More firefighters headed Tuesday to Yellowstone and Grand Teton national parks, where large, growing wildfires have closed or are threatening key roads and forced the evacuation of several campgrounds during one of the busiest times of the summer tourist season.

A fire in Grand Teton park made a 5-mile run on Monday, forcing the closure of a 10-mile stretch of road leading into Yellowstone's South Entrance. More than 4,000 vehicles a day, on average, pass through the South Entrance this time of year, heading into and out of the park.

"It's the main thoroughfare between Jackson and Yellowstone National Park, and this is the 100-year centennial of the National Park Service, so there's a lot of celebrations going on," fire spokesman Brian Lawatch said Tuesday. "So it's definitely that tourist season where there's a lot of people who won't be able to go through for now."

The road was expected to remain closed Tuesday as firefighters cleared debris and any burned trees that might pose a hazard, he said.

Travelers coming from the south can still access Yellowstone through Idaho by heading west from Jackson and entering through the West Entrance.

The main fire had burned over and past the road and toward a wilderness area, Lawatch said. It grew by about 7 square miles on Monday and has now burned about 10 square miles since it was started by lightning last month.

"We're suppressing it where necessary, such as along the roads, to protect structures, things like that. But when it comes to burning toward the wilderness it's mostly being allowed to just burn," he said.

In addition, about 50 people were evacuated from several campgrounds and a lodge that rents cabins in the area.

In neighboring Yellowstone, a fire burning near the West Entrance Road grew Monday and a new team of fire managers was being brought in to help, although the fire was not being actively suppressed yet.

All roads and major tourist areas in Yellowstone remain open, but firefighters are thinning trees and underbrush near the road and the Madison Junction area as the fire grows. The fire was less than 3 miles from Madison Junction, an area that includes a campground, visitor facilities and staff housing.

The fire has burned about 42 square miles since was ignited by lightning on Aug. 8.

The weather was expected to be conducive to more fire growth Tuesday, but cooler temperatures and lighter winds were expected Wednesday, fire spokeswoman Sarah Gracey said.