Since it was spotted Sunday, the Thompson Fire in Glacier National Park has spread to almost 2,000 acres/NPS

Editor's note: This updates the story to reflect that the acreage burned now stands at about 5,500 acres, provides details on firefighting effort.

A wisp of smoke spotted by a backcountry lookout in Glacier National Park blossomed Tuesday into a roughly 5,500-acre wildfire burning in thick stands of timber on a rugged landscape that so far has precluded the use of ground crews to battle the flames.

The Thompson Fire, burning in the southern portion of the park far south of the Going-to-the-Sun Road, was reported Sunday by a spotter in the park's Loneman Lookout, which stands atop 7,181-foot Loneman Mountain.

"The lack of moisture, high temperatures, low humidity, and dry vegetation have resulted in extreme fire behavior and fire growth," Glacier spokeswoman Denise German wrote in a release. "The smoke plume is visible from areas on both the west and east sides of the Continental Divide."

Thirty-five personnel were being aided Tuesday afternoon by two helicopters for water drops, with additional air resources available. 

Fire bosses were working to "contain and confine" the fire.

"This is being accomplished by using aerial resources with water bucket drops to cool hotspots. Fire managers are using natural (rocky) barriers to confine the fire," wrote Ms. German. "The National Park Service Upper and Lower Nyack Backcountry Patrol Cabins remain threatened by the fire. Containment actions are being taken to protect the lower cabin. These actions include fuel mitigation actions around the site and structure wrap of the cabin."

The Thompson Fire has prompted closure of the Nyack and Cole Creek area, including trails and backcountry campsites and the section of trail from Nyack Creek to Cutbank Pass.

The Reynolds Creek Fire, meanwhile, remained 67 percent contained. Spotted on July 21 and possibly caused by campers, the fire burning about 4-5 miles east of Logan Pass has covered an estimated 4,311 acres. Progress on battling this fire has allowed for reopening of the entire Going-to-the-Sun Road.