(Adds weather forecast, evacuation, rainfall totals)
By Keith Coffman
BOULDER, Colo., Sept 13 (Reuters) - The National Guard on Friday evacuated a Colorado town cut off by raging floodwaters, while forecasters called for some let-up in record rains that have killed three people, washed out dams and swamped roads across the state.
The unusually intense late-summer storms drenched Colorado's biggest urban centers, stretching 130 miles (210 km) along the eastern slopes of the Rockies from Fort Collins near the Wyoming border south through Boulder, Denver and Colorado Springs.
Kari Bowen, a meteorologist with the National Weather Service, said the rains should slow on Friday, but intermittent showers may bring up to an inch (2.5 cm) of rain to hard-hit Boulder and Larimer counties on Friday as the bulk of the rainfall moved east.
"We're still expecting some flooding to occur, but not quite as bad as the last couple of days in terms of the amount of rain that we've been getting," she said.
National Guard troops were using trucks to evacuate the remote town of Lyons, north of Boulder, Mike Banuelos, a spokesman for the Boulder County Emergency Operations Center.
The town of 1,600 people was virtually cut off when floodwaters washed out U.S. Route 36, and residents have been without water and power for 48 hours, he said.
"It's a pretty dangerous situation," he said.
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.
President Barack Obama approved a federal disaster assistance request, which will release funds to help with emergency protection.
In Boulder, the storms shattered the rainfall record for September 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.
Boulder Creek, which runs through the heart of the city, 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. The city opened two emergency shelters for displaced residents.
A dozen major roads in northeastern Colorado remained shut with significant damage from flooding, mudslides, rockfalls and other debris, the Colorado Department of Transportation said late on Thursday.
Heavy summer rains are not unusual for Colorado, but the intensity and duration of the downpour that began on Monday night was unprecedented.
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.
A flood watch was extended until 12 p.m. MDT (1800 GMT) on Friday for the entire Front Range, the NWS said.
One 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.
Nearly 150 people were killed near Boulder in 1976 by a flash flood along the Big Thompson Canyon. (Editing by Tim Gaynor, John Stonestreet and James Dalgleish)