"They're hedging their bets," Bertram said.
The town is a popular spot for hiking and camping. The fictional Griswold family camped in South Fork in 1983's "National Lampoon's Vacation." The famous scene where a dog urinates on a picnic basket was filmed at South Fork's Riverbend Resort, called "Kamp Komfort" in the movie.
Residents were being sent to a high school in a neighboring town.
South Fork's mayor, Kenneth Brooke, sent his children and grandchildren to a safe location and stayed behind, helping several dozen area fire responders prepare for hosing down structures.
Brooke said authorities are allowing him to stay in South Fork until the blaze crests a nearby mountain, expected Friday afternoon. Until then, the mayor was taking phone calls from nervous neighbors and telling them the town's grim forecast.
"I just tell them it doesn't looks good," Brook told The Associated Press by phone Friday. "I tell them the truth, that the fire is coming. I just tell them to keep themselves safe, evacuate as need be and don't come back.
"We're just watching the fire and doing what we can for people's properties, but mostly it's just waiting. Right now I'm saying, 'Stay out.'"
Bertram said the hot, dry and windy weather along with large stands of beetle-killed trees are causing extreme fire behavior. While most fires actively burn four hours a day, this one is burning for 12 hours a day, helping it to mushroom in recent days.
Firefighters have largely let the lightning-sparked fire burn because it's too hot and erratic to fight on the ground. Water and slurry drops from air tankers also haven't been effective, with pilots reporting that their drops largely evaporated before hitting the ground.
"There's no stopping it," Bertram said.
The town was incorporated in 1992, making it Colorado's youngest municipality. The town lost 11 structures in the 2002 Million Fire, which charred more than 14 square miles.
South Fork residents are used to damaging wildfires, but this year's is shaping up to be the worst, Brooke said.
"Our tourists are what support the town. The fires are going to run everybody away. So that's going to hurt," he said.