(Adds details on rescue, weather forecast)

By Keith Coffman

BOULDER, Colo., Sept 13 (Reuters) - The U.S. National Guard drove military vehicles into a small Colorado town on Friday to rescue up to 2,000 residents cut off by raging floods that have already killed three people in the state, hauling them out through waist-high waters.

Taking advantage of a break in record rains that caused Colorado's worst flooding in over three decades, Guard members streamed into the remote town of Lyons in high profile vehicles and were going door to door to ferry residents to safer ground.

"These individuals are not only coming with just themselves, but with their suitcases and their precious household items along with their pets and everything, all getting loaded in the back of these vehicles," said First Lieutenant Skye Robinson, a spokesman for the Colorado National Guard.

The flooding, which toppled buildings and forced thousands from their homes, was triggered by unusually intense late-summer storms that drenched Colorado's biggest urban centers, from Fort Collins near the Wyoming border south through Boulder, Denver and Colorado Springs.

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

Lyons, north of Boulder, was virtually cut off when floodwaters washed out U.S. Route 36, and residents have been without water and power for 48 hours, said Mike Banuelos, a spokesman for the Boulder County Emergency Operations Center.

"It's a pretty dangerous situation," he said.

Those evacuated from the town were later transferred to school buses at a staging area outside the flood zone and driven to shelters in nearby Longmont. It was not clear when they could return home, with more rainfall forecast for later on Friday.

"We're taking them to a safe place," Robinson said. "It's not raining at this moment but more floodwaters could be coming."

Governor John Hickenlooper said evacuations were the highest priority and advised people to stay out of debris- and sand-filled floodwaters that were "almost like liquid cement."

"It's got to be the largest storm that I can imagine in the state's history," he told a televised news conference.

The flooding was the worst in the state since nearly 150 people were killed near Boulder in 1976 by a flash flood along the Big Thompson Canyon. President Barack Obama approved a federal disaster assistance request, which will release funds to help with emergency protection.

Robinson said the respite from the storm had also allowed Blackhawk and CH-47 Chinook helicopters to fly over the devastated area for the first time, bringing humanitarian aid to stranded communities and to help with the search and rescue efforts.

CREEK BECOMES RAGING TORRENT

In Boulder, the storms shattered a September rainfall record set in 1940, officials said, unleashing surging floodwaters in Boulder Canyon above the city that triggered the evacuation of some 4,000 residents late on Thursday.

Also on Friday, flooding forced the Colorado Department of Transportation to shut down a 70-mile stretch of Interstate 25 from north of Denver to the Wyoming border, said agency spokeswoman Mindy Crane.

In Denver on Friday, a man who was swept into a culvert by raging waters was rescued about four blocks away along with his dog, said Denver Fire Department spokesman Mark Watson. "He was pretty beat up, but conscious and talking, certainly was glad to be out of there," he said.

Boulder County Sheriff's Commander Heidi Prentup told reporters on Friday that about 20 people in the county were out of contact with loved ones but that none of them had yet been classified as missing or in danger.

"We are still actively trying to locate them," Prentup said. "We have people calling in that they have relatives in Boulder County that they haven't been able to reach."

Boulder Creek, which runs through the heart of Boulder, became a raging torrent that burst its banks and flooded adjacent parking lots and streets as warning sirens wailed.

In Longmont, about 14 miles (23 km) northeast of Boulder, the St. Vrain River jumped its banks, cascading across main thoroughfares and cutting the city in two.

Assistant city manager Shawn Lewis said 7,000 households were under mandatory evacuation orders, and that the city opened two emergency shelters for displaced residents.

A dozen major roads in northeastern Colorado remained shut with significant damage from flooding, mudslides, rock falls and other debris, the Colorado Department of Transportation said late on Thursday.

A flood watch was extended until 12 p.m. MDT (1800 GMT) on Friday for the entire Front Range. The National Weather Service said at least 12.3 inches (31.24 cm) of rain have fallen on Boulder this month, smashing a 73-year-old record of 5.5 inches (14 cm) for September.

Among the casualties of the floods was a person whose body was found in a collapsed building near Jamestown, an evacuated enclave north of Boulder.

A couple were swept away in floodwaters after stopping their car northwest of the city. The man's body was recovered but the woman was missing and feared dead, said Commander Heidi Prentup of the Boulder County Sheriff's Office.

Police found the body of a third confirmed fatality, a man, during flood-watch patrols in Colorado Springs, about 100 miles (160 km) to the south, officials said. (Additional reporting by Alex Dobuzinskis and Dan Whitcomb; Editing by Cynthia Johnston and Diane Craft)

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