More than 30,000 Boulder County residents were forced to evacuate their homes Thursday afternoon to escape two wildfires sweeping the area.

Unusually dry, windy conditions fueled two swiftly moving grass fires, one of which destroyed more than 500 homes in Superior and Marshall, making it the most destructive wildfire in Colorado history in terms of number of homes burned. “Gusts of 100, 110 miles an hour can and have moved this fire down a football field in a matter of seconds,” Governor Jared Polis said during a press briefing held Thursday evening at the Boulder County Sheriff headquarters.

The Boulder Office of Emergency Management ordered the residents of Superior, a town of 12,000 east of the city of Boulder, to evacuate around 1 p.m. Louisville’s 20,000 residents were told to evacuate not long after, at 1:24 p.m. Avista Adventist Hospital and Centennial Peaks Hospital in Louisville evacuated all patients by the afternoon.

The more destructive Marshall Fire destroyed the Sagamore subdivision, a neighborhood on the west side of Superior, Boulder County sheriff Joe Pelle said during the Thursday evening press conference. “It appears that all the homes there were lost,” he said. “That’s 370 homes.”

Pelle also said his team believes 210 homes were lost in Old Town Superior, and that homes in the small subdivisions between Superior and the unincorporated town of Marshall may have been burned as well. The Target shopping complex suffered damage, and the Element Hotel in Superior was destroyed.

No casualties had been reported as of early Thursday evening, Pelle said. However, he warned it was too soon to know for sure that no one was severely hurt or killed in the flames, and that he would not be surprised to learn of significant injuries and fatalities.

Though wildfires of this magnitude are rare in the winter, Dan Dallas, the incident commander for the Rocky Mountain Type 1 Incident Management Team, says he was unsurprised by the speed of the flames. “There’s been a lot of sunny, windy days, and that just creates a deficit. It’s dry in the soil. The grass has dried out,” he says. “So it’s very receptive to fire.”

Boulder County typically gets an average of 12.69 inches of snow in November and 12.33 inches in December, according to data from the National Oceanic and Atmospheric Administration. The area only received 0.7 inches of snow this November, and while December numbers haven’t been finalized, they will certainly be well below average.

In these types of conditions, Dallas says, a spark can easily light dried fuel (such as brush and grasses) and be spread by the wind. He says that makes the fire challenging to put out. “It’s fast moving. It’s hard to get out in front of it. You’re more worried about getting people out of the way of the fire.”

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Fire crews responded to reports of downed power lines and blown transformers just after 10 a.m., according to the Boulder County Sheriff’s Twitter account. The first fire, called the Middle Fork Fire, started near the intersection of North Foothills Highway and Middle Fork Road a bit after 10:30 a.m., Pelle said. Fire crews controlled the blaze by the afternoon and are now monitoring the flames. The Middle Fork Fire did not destroy any buildings.

The Marshall Fire was reported just after 11 a.m. at South Foothills and Marshall Road, and the wind helped it spread rapidly east over the course of the day. It currently covers an estimated 1,600 acres and is still considered dangerous. Residents of the surrounding areas should be “alert and aware,” Pelle said, in case evacuation orders change.

To monitor the size of the fire, visit bouldercounty.maps.arcgis.com, and look for updates at the Boulder Office of Emergency Management webpage.