BOULDER, Colorado (REUTERS) - Rare torrential downpours unleashed flash flooding in Colorado that killed at least three people, left one missing and forced thousands to flee to higher ground on Thursday as rising water toppled buildings and stranded motorists, officials said.

The unusually heavy late-summer rains drenched Colorado's biggest urban centers, stretching 210 km along the eastern slopes of the Rockies from Fort Collins near the Wyoming border south through Boulder, Denver and Colorado Springs.

"There is water everywhere," said Andrew Barth, emergency management spokesman in Boulder County, which bore the brunt of tropical-like rains one meteorologist called unprecedented.

"We've had several structural collapses. There's mud and muck and debris everywhere. Cars are stranded all over the place."

The city of Boulder and a string of other towns nestled along the so-called Front Range of the Rockies north of Denver were especially hard hit as floodwaters streamed down rain-soaked mountainsides and spilled through canyons that funneled the runoff into populated areas below.

"The street was like a river, and I knew it was time to go," said Kitty Kintzing, 65, who fled her ground-floor apartment at the mouth of Boulder Canyon on Thursday morning and took shelter at an American Red Cross facility in town.

"I've never seen anything like this," said Ms Kintzing, an artist who has lived in Boulder, a city of roughly 97,000 people northwest of Denver, since 1969.

National Guard troops were dispatched with emergency supplies to the remote town of Lyons, north of Boulder, which was virtually cut off from surrounding areas when floodwaters washed out US Route 36, county officials said.

Boulder County Sheriff Joe Pelle said Lyons reported that its residents were without fresh running water and that its sewage treatment plant had been knocked out. Two other towns, Jamestown and Estes Park, were also cut off by road washouts.

Heavy summer rains known as monsoons are not unusual for Colorado, but the intensity and duration of this week's downpour was extraordinary, especially this late in the season, said Todd Dankers, a National Weather Service meteorologist in Boulder.

The latest deluge began on Monday night with steady rains that persisted through Wednesday and grew heavier overnight.

Driving, steady showers continued almost unabated in Boulder through late Thursday afternoon, and Mr Dankers said rain was not expected to taper off much before midday on Friday.

A flood watch was extended until 6 am on Friday for the entire Front Range, he said.

Low-lying areas beyond the foothills were also susceptible.

An estimated 2,000 to 3,000 people were ordered evacuated from Commerce City, a blue-collar suburb of Denver, when a retaining pond overflowed and flooded surrounding streets, police said.

Additional evacuations might be necessary due to a separate dam breach at a retaining pond in the nearby Rocky Mountain Arsenal National Wildlife Refuge, police spokesman Christian Rasmussen said.

The prolonged downpour originated from an atmospheric low-pressure system parked over Nevada and western Utah that was drawing extremely moist air out of Mexico and streaming it north into the southern Rockies, Mr Dankers said.

He said the last multi-day rainfall that spawned widespread flooding in the Boulder area occurred in 1969, but an overnight deluge in 1976 from a thunderstorm in mountains northwest of Boulder triggered a flash flood along the Big Thompson Canyon that killed nearly 150 people.

The death toll from the latest floods was much lower. Still, Mr Dankers said the scope of this week's rainfall was unprecedented in the Front Range.

One body was found in a collapsed building near Jamestown, an evacuated enclave of about 200 people just north of Boulder.

A couple were swept away in floodwaters after stopping their car along a rain-swollen creek northwest of Boulder.

The man's body was later recovered, but the woman was missing and feared dead, according to Commander Heidi Prentup of the Boulder County Sheriff's Office.

The body of a third confirmed fatality, a man, was found by police on flood-watch patrols in Colorado Springs, about 160 km to the south, officials said.

At least 24 cm of rain was measured over the past three days in Boulder, a city that typically averages just 4 cm of rainfall for all of September.

The rains transformed Boulder Creek, which runs through the heart of the city and the University of Colorado's Boulder campus, into a raging torrent that spilled over its banks and flooded adjacent parking lots and streets.

Sirens wailed and public-address loudspeakers urged residents to stay clear of high water: "Warning: Flash flood. Please proceed to higher ground. Do not cross standing or running water. Do not cross Boulder Creek."

The university campus was closed for the day, as were Boulder-area public schools and all municipal office buildings.

More than 400 students were evacuated from ground-floor campus housing overnight, campus police spokesman Ryan Huff said. Red Cross spokesman Chip Frye said two large nursing homes in town were also evacuated.

"It's really something here. I tell you, I've never seen rain like this. It's endless," said Boulder resident Lauren Sundstrom, 48, who began moving belongings out of her basement on Thursday as water began creeping in.

Roads across the region were flooded, and standing water throughout Denver snarled rush-hour commutes. On the east side of the city, a voluntary evacuation alert was issued for a neighbourhood of about 200 people when a retaining pond in an adjacent town overflowed, Denver police said.

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