A fierce, wind-driven wildfire destroyed hundreds of homes, injured at least a half dozen people and prompted the evacuation of tens of thousands of residents in and around two towns east of the Rockies near Denver.
The swiftly spreading prairie grass fire was believed to have been ignited by sparks from power lines and transformers toppled by high winds on Colorado's drought-parched Front Range, according to Boulder County Sheriff Joe Pelle.
Evacuation orders were first issued for all residents in the town of Superior, Colorado, with a population of about 13,000, and a short time later for the adjacent municipality of Louisville, home to more than 18,000 residents, the Boulder County emergency management office said on Twitter.
Separately, the National Weather Service office in Boulder tweeted: "All Superior under an Evacuation ORDER. LEAVE NOW!"
Within hours, the blaze had swept an estimated 1,600 acres and destroyed more than 500 homes, Mr Pelle told reporters at a news briefing.
He said an entire subdivision of 370 homes went up in flames west of Superior, and that 210 dwellings were lost in the Old Town area of Superior, along with additional residences in the area.
Property losses also included a shopping centre and hotel in Superior, officials said.
Governor Jared Polis said flames were consuming football fields of landscape in a matter of seconds, calling the conflagration "a force of nature".
Multiple buildings and vehicles were alight. Source: Trevor Hughes-USA TODAY/Sipa USA
The sheriff said gale-force winds made it impossible to halt the fire's rapid advance, adding that fire and emergency personnel were "essentially running ahead of this just trying to get people out of the way. That's all you can do".
Mr Pelle said tens of thousands of area residents were under evacuation orders.
Among them were about 30 patients, along with staff, from Avista Adventist Hospital in Louisville, which was shut down and fully evacuated as of the late afternoon, according to the medical network Centura Health.
Some patients were discharged, and the rest were transferred to other Centura hospitals.
The sheriff said he had no reports of people missing and no deaths reported.
Six people were treated for injuries from the wildfires at the UCHealth hospital in the neighbouring city of Broomfield, spokesperson Kelli Christensen said.
Mr Polis declared a state of emergency allowing the use of disaster funding to support emergency response efforts in Boulder County and to allow mobilisation of the Colorado National Guard and other state resources as needed.
The fire on the outskirts of the Denver metropolitan area, left bone dry from an extreme drought gripping eastern Colorado, follows several days of heavy snow in the Rocky Mountains to the west. However, forecasts call for snow to hit Denver and eastern Colorado starting on Friday.
