An estimated 580 homes, a hotel and a shopping centre have burned and tens of thousands of people were evacuated in wind-fuelled wildfires outside Denver, officials say.
At least one first responder and six others were injured, though Boulder County Sheriff Joe Pelle acknowledged there could be more injuries and deaths could be possible due to the intensity of fires that quickly swept across the region as winds gusted up to 169km/h.
The first fire erupted just before 10.30am on Thursday and was "attacked pretty quickly and laid down later in the day and is currently being monitored" with no structures lost, Pelle said.
A second wildfire, reported just after 11am, "ballooned and spread rapidly east", Pelle said. The blaze spans 6.5 square kilometres and has engulfed parts of the area in smoky, orange-hued skies and sent residents scrambling to get to safety.
The activity of the fires, which are burning unusually late into the winter season, will depend on how the winds behave overnight and could determine when crews are able to go in and begin assessing the damage and searching for any victims.
"This is the kind of fire we can't fight head on," Pelle said. "We actually had deputy sheriffs and firefighters in areas that had to pull out because they just got overrun," he added.
Evacuations have been ordered for the city of Louisville, home to about 21,000 people, and Superior, which has another 13,000 residents.
The neighbouring towns are roughly 32 kilometres northwest of Denver in an area filled with middle and upper-middle class subdivisions surrounded by shopping centres, parks and schools.
The area is in between Denver and Boulder, a foothills college town home to the University of Colorado.
Residents evacuated fairly calmly and orderly, but the winding streets in the suburban subdivisions quickly became clogged as people tried to get out. It sometimes took cars as long as 45 minutes to advance less than a kilometre.
Small fires cropped up here and there in surprising places -- on the grass in a median or in a dumpster in the middle of a parking lot -- as wind gusts caused the fire to jump and spread.
Shifting winds caused the skies to turn from clear to smoky and then back again as emergency sirens blared nearby.
Some of the several blazes in the area on Thursday were sparked by downed power lines.
Six people who were injured in the fires were being treated at UCHealth Broomfield Hospital, spokesperson Kelli Christensen said. A nearby portion of US Highway 36 also was shut down.
Colorado's Front Range, where most of the state's population lives, had an extremely dry and mild fall, and winter so far has continued to be mostly dry.
Denver set a record for most consecutive days without snow before it got a small storm on December 10. It hasn't snowed since, though snow was expected in the region Friday.
The fires prompted Governor Jared Polis to declare a state of a emergency, allowing the state to access disaster emergency funds.
The evacuations come as climate change is making weather more extreme and wildfires more frequent and destructive, scientists say. A historic drought and heat waves have made wildfires harder to fight in the US West.
Ninety per cent of Boulder County is in severe or extreme drought, and hasn't seen substantial rainfall since mid-summer.
The National Weather Service predicts up to a foot of snow could fall tomorrow in Boulder, and that moisture would bring substantial relief, Musselman said.