The specific cause was not clear as of early Friday morning. Sheriff Joe Pelle of Boulder County said on Thursday that he believed the fires had been started by downed power lines.
The wildfires came unusually late in the year for Colorado, where severe drought conditions in recent months have set the stage for such blazes to spread with ease. And they capped a year of intense wildfires and other extreme weather events across the American West.
A state of emergency was declared.
The fires on Thursday burned at least 500 homes, a shopping complex and a hotel, the authorities said. No deaths or major injuries had been reported as of early Friday. But some residents lost power, and others likened the damage to that of a 2013 wildfire that, at the time, was the most destructive in Colorado’s history.
Gov. Jared Polis on Thursday declared a state of emergency, a decision that allows the state to tap emergency funds and to deploy the Colorado National Guard and other resources. He called the fires a “force of nature.”
The Boulder County authorities also issued evacuation orders for Superior, Louisville and for some residents in Broomfield and Westminster, a city with about 116,000 people. All of those communities lie between Boulder and Denver, the capital.
The high winds caused power outages in the Boulder area and complicated the evacuation of residents, but had mostly died down by early Friday.
The winds came two weeks after a powerful storm system generated dust clouds in Colorado and other extreme weather across the Midwest.
What happens next?
Representatives from the Office of Emergency Management were expected to address the news media at 10 a.m. local time on Friday to provide an update on the fires and the damages.
Snow was in the forecast for the Boulder area on Friday, and a big snowfall would bring substantial relief to a parched area, the Boulder-based snow hydrologist Keith Musselman told The Associated Press.
Still, an air quality advisory for areas affected by the fires remained in effect until noon on Friday. And many of the people who were evacuated on Thursday were still wondering if their homes had survived the onslaught.