Firefighters have extinguished other incursions north of the river, but the most recent one appeared to be more serious.
National Weather Service meteorologist Kyle Fredin said some rain was expected over the weekend, but it will not be enough to put the fire out.
"We need a rain that will really last all day," he said. "But it's better than dry wind at this point."
Crews were bracing for difficult conditions Sunday with wind gusts expected to hit 50 mph along ridge tops and in Poudre Canyon and temperatures in the 90s.
The fire was reported 9 June and has since raced through large swaths of private and US Forest Service land. It was 45% contained late Saturday.
Agriculture secretary Tom Vilsack, who oversees the Forest Service, met with fire managers in Fort Collins on Saturday and said "fighting this fire is going to require us to be aggressive, persistent and also patient.
"We're going to continue to work to make our forests more resilient. We're going to continue to ensure that adequate resources are provided for fighting fires and we are going to continue to make sure that we encourage appropriate stewardship of our forests," he said.
Vilsack praised Congress for allowing the government to contract additional aircraft – particularly heavy tankers – to fight wildfires across the west; but he called on law makers for budget certainty to help plan for future fires.
Meanwhile in New Mexico, questions were being raised about whether bureaucratic red tape prevented firefighters from saving more homes affected by the Little Bear Fire after federal officials released transcripts of the firefighters' response.
U.S. Rep. Steve Pearce said Friday in an interview with KOB-TV that he believed federal officials could have done more after lightning sparked the fire outside the resort town of Ruidoso on June 4.
Days later, high winds sent embers more than a mile from the blaze's end, causing the inferno to grow.
Officials released transcripts of the response on the Lincoln National Forest website that suggested firefighters were attacking the blaze as soon as it was a quarter of an acre.
The fire has destroyed 242 homes and commercial structures. It had burned 59 square miles and was 60% contained as of Saturday night.
In Arizona, the Northern Arizona Incident Management Team took command of a blaze in the Tonto National Forest that doubled in size over the past 24 hours to 3,100 acres.
Officials said Saturday night that the fire was 15% contained and firefighters continued to battle unseasonably dry fuels, high temperatures and low humidity.
Intellpuke: This article is a compilation of reporting by Guardian journalists and various news agencies; you can read it in context here: www.guardian.co.uk/world/2012/jun/17/colorado-wildfires-containment