Popular tourist areas in Yellowstone and Grand Teton national parks will be open for the 100th anniversary of the National Park Service on Thursday, but a wildfire is forcing some visitors to drive a little farther than they expected to get to the celebration.

The fire in Grand Teton National Park has shut down a route leading to Yellowstone’s South Entrance, so visitors coming from the south through Wyoming will have to take an hourlong detour into Idaho. Firefighters are hoping cooler weather will help slow the flames Thursday, when U.S. Interior Secretary Sally Jewell is set to visit Yellowstone to mark the centennial celebration.

“Yellowstone National Park is open for business,” and no changes have been made to Jewell’s plans because of the blaze, fire spokesman Bill Swartley said Wednesday.

The wildfire in Grand Teton has burned about 11 square miles since lightning sparked it last month, forcing about 50 people to flee a lodge and cabin operation and several campgrounds. Thirty horses also were taken out of the area.

Meanwhile, four fires were burning inside Yellowstone, but all major visitor areas and roads were open.

Julie Guardado of Sacramento, California, and her boyfriend were visiting Yellowstone as part of a cross-country trip but decided to leave a day early over fears of road closures.

“When we were driving around Yellowstone, we could just see fire around the mountains,” she said Wednesday. “Our car still smells like smoke.”

The wildfires may have cut short Guardado’s trip, but she said they didn’t ruin it.

“Even with the smoke, it was still pretty amazing,” she said.

The largest blaze spans about 42 square miles between the community of West Yellowstone, Montana, and Madison Junction, an area in the park with a campground, visitor facilities and staff housing. Lightning ignited it Aug. 8.

Swartley said the fires were mostly being allowed to burn because they were not threatening major tourist areas.

“These fires are being managed as basically a good fire for the ecology and then, of course, if there’s anything getting close to cultural resources or public resources, structures, the fire is being engaged,” he said.

A 10-mile stretch of U.S. 89/191/287 has been closed since Monday after flames leaped across it, and they were still burning near the highway. The prospects for reopening the route are “not good in the near future” because the fire was close to the highway, fire spokeswoman Karen Miranda said.

Travelers coming from the south via the Jackson Hole area can still access Yellowstone through Idaho and the West Entrance. The detour adds a little over an hour to the drive to Old Faithful.

Grand Teton park and the Wyoming Department of Transportation have posted signs warning drivers of the closure. Grand Teton also is handing out notices and maps of the detour to Yellowstone.

The fire is burning on both sides of the northern portion of Jackson Lake, but no buildings have been lost. More than 100 firefighters, supported by helicopters, are battling the flames near the closed highway.

Cooler temperatures and higher humidity were expected over the next couple of days and should help firefighters, Miranda said.