Trevor Hughes and William M. Welch, USA TODAY
BOULDER, Colo. - Massive flash flooding along Colorado's picturesque Front Range mountains, triggered by what the National Weather Service termed "biblical rainfall amounts,'' killed at least three people Thursday, cut off small towns and forced countless residents to scramble for high ground.
Boulder, home to the University of Colorado, was among the hardest hit by the devastating waters. Classes were canceled, hundreds of students evacuated and a quarter of the campus buildings damaged by rising water, authorities said.
"This is not an ordinary day. It is not an ordinary disaster,'' Boulder County Sheriff Joe Pelle said, describing walls of water as high as 20 feet that tore down mountainsides and canyons already scarred and denuded from wildfires.
LIVE VIDEO: Follow the flash flooding in Colorado
"All the preparation in the world ... can't put people up those canyons while these walls of water are coming down," he said.
Flooding extended all along the Front Range mountains - including in Denver, Colorado Springs, Fort Collins, Greeley and Aurora - as well as scores of small towns and mountain communities.
Much of eastern and northern Colorado remained under a flash flood warning, meaning flooding is imminent or already reported, the National Weather Service said. The warning zone included the entire Denver/Boulder metropolitan area, where more than 3 million people live.
Floodwaters broke through a dam at the Rocky Mountain Arsenal National Wildlife Refuge, prompting evacuations downstream in Commerce City, KUSA-TV reported. In Denver officials ordered the evacuation of an area off East Colfax Avenue, a main thoroughfare, because of rising waters.
The fast, torrential rainfall came from a storm system that has been soaking parts of the Mountain West for much of the week. Up to 10 inches fell in an area from the Wyoming border southward to the foothills west of Denver.
The Weather Channel reported Thursday evening that rain was still falling at a rate of 1 to 2 inches an hour in some parts of the area including Colorado Springs, an hour's drive south of Denver.
Officials feared that water levels, rainfall and the victim count could increase as the storm kept battering the region into the evening and some areas remained inaccessible.
The National Weather Service issued a terse bulletin at 9:41 a.m. on Thursday that attempted to convey the scale of the threat: "Major flooding/flash flooding event underway at this time with biblical rainfall amounts reported in many areas in/near the foothills.''
In Boulder, residents used sandbags, table tops and anything they could find to divert the fast-moving floodwaters away from homes.
One person was killed in the collapse of a home in Jamestown, a mountain community isolated by washed out roads, KUSA-TV in Denver reported. Colorado Springs police found a second victim in Fountain Creek, and a third victim was recovered from a north Boulder home Thursday morning. At least one woman remained missing.
Boulder saw more than 7 inches of rain in 24 hours, shattering the town's 95-year record for rainfall.
Near the town of Lyons, rescuers were kept back when St. Vrain Creek swelled and a dam failed near Pinewood Springs.
"It's just raging, gushing water,'' Lyons resident Carin Gray said. "We're totally isolated.''
Welch reported from Los Angeles
Contributing: Gary Strauss, Doyle Rice in McLean, Va.; Associated Press.