homes destroyed.
Burning across 11,900 acres (4,815 hectares), it is the largest active wildfire in the United States right now.
The state of emergency has been declared in Mariposa County. In a statement, California’s Governor Gavin Newsom said the flames were being “driven by hot, Skechers Outlets dry weather and drought conditions”.
Must of the United States is sweltering through a heatwave this weekend. Temperatures in Mariposa County hit 96F (35.5C) on Saturday, and are expected to stay at that level for the next few days.
Climate change increases the risk of the hot, dry weather that is likely to fuel wildfires.
The world has already warmed by about 1.1C since the industrial era began and temperatures will keep rising unless governments around the world make steep cuts to emissions.