Sheriff Joe Pelle estimated that 580 homes or other structures in and around the Superior may have been lost.
A shopping mall and a Superior hotel were also engulfed in flames.
The Marshall fire had burned at least 1,600 acres and had spread east over Superior and Louisville, the sheriff said. The second brand is known as Middle Fork Fire, but there were no details about its size.
There are currently no reports of injured or missing persons, Pelle said.
“I would like to emphasize that due to the scale of this fire, the intensity of this fire and its presence in such a densely populated area, we would not be surprised if there are injured or killed.”
Governor Jared Polis, who declared a state of emergency for the area, said the flames traveled along a football field in seconds.
“Very little time to get out, very little time to even get the most important parts of your life, and yes, it will be a difficult process for Colorado families directly affected to rebuild their lives,” he said. .
Louisville and Superior are about 4 miles apart, not far from Boulder.
At least six people are being treated for injuries related to one of the fires, a UCHealth spokesman told CNN. Kelli Christensen said there was no further information available to patients who are at UCHealth Broomfield Hospital.
Another hospital, Avista Adventist Hospital, was asked to evacuate its patients and staff. The facility has 114 beds, according to its website.
The U.S. Census Bureau says Louisville has about 21,000 residents, and the Superior has 13,000.
“We are in the middle of a truly historic windstorm across the front line, the foothills and the city corridor,” said the National Weather Service office in Boulder.
Widespread gusts of 80-100 mph have been observed, the weather service said, and “a handful of places in the Rocky Flats have seen several hours of gusts of 100-115 mph.”
County residents have been asked to evacuate if they see flames from one of several wildfires.
The hurricane-strength gusts reported are attributed to “exceptional mountain wave amplification”, which is a large increase in wind speed as air flows over and down the mountains at the lower altitudes that can occur under very specific atmospheric conditions.
Warnings of strong winds are in effect for parts of Colorado and Wyoming through this evening. Sustained winds of 25 to 40 mph with gusts of over 80 mph are forecast.
The “strongest winds below 6,000 feet are expected to be near the base of the foot from Boulder north to Fort Collins to the Wyoming border,” the weather service said.
In addition to spreading fires quickly, these strong winds are likely to blow down trees and power lines, which can lead to extensive power outages and difficult travel.
There will likely be a “rapid withdrawal of the strongest winds back to the foot” Thursday night, the NWS office said.
In the morning, the region will experience “weather whiplash” going “from fire to snow,” according to the weather service. A advancing cold front will push into the area and bring snow showers to the foot and front line at sunrise.
A winter storm warning is in effect for the region, which begins at 7 p.m. 05.00 MT Friday and requires 5-10 inches of snow on Saturday. The combination of heavy snow and wind associated with the front and terrain will create treacherous travel conditions at times.
CNN’s Haley Brink and Brandon Miller contributed to this report.