The fire started on Monday in steep terrain about eight miles north of Idaho City.
Changes in wind direction and low humidity helped the fire to spread rapidly on Thursday, officials say.
With a likely increase to wind speeds expected overnight, firefighters are preparing for the fire to continue to grow.
Almost 300 firefighters, including five hotshot crews and five helicopters, are working to douse the fire, which was 20 percent contained Thursday evening.
Investigators are still trying to find out what sparked the fire.