A massive, unpredictable fire that sent ash plumes hundreds of miles forced authorities Tuesday to shut down most of the Boundary Waters Canoe Area Wilderness and urge hundreds of nearby homeowners to flee.

Fanned by unanticipated winds and fueled by dry wood, the blaze has spread rapidly to consume more than 100,000 acres, making it one of the biggest forest fires in Minnesota history. One U.S. Forest Service official called the speed of the fire "pretty much unprecedented."

Defying firefighters' efforts to contain it to the wilderness area, the blaze broke through the southern edge of the BWCA in northeastern Minnesota to threaten private property.

Firefighters also were closely watching the eastern edge of the fire, which reached a small portion of the forest knocked down by a major windstorm in 1999. The blowdown left miles of dry timber in its path.

"Already severe and erratic fire behavior ... is going to be more severe and more erratic if it's in the blowdown," said Jean Bergerson, lead public information officer for the Forest Service and state and local agencies fighting the fire. "It's going to be an additional cause for concern."

There were no reports of injuries or serious property damage Tuesday, as light rain dampened some of the area and prevented the fire from growing much in the last 24 hours. But authorities fear that more wind will spread it further.

"It's a big beast out there," said Doug Anderson, another public information officer for the fire-fighting crew. "It's an uncontained fire. It took a nap today but it didn't go away."

Forest Service spokeswoman Mary Shedd said it will take a "season-ending" rain or snowfall to put the fire completely out.

The forecast for Wednesday calls for gusty northwest winds, but a high of only 47 degrees and scattered showers, said National Weather Service forecaster Byron Paulson, who has been assigned from the Twin Cities office to work at the fire.

At a meeting Tuesday night in Ely, west of the fire area, officials told an audience of more than 120 people that they've been caught off guard by the fire's rapid spread east and south after fighting to keep it out of public areas near the town over the weekend.

Since the fire began Aug. 18, none of the forecast rain has materialized, and winds have been stronger than predicted, said Mark Van Every, U.S. Forest Service ranger from the Kawishiwi Ranger District. He said models indicated the fire had only a 0.2 percent probability of reaching its current size.

"What we've had happen doesn't fit anything we've seen historically," he said.

Authorities decided to shut all but a few entry points on the far northern end of the Gunflint Trail in the eastern BWCA for fear that campers could become trapped if the fire spread.

"We do not want to take a chance of campers in there who we cannot reach," said Lisa Radosevich-Craig, a spokeswoman for the U.S. Forest Service.

She said safety crews would paddle lake to lake to urge campers to leave and help them cover portage trails quickly. Entry to the western part of the BWCA near Ely will still be allowed.

Advised to leave

Meanwhile, hundreds of homeowners who live along a 5-mile corridor between Hwy. 1 and the southern edge of the fire were being advised to leave. Thirty-six homeowners left late Monday and early Tuesday.

"We're not going to drag them kicking and screaming out of there," Lake County Sheriff Carey Johnson said. "But we can't promise we can get back in there to help them."

At the Knotted Pine tavern near Isabella, Minn., Joyce Kuehl, who has lived in the area for 15 years, said residents have worried about fire danger since the 1999 blowdown. The past several nights, she said, she's been "sleeping with one eye open."

She's already packed up her photographs, papers and jewelry and plans to stay with relatives in Two Harbors if necessary.

"We've got so much stuff, you can't pack it all," she said.

Fighting the fire

About 300 people were fighting the fire. Four Minnesota National Guard Black Hawk helicopters and Army and Air National Guard aviation support teams were to be used to dump water on the fire. Gov. Mark Dayton issued an emergency order authorizing the Guard to act.

Authorities early Tuesday estimated that the fire had consumed 60,000 acres, but that estimate grew to 100,000 acres after they were able to get a better look from the air.

"We couldn't see the fire from the smoke," Anderson said.

The dimensions exceed the 2007 Ham Lake blaze, the state's most destructive in 90 years. The fire at Ham Lake, which is just outside the BWCA, burned 76,000 acres.

The fire sent a smoky odor and ash into parts of northern Michigan, hundreds of miles away, and Chicago, according to the Associated Press. The weather service said the smell was noticeable in Traverse City and reported ash falling from the sky in Marquette.

The Milwaukee Journal Sentinel reported that the fire sent ash and smoke to large parts of Wisconsin, including Milwaukee. The paper said the Milwaukee Brewers decided to close the roof of Miller Park for Tuesday night's baseball game against the Colorado Rockies. The paper said the team linked the roof closing to haze and cooler weather.

According to the U.S. Forest Service, the BWCA fire began burning on a much smaller scale on Aug. 18. The fire took on greater urgency Monday.

"That cold front came in, and there were high winds and it spread to the south and the east," said Jean Goad, a spokeswoman for the Minnesota Interagency Fire Center. "It made a 16-mile run to the east."

Radosevich-Craig added, "It's pretty much unprecedented, the distance of the run."

mcaul@startribune.com • 612-673-7646 pdoyle@startribune.com • 612-673-4504