Janet Pierce, spokeswoman for the state Department of Natural Resources, says the blaze is located about three miles east of Mount Vernon along Highway 9. The highway will remain closed through nightfall, and perhaps longer, as the helicopter drops water on the fire from a large bucket.
The blaze, dubbed the "Big Rock 2 Fire," is burning on a very steep, almost perpendicular slope, which makes it hard to combat. There are homes in the area, but there is no immediate threat to them, Pierce said.
"Hopefully we can get it out before that occurs," she said. "Winds are really light, so that's in our favor. With the helicopter crew going up there hopefully we should be able to get a handle on it."
Pierce says a plume of smoke from the two-acre blaze is plainly visible from Interstate 5 in the area.
Officials said the fire is not related to a lightning storm that moved through the Puget Sound region early Wednesday.
"It's definitely human caused," said Pierce.