The Rocky Mountain Incident Management Team said nearly 400 firefighters were battling the Hewlett Gulch fire, northwest of Fort Collins, and additional firefighters have been requested, The Denver Post reported.
A thunderstorm moved through the region Thursday, bringing winds that complicated efforts to fight the fire. Authorities said they had no estimate on when the fire would be contained.
An evacuation center was being set up at an elementary school in LaPorte, where a community meeting was planned Thursday night.
The blaze began Monday afternoon and is believed to have been human-caused. It remains under investigation.
The fire was burning brush, open timber and grass in Roosevelt National Forest.
Shifting winds caused it to grow dramatically Wednesday.
Part of Highway 14 was closed.
Two heavy air tankers, a heavy helicopter and one light helicopter were assisting crews on the ground.
The Colorado Department of Public Health and Environment said a smoke health advisory for areas around the fire would be in place until noon Friday for the Poudre Canyon area, including LaPorte and Fort Collins.
Older adults and children, and anyone with heart or lung disease, should avoid exposure to the smoky air for long periods. If visibility is less than 5 miles, residents were urged to stay indoors.